Near-Infrared Plasmon-Assisted Water Oxidation - The Journal of

Letter pubs.acs.org/JPCL

Near-Infrared Plasmon-Assisted Water Oxidation Yoshiaki Nishijima,† Kosei Ueno,†,‡ Yuki Kotake,† Kei Murakoshi,§ Haruo Inoue,∥ and Hiroaki Misawa*,† †

Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0021, Japan PRESTO, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan § Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan ∥ SORST/JST, Tokyo Metropolitan University, 1-1 minami-ohsawa, Hachiohji, Tokyo 192-0397, Japan ‡

ABSTRACT: We report the stoichiometric evolution of oxygen via water oxidation by irradiating a plasmon-enhanced photocurrent generation system with near-infrared light (λ: 1000 nm), in which gold nanostructures were arrayed on the surface of TiO2 electrode. It is considered that multiple electron holes generated by plasmon-induced charge excitation led to the effective recovery of water oxidation after the electron transfer from gold to TiO2. The proposed system containing a gold nanostructured TiO2 electrode may be a promising artificial photosynthetic system using near-infrared light.

SECTION: Plasmonics, Optical Materials, and Hard Matter

A

water. Alternatively, in the proposed system, near-infrared light with a wavelength of 1000 nm oxidizes water, and thus the system described here may be used to develop artificial photosynthetic systems that are superior to those in plants. Gold nanorods (Au-NRs, 220 nm × 110 nm × 40 nm) that display localized surface plasmon resonance (LSPR) were fabricated on n-type rutile TiO2 single crystals (0.05 wt.% niobium doped) using electron beam lithography (EBL)/lift-off processes.24,25 The inset in Figure 1 shows a scanning electron microscope (SEM) image of Au-NRs fabricated on TiO2. To determine the energetics of water oxidation on the proposed gold nanostructured TiO2 photoelectrode (Au-NRs/TiO2 electrode), we performed photocurrent measurements using a three-electrode photoelectrochemical measurement system containing Au-NRs/TiO2 as the working electrode. An aqueous electrolyte solution containing Na2SO4 (0.1 mol/dm3, pH 7.3) was used. The extinction spectrum of Au-NRs fabricated on a TiO2 single crystal was measured in water under nonpolarized conditions and is shown in Figure 2a. Two distinct LSPR bands were observed at 780 and 1040 nm, which correspond to the LSPR bands obtained in the transverse mode (T-mode, λmax: 780 nm) and the longitudinal mode (L-mode, λmax: 1040 nm), respectively.26 Figure 2a also shows the photocurrent action spectrum (IPCE), which nearly reproduces the shape of the LSPR bands, except at wavelengths less than 500 nm. Therefore, a plasmonic photocurrent was generated by

system that changes solar energy to electrical or chemical energy is indispensable for solving the world’s energy problems. To obtain stable light energy conversion that is not affected by climatic conditions, we must develop chemical energy sources that can be stored for long periods. Owing to the Honda-Fujishima effect, which was discovered in 1972, the simultaneous generation of oxygen (O2) and hydrogen (H2) from a titanium dioxide (TiO2) anode and a platinum cathode can be achieved by irradiating the TiO2 electrode with ultraviolet light accompanying the photovoltaic effect.1 However, only 6% of the solar energy that arrives at the surface of the earth can excite the wide band gap of TiO2. Thus far, various photocatalysts composed of semiconductor particles with narrow band gaps that respond to light in the visible spectrum have been developed.2−10 The main drawbacks of artificial photosynthetic systems for the conversion of photoenergy to chemical energy are associated with the oxidation of water via four-electron transfer.11,12 Herein, we report the successful stoichiometric evolution of oxygen via water oxidation by irradiating a plasmon-enhanced photocurrent generation system with near-infrared light. Although the systems with response to visible light showing photocatalytic activity have already been reported so far,13−23 this is a first observation as a water oxidation with a near-infrared light irradiation. In the proposed system, gold nanostructures were elaborately arrayed on the surface of a TiO2 single-crystal electrode, which enabled smooth water oxidation owing to the stabilization of multiple electron holes in a local space by plasmonically enhanced optical near-fields. In higher plants, visible light with a wavelength of 680 nm drives photosynthetic system II, which generates oxygen by abstracting electrons from © 2012 American Chemical Society

Received: March 17, 2012 Accepted: April 26, 2012 Published: April 26, 2012 1248

dx.doi.org/10.1021/jz3003316 | J. Phys. Chem. Lett. 2012, 3, 1248−1252

The Journal of Physical Chemistry Letters

Letter

irradiating the system with visible to near-infrared light.26 At wavelengths less than 500 nm, the direct excitation of gold (interband transition) was strongly related to the photocurrent. I−V measurements were performed under irradiation at several ranges of wavelengths (indicated by arrows in the IPCE action spectrum), as shown in Figure 2b. Anodic photocurrents were observed at positive potentials greater than −0.3 V. No photocurrent was observed at the TiO2 single crystal without Au-NRs under irradiation of light with a wavelength of 450 nm or longer, although data are not shown here.26 In the system described here, irradiation with near-infrared light at wavelengths between 850 and 1150 nm generated an anodic photocurrent of 20 μA/cm2. Figure 2c depicts irradiation times plotted as a function of the number of O2 molecules that evolved from the Au-NRs/TiO2 working electrode. At all of the studied wavelengths, the number of evolved O2 molecules increased linearly with increasing irradiation times, indicating that water oxidation proceeded with high stability. As shown in Figure 2c, the number of evolved O2 molecules was quantitatively determined after irradiation with near-infrared light at wavelengths between 850 and 1150 nm. As the spectral width increased, an increase in the number of evolved O2 molecules was observed because the photocurrent density increased with the range of wavelengths used, as shown in Figure 2b. Therefore, the evolution of O2 is closely related to the generation of a photocurrent. The relationship between the number of electrons calculated from the observed anodic photocurrent at each wavelength

Figure 1. Schematic illustration of the proposed three-electrode photoelectrochemical cell and the oxidation of water on Au-NRs. The inset shows a scanning electron microscope image of Au-NRs fabricated on TiO2.

Figure 2. (a) Extinction spectrum of Au-NRs fabricated on a single TiO2 crystal measured in water (upper panel) and the corresponding photocurrent action spectrum (IPCE, lower panel). Both spectra were obtained under nonpolarized conditions. The arrows show the wavelength ranges of light used to obtain the I−V curve and to measure O2 evolution (see Methods Summary). (b) I−V curves obtained by the irradiation of light at several wavelength ranges, which are indicated by the inset arrows shown in panel a. The dotted curve indicates dark current. (c) Relationship between irradiation time and the number of O2 molecules that evolved at several wavelength ranges.

1249



Letter

Table 1. Relationship between the Number of Electrons Calculated from the Observed Anodic Photocurrent at Each Wavelength Range and the Absolute Quantity of O2 and H2O2 That Evolved incident wavelength

500 ± 50 nm

600 ± 50 nm

700 ± 50 nm

800 ± 50 nm

1000 ± 150 nm

number of electrons O2 (molecules) H2O2 (molecules)

1.4 × 1019 3.3 × 1018 nd

1.1 × 1018 2.7 × 1017 nd

9.3 × 1018 2.0 × 1018 nd

4.4 × 1018 6.6 × 1017 4.5 × 1017

7.1 × 1018 7.2 × 1017 1.49 × 1018

range and the absolute quantity of O2 and H2O2 that evolved is summarized in Table 1. To compare the unified irradiation time to the absolute quantity of O2 and H2O2 produced, the number of electrons produced in the reaction and the quantity of evolved O2 and H2O2 were converted into per-hour units. To demonstrate the dependence of O2 and H2O2 evolution on the incident wavelength, we determined the yield of O2 and H2O2 as a function of the observed photocurrent, and the results are summarized in Figure 3. As shown in Figure 3, upon irradiation

Figure 4. Energy diagram of the Au-NRs/TiO2 electrode system. UFB and UO show flat band potential of TiO2 and redox potentials versus RHE (reversible hydrogen electrode), respectively. The symbols [−] and [+] indicate electron and hole.

water oxidation in the present system. Therefore, the mechanism of water oxidation by low-energy light in the current Au-NRs/TiO2 system must be considered. A catalytic electrode that reduces the activation energy of the reaction is necessary to transfer four electrons from two water molecules. In the present system, multiple electron holes play an important role in the photocatalytic reaction after plasmoninduced charge excitation and electron transfer through direct interband excitation or the decay of the plasmon to hot electrons.13,27,28 As previously reported by Furube and coworkers, the time of electron transfer to TiO2 is 82% in the presence of light at a wavelength of 1000 ± 150 nm. Therefore, the oxidation of water also proceeded nearly stoichiometrically under near-infrared irradiation. Figure 4 shows an energy diagram of the Au-NRs/TiO2 electrode system. Interestingly, the oxidation of water was achieved by irradiating the system with near-infrared light. The IPCE spectrum illustrated in Figure 2 proves that even lowenergy irradiation (1000 ± 150 nm (E = 1.24 eV)) promotes 1250



Letter

oxo[5,10,15,20-tetra(4-pyridyl)porphinato]titanium(IV) as an indicator of H2O2. To analyze the quantity of O2 and H2O2 evolved at an observed photocurrent, O2, H2O2, and the number of electrons were quantitatively measured to assess the relationship between the irradiation time and the photocurrent. To measure quantitatively the evolution of O2 and H2O2 and obtain I−V curves, the Au-NRs/TiO2 electrode was irradiated using broadband spectra of xenon light spectrally filtered to wavelength ranges of 450−1150, 550−1150, 650−1150, 750− 1150, and 850−1150 nm. To assess the evolution of O2 and H2O2 at 500 ± 50 nm, we estimated the difference between the quantities of O2 and H2O2 evolved at 450−1150 and 550−1150 nm. Alternatively, monochromatic xenon light with a bandwidth of 10 nm was used to obtain photocurrent action spectra (IPCE). To produce IPCE action spectra and I−V curves, we used a saturated calomel electrode (SCE) as the reference electrode. Platinum wires were used as counter and quasi-reference electrodes at the photoelectrochemical cell for quantitative measurement on oxygen evolution. For the IPCE measurement, applied potential of the Au-NRs/TiO2 working electrode was set at +0.3 V versus SCE. The present system does not show significant potential dependence on the IPCE under sufficient positive polarization of the working electrode (>0 V vs SCE) because of efficient charge separation at the space charge layer of the TiO2 electrode. We confirmed that this sufficient positive polarization (+0.3 V vs SCE) was applied to the system using the Pt quasi-reference electrode.

mechanism of photocurrent generation that is due to LSPR excitation and the direct excitation of gold (interband transition) should be different. Importantly, the potential of the conduction band edge does not change during LSPR excitation and interband transition because the onset potential of anodic photocurrent remained constant at −0.3 V in each wavelength region (Figure 2b). Therefore, a particular reaction process that reduces the activation energy was induced. It is difficult to explain the difference of the activation energy only from the viewpoint of thermodynamics. In the case of the interband transition of gold, it is considered that the hole is delocalized to the whole area of the interface between gold nanorod and TiO2. Therefore, O2 and H2O2 evolution is not induced if intermediates resulting from twoelectron transfer reaction do not migrate and collide with each other on the surface of gold. Therefore, a kinetic energy is required for migration, which leads to the increasing of the activation energy. However, IPCE shows a high value because the wavelength of the light that induces interband transition of gold is shorter with higher energy. In the case of plasmon excitation, multiple holes are generated and stored at the local space by plasmonically enhanced optical near-field, so that it can be considered that two-electron transfer occurs from a water molecule to the holes. Namely, intermediates can react without migration. Therefore, it can be concluded that the activation energy becomes lower compared with the case of interband transition. On the basis of the proposed mechanism, plasmonic optical near-fields would promote water oxidation, resulting in the generation of O2 and H2O2, even in the wavelength region of low-energy near-infrared light. In conclusion, plasmon-assisted water oxidation using a AuNRs/TiO2 electrode was verified by analyzing the evolution of O2 and H2O2 from the electrode. Interestingly, the results of the present study indicate that plasmonic-enhanced optical near-fields may reduce the activation energy of the reaction, and stable plasmon-induced electron−hole pairs may promote water oxidation. The proposed plasmon-assisted photocurrent generation system, which contains a Au-NRs/TiO2 electrode, may be a promising artificial photosynthetic system to generate oxygen using visible and near-infrared light.

■

AUTHOR INFORMATION

Corresponding Author

*Phone: +81-11-706-9358. Fax: +81-11-706-9359. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This study was supported by funding from the Ministry of Education, Culture, Sports, Science, and Technology of Japan: KAKENHI Grant-in-Aid for Scientific Research on the Priority Area, “Strong Photon−Molecule Coupling Fields” (no. 470 (no. 19049001) and no. 23225006), and the Low-Carbon Research Network of Japan.

■

METHODS SUMMARY Large periodic arrays of Au-NRs were defined on a TiO2 electrode using EBL conducted on an SEM instrument (ELSF125; Elionix) operating at 125 kV. A conventional copolymer resist (ZEP520A; Zeon Chemicals) diluted with ZEP thinner (1:1) was spin-coated (1000 rpm for 10 s and 4000 rpm for 90 s) and prebaked on a hot plate for 3 min at 180 °C. EBL was conducted at an electrical current of 3 nA. After development, a gold−titanium bilayer was deposited by sputtering (MPS-4000; ULVAC), and a 2 nm titanium/40 nm gold bilayer was obtained. Lift-off was carried out by immersion in a solution of anisole for 2 min in an ultrasonic bath. The fabricated structures were characterized using field emission-SEM (FESEM; JSM-6700FT; JEOL) at a resolution of 1 nm. By forming an In−Ga alloy film (ratio by weight: 1:1) on the back of the TiO2 substrate, ohmic contact was maintained between a lead wire connected to the electrochemical analyzer and the Au-NRs/TiO2 working electrode.26 The amount of evolved O2 and hydrogen peroxide (H2O2) induced by the oxidation of water via photoexcitation of the Au-NRs/TiO2 electrode was quantitatively determined by GC-MS using water-(18-O) (10% isotopic purity) and absorptiometry using

■

REFERENCES

(1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37−38. (2) Krasnovsky, A. A.; Brin, G. P. Inorganic Models of Hill’s Reaction. Dokl. Akad. Nauk SSSR 1962, 147, 656−659. (3) Erbs, W.; Desilvestro, J.; Borgarello, E.; Gräetzel, M. VisibleLight-Induced Oxygen Generation from Aqueous Dispersions of Tungsten(VI) Oxide. J. Phys. Chem. 1984, 88, 4001−4006. (4) Matsumura, M.; Saho, Y.; Tsubomura, H. Photocatalytic Hydrogen Production from Solutions of Sulfite Using Platinized Cadmium Sulfide Powder. J. Phys. Chem. 1983, 87, 3807−3808. (5) Yoshimura, J.; Ebina, Y.; Kondo, J.; Domen, K.; Tanaka, A. Visible Light-Induced Photocatalytic Behavior of a Layered PerovskiteType Rubidium Lead Niobate, RbPb2Nb3O10. J. Phys. Chem. 1993, 97, 1970−1973. (6) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. VisibleLight Photocatalysis in Nitrogen-Doped Titanium Oxides. Science 2001, 293, 269−271. (7) Kato, H.; Kudo, A. Visible-Light-Response and Photocatalytic Activities of TiO2 and SrTiO3 Photocatalysts Codoped with Antimony and Chromium. J. Phys. Chem. B 2002, 106, 5029−5034. 1251



Letter

Infrared Wavelength Using a Au-Nanorods/TiO2 Electrode. J. Phys. Chem. Lett. 2010, 1, 2031−2036. (27) Knight, M. W.; Sobhani, H.; Nordlander, P.; Halas, N. J. Photodetection with Active Optical Antennas. Science 2011, 332, 702− 704. (28) Warren, S. C.; Thimsen, E. Plasmonic Solar Water Splitting. Energy Environ. Sci 2012, 5, 5133−5146. (29) Furube, A.; Du, L.; Hara, K.; Katoh, R.; Tachiya, M. Ultrafast Plasmon-Induced Electron Transfer from Gold Nanodots into TiO2 Nanoparticles. J. Am. Chem. Soc. 2007, 129, 14852−14853.

(8) Yu, J. C.; Yu, J. G.; Ho, W. K.; Jiang, Z. T.; Zhang, L. Z. Effects of F-Doping on the Photocatalytic Activity and Microstructures of Nanocrystalline TiO2 Powders. Chem. Mater. 2002, 14, 3808−3816. (9) Irie, H.; Wanatabe, Y.; Hashimoto, K. Nitrogen-Concentration Dependence on Photocatalytic Activity of TiO2−xNx Powders. J. Phys. Chem. B 2003, 107, 5483−5486. (10) Ohno, T.; Akiyoshi, M.; Umebayashi; TAsai, K.; Mitui, T.; Matsumura, M. External Link Preparation of S-Doped TiO 2 Photocatalysts and Their Photocatalytic Activities under Visible Light. Appl. Catal., A 2004, 265, 115−121. (11) Dau, H.; Limberg, C.; Reier, T.; Risch, M.; Roggan, S.; Strasser, P. The Mechanism of Water Oxidation: From Electrolysis via Homogeneous to Biological Catalysis. ChemCatChem 2010, 2, 724− 761. (12) Inoue, H.; Shimada, T.; Kou, Y.; Nabetani, Y.; Masui, D.; Takagi, S.; Tachibana, H. The Water Oxidation Bottleneck in Artificial Photosynthesis: How Can We Get through It? An Alternative Route Involving a Two-Electron Process. ChemSusChem 2011, 4, 173−179. (13) Tian, Y.; Tatsuma, T. Mechanisms and Applications of PlasmonInduced Charge Separation at TiO2 Films Loaded with Gold Nanoparticles. J. Am. Chem. Soc. 2005, 127, 7632−7637. (14) Awazu, K.; Fujimake, M.; Rockstuhl, C.; Tominaga, J.; Murakami, H.; Ohki, Y.; Yoshida, N.; Watanabe, T. A Plasmonic Photocatalyst Consisting of Silver Nanoparticles Embedded in Titanium Dioxide. J. Am. Chem. Soc. 2008, 130, 1676−1680. (15) Kowalska, E.; Abe, R.; Ohtani, B. Visible Light-Induced Photocatalytic Reaction of Gold-Modified Titanium (IV) Oxide Particles: Action Spectrum Analysis. Chem. Commun. 2009, 241−243. (16) Christopher, P.; Ingram, D. B.; Linic, S. Enhancing Photochemical Activity of Semiconductor Nanoparticles with Optically Active Ag Nanostructures: Photochemistry Mediated by Ag Surface Plasmons. J. Phys. Chem. C 2010, 19, 9173−9177. (17) Zhou, X.; Hu, C.; Hu, X.; Peng, T.; Qu, J. Plasmon-Assisted Degradation of Toxic Pollutants with Ag-AgBr/Al2O3 under VisibleLight Irradiation. J. Phys. Chem. C 2010, 114, 2746−2750. (18) Primo, A.; Corma, A.; García, H. Titania Supported Gold Nanoparticles as Photocatalyst. Phys. Chem. Chem. Phys. 2011, 13, 886−910. (19) Kumar, M. K.; Krishnamoorthy, S.; Tan, L. K.; Chiam, S. Y.; Tripathy, S.; Gao, H. Field Effects in Plasmonic Photocatalyst by Precise SiO2 Thickness Control Using Atomic Layer Deposition. ACS Catal. 2011, 1, 300−308. (20) Silva, C. G.; Juárez, R.; Marino, T.; Molinari, R.; García, H. Influence of Excitation Wavelength (UV or Visible Light) on the Photocatalytic Activity of Titania Containing Gold Nanoparticles for the Generation of Hydrogen or Oxygen from Water. J. Am. Chem. Soc. 2011, 133, 595−602. (21) Liu, Z.; Hou, W.; Pavaskar, P.; Aykol, M.; Cronin, S. B. Plasmon Resonant Enhancement of Photocatalytic Water Splitting Under Visible Illumination. Nano Lett. 2011, 11, 1111−1116. (22) Ingram, D. B.; Linic, S. Water Splitting on Composite Plasmonic-Metal/Semiconductor Photoelectrodes: Evidence for Selective Plasmon-Induced Formation of Charge Carriers near the Semiconductor Surface. J. Am. Chem. Soc. 2011, 133, 5202−5205. (23) Primo, A.; Marino, T.; Corma, A.; Molinari, R.; García, H. Efficient Visible-Light Photocatalytic Water Splitting by Minute Amounts of Gold Supported on Nanoparticulate CeO2 Obtained by a Biopolymer Templating Method. J. Am. Chem. Soc. 2011, 133, 6930−6933. (24) Ueno, K.; Juodkazis, S.; Mizeikis, V.; Sasaki, K.; Misawa, H. Clusters of Closely-Spaced Gold Nanoparticles as a Source of TwoPhoton Photoluminescence at Visible Wavelengths. Adv. Mater. 2008, 20, 26−30. (25) Ueno, K.; Juodkazis, S.; Shibuya, T.; Yokota, Y.; Mizeikis, V.; Sasaki, K.; Misawa, H. Nanoparticle Plasmon-Assisted Two-Photon Polymerization Induced by Incoherent Excitation Source. J. Am. Chem. Soc. 2008, 130, 6928−6929. (26) Nishijima, Y.; Ueno, K.; Yokota, Y.; Murakoshi, K.; Misawa, H. Plasmon-Assisted Photocurrent Generation from Visible to Near1252


Near-Infrared Plasmon-Assisted Water Oxidation - The Journal of

Recommend Documents