ChemComm
Published on 26 May 2014. Downloaded by University of Hawaii at Manoa Library on 21/08/2014 06:34:59.
COMMUNICATION
Cite this: Chem. Commun., 2014, 50, 7614 Received 8th April 2014, Accepted 26th May 2014
View Article Online View Journal | View Issue
Nitrogen and transition-metal codoped titania nanotube arrays for visible-light-sensitive photoelectrochemical water oxidation† Tomiko M. Suzuki,* Gaku Kitahara, Takeo Arai, Yoriko Matsuoka and Takeshi Morikawa
DOI: 10.1039/c4cc02571g www.rsc.org/chemcomm
Vertically aligned titanium dioxide nanotube (TNT) arrays codoped with nitrogen and 3d transition metals were successfully fabricated using anodization and nitridation processes. The codoping of N and Fe yielded the highest visible-light-induced photoelectrochemical water oxidation due to bandgap narrowing of impurity levels by N and Fe.
Photoinduced oxygen production by water-splitting, which extracts electrons and protons from water molecules, is one of the important photoreactions to create renewable energy sources using sunlight for both hydrogen generation1 and CO2 conversion to fuels.2 To date, it has been recognized that semiconductor nanotubes, such as titanium dioxide nanotubes (TNTs), which are well ordered and densely packed are attractive for constructing efficient photoanodes for water oxidation due to their aligned nanostructure and high surface area.3 Owing to the wide band gap (3.2 eV) of titanium dioxide (TiO2), TNTs can only utilize UV light, so that it is important to develop new TNTs with enhanced photocatalytic activity under visible-light irradiation. In TiO2 particles and films, single-doping of anions such as nitrogen (N)4 or 3d transition metals (TMs) such as vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), and nickel (Ni)5 has been successfully demonstrated as one of the approaches to extend the spectral response toward the visible-light region. It was suggested that substitutional doping of N to the O-site creates a new level above the valence band maximum (VBM) in the band gap,4,6 and doping of some 3d TM ions creates deep levels below the conduction band minimum (CBM).5,7 Furthermore, codoping of anionic N and cationic V, Cr or cobalt (Co) is promising because first-principles calculations have suggested that these have the potential to form new states close to the valence band and conduction band edges, respectively.7,8 Although some reports have indicated the enhancement of visible-light absorption of TiO2 Toyota Central Research & Development Labs. Inc., Nagakute, Aichi, 480-1192, Japan. E-mail: [email protected]; Fax: +81-561-63-6137 † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cc02571g
7614 | Chem. Commun., 2014, 50, 7614--7616
powder or film by codoping both anionic and cationic species,9 these were insufficient because the effect of codoping in the TiO2 lattice may be limited to extend photoabsorption in the visible-light range without deteriorating photocatalytic reaction rates. The rate deterioration is responsible for charge recombination centers created by dopant aggregation and/or unbalanced total charge induced by doping. Dopant aggregation tends to occur for cationic doping rather than anionic doping. For preventing aggregation of cationic dopants, nanotubes are excellent because TNTs in which dopants are highly dispersed can be fabricated in a relatively simple way by electrochemical anodization of Ti-based alloys in which metallic dopants are highly dispersed in a manufacturing process.3c,d In addition, the thin wall thickness of TNTs (ca. 15 nm) is beneficial for subsequent doping with N under an ammonia atmosphere in a dry process.4 As for codoped TNTs composed of elements mentioned above, only N–Nb codoped TNTs10 and Ti–Pd oxynitride nanotubes11 have been demonstrated to date. However, detailed studies on the effects of codoping of N and various 3d-TMs into TNTs for enhanced visible-light-induced oxidation of water have not been reported so far. In this work, we report on fabrication of TNT arrays codoped with N and earth-abundant 3d TMs for visible-light driven photoelectrochemical water oxidation. To the best of our knowledge, this is the first report on codoped TNTs wherein the effects of codoping of N and 3d TMs into TNT arrays have been studied systematically, and O2 evolution by water-splitting is also demonstrated under visible-light irradiation. Fig. 1a demonstrates the fabrication method for N and TM (TM: Fe, V, Cr, or Co) codoped TNT [(N,TM)-TNT] arrays via anodization of alloys composed of Ti and low-concentration TM (Ti-TM alloy) and a subsequent N-doping process. Fig. 1b and c show a field emission scanning electron microscope (FE-SEM) image of (N,Fe)-TNTs grown on Ti–Fe (0.13 at% of Fe) alloys [denoted as (N,Fe0.13)-TNTs hereafter]. The external diameter and length of the nanotubes were ca. 100 nm and ca. 3000 nm, respectively. The structures of the nanotubes were not changed by nitridation and we also confirmed the fabrication of various
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 26 May 2014. Downloaded by University of Hawaii at Manoa Library on 21/08/2014 06:34:59.
Communication
ChemComm
Fig. 1 (a) Schematic process for the fabrication of N and transition-metal codoped TiO2 nanotube [(N,TM)-TNT] arrays. (b) Top view and (c) crosssectional view FE-SEM images of (N,Fe0.13)-TNT.
(N,TM)-TNTs grown on Ti-alloy substrates with different TMs containing 0.05 at% Fe, V, and Cr, and 0.06 at% Co, respectively [denoted as the (N,Fe0.05)-TNT, (N,V)-TNT, (N,Cr)-TNT, and (N,Co)-TNT, Fig. S1, ESI†]. XRD patterns of all (N,TM)-TNTs have broad peaks assignable to the typical anatase phase and the patterns of the samples after nitridation remained unchanged from those of TM-TNTs [Fig. S2, ESI†]. In all samples, metal oxide could not be observed in XRD patterns. We also confirmed the N and TM doping in the TNT samples by X-ray photoelectron spectroscopy (XPS), dynamic secondary ion mass spectrometry (D-SIMS) and time-of-flight (TOF)-SIMS (Fig. S3–6, ESI†). The nitrogen concentrations were determined to be 1.9–2.3 at% for (N,TM)-TNTs, according to a previously reported calculation method (Table S2, ESI†).12 These results indicate that vertically aligned TNTs codoped with N and various TMs were indeed successfully prepared. To investigate the characteristics of the codoped TNT arrays, photoelectrochemical measurements were performed in a threeelectrode cell containing 0.1 M KOH with a (N,TM)-TNT photoanode, a platinum cathode, and an Ag/AgCl reference electrode. Fig. 2 shows the time courses for the photocurrent of various (N,TM)-TNTs which were fabricated using Ti-TM alloys containing the same amounts of TMs (0.05–0.06 at% of TMs) under visiblelight irradiation at +0.6 V (vs. Ag/AgCl). Although N-doping in the TNT provides enhanced photocurrent under visible-light irradiation, as previously reported,3b,13 TM-doping (Fe-TNT data shown as a representative example) was not effective for improving the visible photoresponse. It is worth noting that codoping with both N and TMs substantially improved the photocurrent of the TNT photoanode. The rate of increase in the photocurrent was dependent on the TM species and the order of the effectiveness in improving the photocurrent was found to be Fe 4 Cr, V 4 Co. These TMs were reported to be effective for band-gap narrowing by the overlap of the conduction band of Ti;5,7 this is consistent with the present results for codoping of N and TM. Moreover, the photocurrent of (N,Fe0.13)-TNT actually increased by 63% compared with the (N,Fe0.05)-TNT (Fig. S7-1, ESI†). Similar results were obtained in the case of (N,Co)-TNT samples (Fig. S7-2, ESI†). This indicates that the amount and ratio of N and TM are strongly correlated with the enhancement in photoactivity under visible light. As for the chemical state of TMs in the
This journal is © The Royal Society of Chemistry 2014
Fig. 2 Time courses for the photocurrent of various TNT samples which contain 0.05–0.06 at% of each metal for doping. The curves were measured in a 0.1 M aqueous KOH solution at +0.6 V (vs. Ag/AgCl) visible light (Z410 nm).
TNTs, although it was not confirmed owing to low concentration of TMs, it could be expected that TMs are substituted in Ti sites, taking into consideration the highly enhanced photoresponse under visible light. In order to improve the stability of the (N,TM)-TNT photoanode, we modified it with a water oxidation co-catalyst, cobalt borate (Co–Bi), which operates in a basic electrolyte.14 As shown in Fig. 3, the (N,Fe0.13)-TNT modified with the Co–Bi co-catalyst enhanced the stability of the photocurrent and yielded visiblelight-induced water oxidation with a photocurrent density of 0.76 mA cm 2, which was 13 times and 5 times, respectively, higher than that of Fe0.13-TNTs and N-TNTs. Optimization of the chemical state and the amount of the co-catalyst will lead to further enhancement in the photocurrent of the (N,TM)-TNT photoelectrode as demonstrated for the Ta3N5 photoanodes.15 We evaluated the incident photon to charge carrier efficiency (IPCE) spectra for the TNT, Fe0.13-TNT, N-TNT, and (N,Fe0.13)TNT which were loaded with the Co–Bi co-catalyst and IPCE in the visible-light region of 400–700 nm was enhanced by doping
Fig. 3 Time courses for the photocurrent TNT, N-TNT, and (N,Fe0.13)-TNT with and without Co–Bi. The curves were measured in a 0.1 M aqueous KOH solution at +0.6 V (vs. Ag/AgCl) visible light (Z410 nm).
Chem. Commun., 2014, 50, 7614--7616 | 7615
View Article Online
Published on 26 May 2014. Downloaded by University of Hawaii at Manoa Library on 21/08/2014 06:34:59.
ChemComm
Fig. 4 (a) Presumed energy-level diagram of N and Fe codoped TNT. (b) Time profile of the current (black line, left vertical axis) and the amount of O2 (red line, right vertical axis) observed over the Co–Bi/(N,Fe0.13)-TNT photoelectrode. O2 was measured in the electrolyte (0.05 M K2B4O7 and 0.2 M H3BO3 solution, pH 8.4) at +0.6 V (vs. Ag/AgCl) visible light (Z410 nm). The blue line indicates the theoretical amount of oxygen calculated by the observed photocurrent with 100% current efficiency.
N and/or Fe into the TNTs (Fig. S8, ESI†). The IPCE at 420 nm of the (N,Fe)-TNT, N-TNT, Fe-TNT, and TNT was measured to be 1.52%, 0.42%, 0.38%, and 0.03%, respectively (Fig. S8b, ESI†), and the codoping of N and Fe was found to be the most effective for enhancement of photoresponse. Fig. 4a shows the presumed energy level diagram of N and Fe doping into the TNTs. Single doping of both N and Fe into the TNTs has the effect of narrowing of the band gap or formation of impurity levels by N and Fe. In addition, it is worth noting that the codoping of both N and Fe not only exhibits a positive effect on the photoresponse in the visible-light region but also suppresses degradation of the photoresponse in the range of UV light compared with that of N single doping. Reducing the amount of N doping will also lead to enhance the IPCE in the UV region as demonstrated for N–TiO2 nanowires.16 The flatband potential was estimated by Mott–Schottky analysis, which clarified that the doping of N or/and Fe caused shifts in the flatband potential to a more negative position (Fig. S10 and Table S3, ESI†). In order to confirm the chemical reaction correlating with the photocurrent, we examined the amounts of O2 evolved by water-splitting over the (N,Fe0.13)-TNT loaded with the Co–Bi co-catalyst. Oxygen detection was conducted in near-neutral borate buffer using a fluorescence measurement system.17 As shown in Fig. 4b, O2 generation and the photocurrent were confirmed repeatedly during visible-light irradiation with high current efficiency (Z95%) and these results indicate that the observed photocurrent originated from the water-splitting reaction. We also observed O2 generation in a basic electrolyte (0.1 M KOH), in which the amount of O2 corresponded to the theoretical amount (Fig. S9, ESI†). In conclusion, a versatile and simple method using anodization and nitridation processes was demonstrated for the fabrication of vertically aligned N and 3d-TM-codoped TNT arrays in which the dopants are highly dispersed. The codoping of N and TM substantially improved the photocurrent of the TNT photoanode under visible-light irradiation due to the band-gap narrowing of impurity levels by N and TM. The photoelectrochemical water oxidation activity of the codoped TNTs could be further improved by optimizing the amount of doping, the
7616 | Chem. Commun., 2014, 50, 7614--7616
Communication
kind of dopant, and the nanotube structure. This scalable method for codoping of TNTs can also be extended to other metal oxide nanotubes for solar-driven photoelectrochemical water-splitting for hydrogen generation and the CO2 reduction reaction. This work was partially supported by a Grant-in-Aid for Scientific Research on Innovative Areas Artificial photosynthesis (AnApple) from the Japan Society for the Promotion of Science (JSPS). The authors thank Dr Tomohiro Suzuki, Ms Mai Asaoka, and Ms Aya Tsuji for the preparation of the Ti-TM alloys, Mr Satoru Kosaka for the ICP measurements, Ms Naoko Takahashi for XPS measurements, Mr Kousuke Kitazumi for XPS and D-SIMS measurements, and Ms Masae Inoue and Mr Yoshitake Suganuma for TOF-SIMS measurements. The authors also thank Dr Toshio Horie for fruitful discussions.
Notes and references 1 (a) O. Khaselev and J. A. Turner, Science, 1998, 280, 425; (b) K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito, Y. Inoue and K. Domen, Nature, 2006, 440, 295; (c) S. Y. Reece, J. A. Hamel, K. Sung, T. D. Jarvi, A. J. Esswein, J. J. H. Pijpers and D. G. Nocera, Science, 2011, 334, 645. 2 S. Sato, T. Arai, T. Morikawa, K. Uemura, T. M. Suzuki, H. Tanaka and T. Kajino, J. Am. Chem. Soc., 2011, 133, 15240. 3 (a) C. A. Grimes, J. Mater. Chem., 2007, 17, 1451; (b) S. C. Roy, O. K. Varghese, M. Paulose and C. A. Grimes, ACS Nano, 2010, 4, 1259; (c) P. Roy, S. Berger and P. Schmuki, Angew. Chem., Int. Ed., 2011, 50, 2904; (d) Y. Jun, J. H. Park and M. G. Kang, Chem. Commun., 2012, 48, 6456. 4 R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science, 2001, 293, 269. 5 (a) H. Yamashita, M. Harada, J. Misaka, M. Takeuchi, K. Ikeue and M. Anpo, J. Photochem. Photobiol., A, 2002, 148, 257; (b) M. Anpo and M. Takeuchi, J. Catal., 2003, 216, 505. 6 (a) H. Irie, Y. Watanabe and K. Hashimoto, J. Phys. Chem. B, 2003, 107, 5483; (b) S. Livraghi, M. C. Paganini, E. Giamello, A. Selloni, C. D. Valentin and G. Pacchioni, J. Am. Chem. Soc., 2006, 128, 15666. 7 Y. Gai, J. Li, S.-S. Li, J.-B. Xia and S.-H. Wei, Phys. Rev. Lett., 2009, 102, 036402. 8 (a) W. Zhu, X. Qiu, V. Iancu, X.-Q. Chen, H. Pan, W. Wang, N. M. Dimitrijevic, T. Rajh, H. M. Meyer III, M. P. Paranthaman, G. M. Stocks, H. H. Weitering, B. Gu, G. Eres and Z. Zhang, Phys. Rev. Lett., 2009, 103, 226401; (b) X. J. Zhang, G. F. Zhang, H. X. Jin, L. D. Zhu and Q. L. Lin, Acta Phys. Sin., 2013, 62, 017102. 9 (a) Y. Cong, J. Zhang, F. Chen, M. Anpo and D. He, J. Phys. Chem. C, 2007, 111, 10618; (b) X. Yang, C. Cao, L. Erickson, K. Hohn, R. Maghirang and K. Klabunde, Appl. Catal., B, 2009, 91, 657; (c) M. Chiodi, C. P. Cheney, P. Vilmercati, E. Cavaliere, N. Mannella, H. H. Weitering and L. Gavioli, J. Phys. Chem. C, 2012, 116, 311. ´alu, P.-A. Gross, S. N. Pronkin, N. Keller, 10 T. Cottineau, N. Be E. R. Savinova and V. Keller, J. Mater. Chem. A, 2013, 1, 2151. 11 N. K. Allam, A. J. Poncheri and M. A. El-Sayed, ACS Nano, 2011, 5, 5056. 12 R. Asahi and T. Morikawa, Chem. Phys., 2007, 339, 57. 13 (a) R. P. Vitiello, J. M. Macak, A. Ghicov, H. Tsuchiya, L. F. P. Dick and P. Schmuki, Electrochem. Commun., 2006, 8, 544; (b) O. K. Varghese, M. Paulose, T. J. LaTempa and C. A. Grimes, Nano Lett., 2009, 9, 731. ˘ and D. G. Nocera, J. Am. Chem. Soc., 14 (a) Y. Surendranath, M. Dinca 2009, 131, 2615; (b) A. J. Esswein, Y. Surendranath, S. Y. Reece and D. G. Nocera, Energy Environ. Sci., 2011, 4, 499. 15 Y. Li, T. Takata, D. Cha, K. Takanabe, T. Minegishi, J. Kubota and K. Domen, Adv. Mater., 2013, 25, 125. 16 S. Hoang, S. Guo, N. T. Hahn, A. J. Bard and C. B. Mullins, Nano Lett., 2012, 12, 26. 17 D. K. Zhong, S. Choi and D. R. Gamelin, J. Am. Chem. Soc., 2011, 133, 18370.
This journal is © The Royal Society of Chemistry 2014
Published on 26 May 2014. Downloaded by University of Hawaii at Manoa Library on 21/08/2014 06:34:59.
COMMUNICATION
Cite this: Chem. Commun., 2014, 50, 7614 Received 8th April 2014, Accepted 26th May 2014
View Article Online View Journal | View Issue
Nitrogen and transition-metal codoped titania nanotube arrays for visible-light-sensitive photoelectrochemical water oxidation† Tomiko M. Suzuki,* Gaku Kitahara, Takeo Arai, Yoriko Matsuoka and Takeshi Morikawa
DOI: 10.1039/c4cc02571g www.rsc.org/chemcomm
Vertically aligned titanium dioxide nanotube (TNT) arrays codoped with nitrogen and 3d transition metals were successfully fabricated using anodization and nitridation processes. The codoping of N and Fe yielded the highest visible-light-induced photoelectrochemical water oxidation due to bandgap narrowing of impurity levels by N and Fe.
Photoinduced oxygen production by water-splitting, which extracts electrons and protons from water molecules, is one of the important photoreactions to create renewable energy sources using sunlight for both hydrogen generation1 and CO2 conversion to fuels.2 To date, it has been recognized that semiconductor nanotubes, such as titanium dioxide nanotubes (TNTs), which are well ordered and densely packed are attractive for constructing efficient photoanodes for water oxidation due to their aligned nanostructure and high surface area.3 Owing to the wide band gap (3.2 eV) of titanium dioxide (TiO2), TNTs can only utilize UV light, so that it is important to develop new TNTs with enhanced photocatalytic activity under visible-light irradiation. In TiO2 particles and films, single-doping of anions such as nitrogen (N)4 or 3d transition metals (TMs) such as vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), and nickel (Ni)5 has been successfully demonstrated as one of the approaches to extend the spectral response toward the visible-light region. It was suggested that substitutional doping of N to the O-site creates a new level above the valence band maximum (VBM) in the band gap,4,6 and doping of some 3d TM ions creates deep levels below the conduction band minimum (CBM).5,7 Furthermore, codoping of anionic N and cationic V, Cr or cobalt (Co) is promising because first-principles calculations have suggested that these have the potential to form new states close to the valence band and conduction band edges, respectively.7,8 Although some reports have indicated the enhancement of visible-light absorption of TiO2 Toyota Central Research & Development Labs. Inc., Nagakute, Aichi, 480-1192, Japan. E-mail: [email protected]; Fax: +81-561-63-6137 † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cc02571g
7614 | Chem. Commun., 2014, 50, 7614--7616
powder or film by codoping both anionic and cationic species,9 these were insufficient because the effect of codoping in the TiO2 lattice may be limited to extend photoabsorption in the visible-light range without deteriorating photocatalytic reaction rates. The rate deterioration is responsible for charge recombination centers created by dopant aggregation and/or unbalanced total charge induced by doping. Dopant aggregation tends to occur for cationic doping rather than anionic doping. For preventing aggregation of cationic dopants, nanotubes are excellent because TNTs in which dopants are highly dispersed can be fabricated in a relatively simple way by electrochemical anodization of Ti-based alloys in which metallic dopants are highly dispersed in a manufacturing process.3c,d In addition, the thin wall thickness of TNTs (ca. 15 nm) is beneficial for subsequent doping with N under an ammonia atmosphere in a dry process.4 As for codoped TNTs composed of elements mentioned above, only N–Nb codoped TNTs10 and Ti–Pd oxynitride nanotubes11 have been demonstrated to date. However, detailed studies on the effects of codoping of N and various 3d-TMs into TNTs for enhanced visible-light-induced oxidation of water have not been reported so far. In this work, we report on fabrication of TNT arrays codoped with N and earth-abundant 3d TMs for visible-light driven photoelectrochemical water oxidation. To the best of our knowledge, this is the first report on codoped TNTs wherein the effects of codoping of N and 3d TMs into TNT arrays have been studied systematically, and O2 evolution by water-splitting is also demonstrated under visible-light irradiation. Fig. 1a demonstrates the fabrication method for N and TM (TM: Fe, V, Cr, or Co) codoped TNT [(N,TM)-TNT] arrays via anodization of alloys composed of Ti and low-concentration TM (Ti-TM alloy) and a subsequent N-doping process. Fig. 1b and c show a field emission scanning electron microscope (FE-SEM) image of (N,Fe)-TNTs grown on Ti–Fe (0.13 at% of Fe) alloys [denoted as (N,Fe0.13)-TNTs hereafter]. The external diameter and length of the nanotubes were ca. 100 nm and ca. 3000 nm, respectively. The structures of the nanotubes were not changed by nitridation and we also confirmed the fabrication of various
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 26 May 2014. Downloaded by University of Hawaii at Manoa Library on 21/08/2014 06:34:59.
Communication
ChemComm
Fig. 1 (a) Schematic process for the fabrication of N and transition-metal codoped TiO2 nanotube [(N,TM)-TNT] arrays. (b) Top view and (c) crosssectional view FE-SEM images of (N,Fe0.13)-TNT.
(N,TM)-TNTs grown on Ti-alloy substrates with different TMs containing 0.05 at% Fe, V, and Cr, and 0.06 at% Co, respectively [denoted as the (N,Fe0.05)-TNT, (N,V)-TNT, (N,Cr)-TNT, and (N,Co)-TNT, Fig. S1, ESI†]. XRD patterns of all (N,TM)-TNTs have broad peaks assignable to the typical anatase phase and the patterns of the samples after nitridation remained unchanged from those of TM-TNTs [Fig. S2, ESI†]. In all samples, metal oxide could not be observed in XRD patterns. We also confirmed the N and TM doping in the TNT samples by X-ray photoelectron spectroscopy (XPS), dynamic secondary ion mass spectrometry (D-SIMS) and time-of-flight (TOF)-SIMS (Fig. S3–6, ESI†). The nitrogen concentrations were determined to be 1.9–2.3 at% for (N,TM)-TNTs, according to a previously reported calculation method (Table S2, ESI†).12 These results indicate that vertically aligned TNTs codoped with N and various TMs were indeed successfully prepared. To investigate the characteristics of the codoped TNT arrays, photoelectrochemical measurements were performed in a threeelectrode cell containing 0.1 M KOH with a (N,TM)-TNT photoanode, a platinum cathode, and an Ag/AgCl reference electrode. Fig. 2 shows the time courses for the photocurrent of various (N,TM)-TNTs which were fabricated using Ti-TM alloys containing the same amounts of TMs (0.05–0.06 at% of TMs) under visiblelight irradiation at +0.6 V (vs. Ag/AgCl). Although N-doping in the TNT provides enhanced photocurrent under visible-light irradiation, as previously reported,3b,13 TM-doping (Fe-TNT data shown as a representative example) was not effective for improving the visible photoresponse. It is worth noting that codoping with both N and TMs substantially improved the photocurrent of the TNT photoanode. The rate of increase in the photocurrent was dependent on the TM species and the order of the effectiveness in improving the photocurrent was found to be Fe 4 Cr, V 4 Co. These TMs were reported to be effective for band-gap narrowing by the overlap of the conduction band of Ti;5,7 this is consistent with the present results for codoping of N and TM. Moreover, the photocurrent of (N,Fe0.13)-TNT actually increased by 63% compared with the (N,Fe0.05)-TNT (Fig. S7-1, ESI†). Similar results were obtained in the case of (N,Co)-TNT samples (Fig. S7-2, ESI†). This indicates that the amount and ratio of N and TM are strongly correlated with the enhancement in photoactivity under visible light. As for the chemical state of TMs in the
This journal is © The Royal Society of Chemistry 2014
Fig. 2 Time courses for the photocurrent of various TNT samples which contain 0.05–0.06 at% of each metal for doping. The curves were measured in a 0.1 M aqueous KOH solution at +0.6 V (vs. Ag/AgCl) visible light (Z410 nm).
TNTs, although it was not confirmed owing to low concentration of TMs, it could be expected that TMs are substituted in Ti sites, taking into consideration the highly enhanced photoresponse under visible light. In order to improve the stability of the (N,TM)-TNT photoanode, we modified it with a water oxidation co-catalyst, cobalt borate (Co–Bi), which operates in a basic electrolyte.14 As shown in Fig. 3, the (N,Fe0.13)-TNT modified with the Co–Bi co-catalyst enhanced the stability of the photocurrent and yielded visiblelight-induced water oxidation with a photocurrent density of 0.76 mA cm 2, which was 13 times and 5 times, respectively, higher than that of Fe0.13-TNTs and N-TNTs. Optimization of the chemical state and the amount of the co-catalyst will lead to further enhancement in the photocurrent of the (N,TM)-TNT photoelectrode as demonstrated for the Ta3N5 photoanodes.15 We evaluated the incident photon to charge carrier efficiency (IPCE) spectra for the TNT, Fe0.13-TNT, N-TNT, and (N,Fe0.13)TNT which were loaded with the Co–Bi co-catalyst and IPCE in the visible-light region of 400–700 nm was enhanced by doping
Fig. 3 Time courses for the photocurrent TNT, N-TNT, and (N,Fe0.13)-TNT with and without Co–Bi. The curves were measured in a 0.1 M aqueous KOH solution at +0.6 V (vs. Ag/AgCl) visible light (Z410 nm).
Chem. Commun., 2014, 50, 7614--7616 | 7615
View Article Online
Published on 26 May 2014. Downloaded by University of Hawaii at Manoa Library on 21/08/2014 06:34:59.
ChemComm
Fig. 4 (a) Presumed energy-level diagram of N and Fe codoped TNT. (b) Time profile of the current (black line, left vertical axis) and the amount of O2 (red line, right vertical axis) observed over the Co–Bi/(N,Fe0.13)-TNT photoelectrode. O2 was measured in the electrolyte (0.05 M K2B4O7 and 0.2 M H3BO3 solution, pH 8.4) at +0.6 V (vs. Ag/AgCl) visible light (Z410 nm). The blue line indicates the theoretical amount of oxygen calculated by the observed photocurrent with 100% current efficiency.
N and/or Fe into the TNTs (Fig. S8, ESI†). The IPCE at 420 nm of the (N,Fe)-TNT, N-TNT, Fe-TNT, and TNT was measured to be 1.52%, 0.42%, 0.38%, and 0.03%, respectively (Fig. S8b, ESI†), and the codoping of N and Fe was found to be the most effective for enhancement of photoresponse. Fig. 4a shows the presumed energy level diagram of N and Fe doping into the TNTs. Single doping of both N and Fe into the TNTs has the effect of narrowing of the band gap or formation of impurity levels by N and Fe. In addition, it is worth noting that the codoping of both N and Fe not only exhibits a positive effect on the photoresponse in the visible-light region but also suppresses degradation of the photoresponse in the range of UV light compared with that of N single doping. Reducing the amount of N doping will also lead to enhance the IPCE in the UV region as demonstrated for N–TiO2 nanowires.16 The flatband potential was estimated by Mott–Schottky analysis, which clarified that the doping of N or/and Fe caused shifts in the flatband potential to a more negative position (Fig. S10 and Table S3, ESI†). In order to confirm the chemical reaction correlating with the photocurrent, we examined the amounts of O2 evolved by water-splitting over the (N,Fe0.13)-TNT loaded with the Co–Bi co-catalyst. Oxygen detection was conducted in near-neutral borate buffer using a fluorescence measurement system.17 As shown in Fig. 4b, O2 generation and the photocurrent were confirmed repeatedly during visible-light irradiation with high current efficiency (Z95%) and these results indicate that the observed photocurrent originated from the water-splitting reaction. We also observed O2 generation in a basic electrolyte (0.1 M KOH), in which the amount of O2 corresponded to the theoretical amount (Fig. S9, ESI†). In conclusion, a versatile and simple method using anodization and nitridation processes was demonstrated for the fabrication of vertically aligned N and 3d-TM-codoped TNT arrays in which the dopants are highly dispersed. The codoping of N and TM substantially improved the photocurrent of the TNT photoanode under visible-light irradiation due to the band-gap narrowing of impurity levels by N and TM. The photoelectrochemical water oxidation activity of the codoped TNTs could be further improved by optimizing the amount of doping, the
7616 | Chem. Commun., 2014, 50, 7614--7616
Communication
kind of dopant, and the nanotube structure. This scalable method for codoping of TNTs can also be extended to other metal oxide nanotubes for solar-driven photoelectrochemical water-splitting for hydrogen generation and the CO2 reduction reaction. This work was partially supported by a Grant-in-Aid for Scientific Research on Innovative Areas Artificial photosynthesis (AnApple) from the Japan Society for the Promotion of Science (JSPS). The authors thank Dr Tomohiro Suzuki, Ms Mai Asaoka, and Ms Aya Tsuji for the preparation of the Ti-TM alloys, Mr Satoru Kosaka for the ICP measurements, Ms Naoko Takahashi for XPS measurements, Mr Kousuke Kitazumi for XPS and D-SIMS measurements, and Ms Masae Inoue and Mr Yoshitake Suganuma for TOF-SIMS measurements. The authors also thank Dr Toshio Horie for fruitful discussions.
Notes and references 1 (a) O. Khaselev and J. A. Turner, Science, 1998, 280, 425; (b) K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito, Y. Inoue and K. Domen, Nature, 2006, 440, 295; (c) S. Y. Reece, J. A. Hamel, K. Sung, T. D. Jarvi, A. J. Esswein, J. J. H. Pijpers and D. G. Nocera, Science, 2011, 334, 645. 2 S. Sato, T. Arai, T. Morikawa, K. Uemura, T. M. Suzuki, H. Tanaka and T. Kajino, J. Am. Chem. Soc., 2011, 133, 15240. 3 (a) C. A. Grimes, J. Mater. Chem., 2007, 17, 1451; (b) S. C. Roy, O. K. Varghese, M. Paulose and C. A. Grimes, ACS Nano, 2010, 4, 1259; (c) P. Roy, S. Berger and P. Schmuki, Angew. Chem., Int. Ed., 2011, 50, 2904; (d) Y. Jun, J. H. Park and M. G. Kang, Chem. Commun., 2012, 48, 6456. 4 R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science, 2001, 293, 269. 5 (a) H. Yamashita, M. Harada, J. Misaka, M. Takeuchi, K. Ikeue and M. Anpo, J. Photochem. Photobiol., A, 2002, 148, 257; (b) M. Anpo and M. Takeuchi, J. Catal., 2003, 216, 505. 6 (a) H. Irie, Y. Watanabe and K. Hashimoto, J. Phys. Chem. B, 2003, 107, 5483; (b) S. Livraghi, M. C. Paganini, E. Giamello, A. Selloni, C. D. Valentin and G. Pacchioni, J. Am. Chem. Soc., 2006, 128, 15666. 7 Y. Gai, J. Li, S.-S. Li, J.-B. Xia and S.-H. Wei, Phys. Rev. Lett., 2009, 102, 036402. 8 (a) W. Zhu, X. Qiu, V. Iancu, X.-Q. Chen, H. Pan, W. Wang, N. M. Dimitrijevic, T. Rajh, H. M. Meyer III, M. P. Paranthaman, G. M. Stocks, H. H. Weitering, B. Gu, G. Eres and Z. Zhang, Phys. Rev. Lett., 2009, 103, 226401; (b) X. J. Zhang, G. F. Zhang, H. X. Jin, L. D. Zhu and Q. L. Lin, Acta Phys. Sin., 2013, 62, 017102. 9 (a) Y. Cong, J. Zhang, F. Chen, M. Anpo and D. He, J. Phys. Chem. C, 2007, 111, 10618; (b) X. Yang, C. Cao, L. Erickson, K. Hohn, R. Maghirang and K. Klabunde, Appl. Catal., B, 2009, 91, 657; (c) M. Chiodi, C. P. Cheney, P. Vilmercati, E. Cavaliere, N. Mannella, H. H. Weitering and L. Gavioli, J. Phys. Chem. C, 2012, 116, 311. ´alu, P.-A. Gross, S. N. Pronkin, N. Keller, 10 T. Cottineau, N. Be E. R. Savinova and V. Keller, J. Mater. Chem. A, 2013, 1, 2151. 11 N. K. Allam, A. J. Poncheri and M. A. El-Sayed, ACS Nano, 2011, 5, 5056. 12 R. Asahi and T. Morikawa, Chem. Phys., 2007, 339, 57. 13 (a) R. P. Vitiello, J. M. Macak, A. Ghicov, H. Tsuchiya, L. F. P. Dick and P. Schmuki, Electrochem. Commun., 2006, 8, 544; (b) O. K. Varghese, M. Paulose, T. J. LaTempa and C. A. Grimes, Nano Lett., 2009, 9, 731. ˘ and D. G. Nocera, J. Am. Chem. Soc., 14 (a) Y. Surendranath, M. Dinca 2009, 131, 2615; (b) A. J. Esswein, Y. Surendranath, S. Y. Reece and D. G. Nocera, Energy Environ. Sci., 2011, 4, 499. 15 Y. Li, T. Takata, D. Cha, K. Takanabe, T. Minegishi, J. Kubota and K. Domen, Adv. Mater., 2013, 25, 125. 16 S. Hoang, S. Guo, N. T. Hahn, A. J. Bard and C. B. Mullins, Nano Lett., 2012, 12, 26. 17 D. K. Zhong, S. Choi and D. R. Gamelin, J. Am. Chem. Soc., 2011, 133, 18370.
This journal is © The Royal Society of Chemistry 2014
