We have successfully demonstrated plasmon-enhanced photocurrent generation using gold nanoparticle-loaded titanium dioxide single-crystal (TiO2) photoelectrodes with visible-light irradiation. Water molecules serve as an electron source in photocurrent generation, and oxygen evolution occurs due to water oxidation from a gold nanostructured TiO2 photoelectrode as a half reaction of water splitting. On the basis of this property, the photocurrent generation system was applied to the plasmon-induced water-splitting system using both sides of the same strontium titanate (SrTiO3) single-crystal substrate without an electrochemical apparatus. The chamber on the side of the gold nanoparticles was the anode side, whereas the chamber on the side of the platinum plate was the cathode side. Platinum was used as a co-catalyst for hydrogen evolution. Hydrogen and oxygen were separately evolved from the anode and cathode chambers, respectively. Water splitting was induced with a relatively low chemical bias of 0.23 V due to plasmonic effects based on efficient water oxidation. Similar to the artificial photosynthesis system, we have also demonstrated ammonia formation via nitrogen fixation using ruthenium as a co-catalyst via an analogous setup of the water-splitting system.
Global environmental and energy problems have motivated research and development of solar cells or artificial photosynthesis systems that can effectively use sunlight as a source of renewable energy [1–4]. We have advocated the concept of “effective utilization of a photon” using plasmonic nanostructures and investigated research on a highly efficient photochemical reaction using an optical antenna function related to the plasmonically enhanced optical near-field exhibited by metallic nanostructures [5–12]. Recently, we have promoted research on plasmon-enhanced light energy conversion, such as by solar cells or artificial photosynthesis systems. We demonstrated plasmon-enhanced photocurrent generation and water oxidation from visible to near-infrared light without deteriorating photoelectric conversion using photoelectrodes in which gold nanostructures were elaborately arrayed on the surface of a titanium dioxide (TiO2) single crystal [13–17]. We recently successfully developed a plasmon-induced artificial photosynthesis system that can split a water molecule into hydrogen (H2) and oxygen (O2) and an ammonia synthesis system based on nitrogen fixation as an evolution of the plasmon-enhanced photoelectric conversion system [18, 19].
Photoelectric conversion systems and photocatalysts using an oxide semiconductor such as TiO2 are mainly driven by ultraviolet irradiation [20–24]. However, ultraviolet radiation represents only approximately 4% of the sunlight energy that arrives at the surface of the earth, and therefore such systems do not use sunlight energy effectively. Accordingly, the development of photoelectrodes and photocatalysts that respond to visible light has been actively studied. Methodologies involving a response to visible light include the use of sensitizers such as organic dyes that absorb visible light [25–29] or the construction of a semiconductor that forms an impurity state by doping a transition metal [30–35]. These methods might be effective for a solar cell or a photocatalyst but are not optimal for water splitting. When the band gap of a semiconductor is narrowed to respond to visible wavelengths, it is difficult for the oxidation and reduction of water to proceed accompanied by a multi-electronic transition process. The thermodynamic reversible potential for water splitting is 1.23 V; however, an experimental oxygen evolution based on four electronic oxidations of water requires relatively high overpotential (leading to the requirements for additional a chemical bias) [36–38]. Thus, simultaneous oxidation and reduction of water using a single photocatalyst activated by visible light irradiation is difficult to achieve [39, 40]. Therefore, a Z-scheme process for water splitting using visible light irradiation is generally employed in which H2 and O2 evolution proceeds using photocatalysts with different energy levels in combination with an appropriate redox mediator [41–47]. However, in the Z-scheme, two different photocatalysts must be simultaneously excited, thus reducing the light irradiation area by half compared with a water splitting system driven by a single photocatalyst.
We previously determined that O2 and hydrogen peroxide (H2O2) were stoichiometrically evolved upon irradiation with visible and near-infrared light based on four or two electronic oxidations of water molecules when photoelectrochemical measurements were performed in an aqueous electrolyte solution using gold nanostructure-loaded TiO2 photoelectrodes. The gold nanoparticles appear to function as a co-catalyst of O2 evolution because the oxidation reaction of water is accelerated in the plasmonically enhanced optical near-field. Therefore, it is also expected that water splitting can be realized with a small bias on a single photocatalyst by combining such a co-catalyst of O2 and H2 evolution. Here, we describe the latest results of our research on a plasmon-induced light energy conversion system based on photocurrent generation and artificial photosynthesis employing a plasmon-induced charge separation between the gold nanoparticles and the oxide semiconductor substrate.
Plasmon-enhanced photocurrent generation and water oxidation as a half reaction of water splitting
Gold nanoislands (Au-Nis) were prepared on a 0.05 wt% niobium-doped TiO2 (Nb-TiO2) single-crystal substrate by sputtering and annealing. A gold thin film with a thickness of 3 nm was deposited on the Nb-TiO2 substrate, and the substrate was subsequently annealed at a temperature of 800 °C under a nitrogen (N2) atmosphere. A scanning electron microscope image of the prepared Au-Nis/Nb-TiO2 photoelectrode is shown in Fig. 1(a). The average estimated Au-Nis diameter was 20 nm, and the standard deviation of nanoisland size was 8 nm. Figure 1(b) shows the extinction spectrum of the Au-Nis/Nb-TiO2 photoelectrode. The Au-Nis exhibit a localized surface plasmon resonance (LSPR) band peaking at 610 nm wavelength.
We performed a photocurrent measurement in an aqueous electrolyte solution (0.1 mol/dm3 KClO4 aq.) using an Au-Nis/Nb-TiO2 photoelectrode as the working electrode (WE) with a conventional three-electrode photoelectrochemical measurement system. Pt wire and a saturated calomel electrode (SCE) were used as the counter electrode (CE) and reference electrode (RE), respectively. Notably, the electrolyte solution in this system did not contain a sacrificial electron donor. The linear sweep voltammogram (current-potential curve) measured under conditions of visible light irradiation of 550 nm, 620 nm and 750 nm on the Au-Nis/TiO2 photoelectrode is shown in Fig. 1(c). An anodic photocurrent was clearly observed above an applied potential of –0.2 V (vs. SCE). The IPCE action spectrum is shown in Fig. 1(d), and the IPCE value was calculated using the following eq. (1).
where J is the photocurrent value at 0.3 V vs. SCE measured by the quasi-steady-state current of the current-time curve, λ is the incident wavelength, and I is the intensity of the incident light. The IPCE action spectrum has a peak at a wavelength of approximately 600 nm, almost corresponding to the LSPR band, and thus plasmon-enhanced photocurrent generation was clearly observed.
Here, we discuss the possible mechanism of photocurrent generation. An energy diagram of electron excitation based on the plasmon-induced charge separation is shown in Fig. 2(a). The interband or intraband transition of gold was induced by the plasmonically enhanced optical near-field. The excited electrons in the Au-Nis are rapidly transferred to the conduction band of Nb-TiO2, and the hole might oxidize water molecules as a result of the evolution of O2 via four electronic transition processes because there is no electron donor in the aqueous electrolyte solution other than water molecules. To verify the evolution of O2 from the Au-Nis/Nb-TiO2 photoelectrode, O2 content was determined quantitatively by a gas chromatography-mass spectrometry (GC-MS) apparatus using an aqueous electrolyte solution containing isotopic Water-18O (H218O). The chromatogram in Fig. 2(b) shows that O2, which has a molecular weight of 34 (34-O2), evolved over various irradiation times when the Au-Nis/Nb-TiO2 photoelectrode was irradiated with visible light [from 450 to 750 nm (32.6 W/cm2)]. The quantity of O2 evolution increased with increasing irradiation time. The number of moles of evolved O2, as estimated from the irradiation dependence of the results of GC-MS analysis and the isotope abundance ratio, is plotted as a function of the observed photocurrent in Fig. 2(c), revealing a linear relationship. One oxygen molecule evolves per four electrons because water oxidation is a four electronic transition process, as shown in the following formula (2).
The yield of evolved O2 at the observed photocurrent was estimated to be 91.6 %. This result demonstrates that water is functioning as an electron source in the photocurrent generation system and that O2 evolves as a half-reaction of water splitting via the four-electron oxidation of water.
Plasmon-induced water splitting using both sides of a single- crystal strontium titanate substrate
To construct a water-splitting system using two electrodes acting as an anode and cathode, the electrochemical potential of H2 evolution is important [48–50]. Therefore, we employed strontium titanate (SrTiO3) as a photoelectrode because its conduction band potential is 0.2 V less than that of TiO2. Au-Nis were formed on a 0.05 wt% niobium-doped SrTiO3 (Nb-SrTiO3) single crystal by sputtering and annealing in the same manner as for the preparation of the Au-Nis/Nb-TiO2 photoelectrode. The LSPR band peaked at a wavelength of 600 nm, as shown in Fig. 4. For efficient H2 evolution, platinum (Pt) was used as a co-catalyst. A Pt board was fixed on the backside of the Au-Nis/Nb-SrTiO3 photoelectrode using an indium-gallium alloy to obtain ohmic contact between Nb-SrTiO3 and Pt. The plasmon-induced water-splitting system using two sides of the same SrTiO3 substrate was constructed using the prepared Au-Nis/Nb-SrTiO3/Pt photoelectrode as shown in Fig. 3(a). The anode and cathode chambers were separated by the substrate; thus, the Au-Nis side of the photoelectrode functioned as the anode, whereas the Pt side functioned as the cathode. The gases were separated because H2 and O2 evolution were induced in the cathode-side and anode-side chambers, respectively. The chemical bias was induced by pH regulation, and the charge balance between the chambers was maintained using a salt bridge.
A plot of the quantity of H2 and O2 evolved as a function of light irradiation time is shown in Fig. 3(b). In this experiment, the pH of the cathode-side chamber was set at 1 and that of the anode-side chamber was set at 13 (that is, the chemical bias was 708 mV), and the irradiation wavelengths and intensity were 550 nm to 650 nm and 0.7 W/cm2, respectively. The quantities of H2 and O2 evolved were linearly dependent on the irradiation time. Water splitting proceeded stoichiometrically; the quantity of evolved H2 was twice that of O2. Figure 4 shows the action spectrum of H2 evolution using a histogram. The pH conditions were the same as in Fig. 3(b). In the 600 ± 50 nm, 700 ± 50 nm, and 800 ± 50 nm wavelength regions, the evolution efficiency (mol per hour) of H2 corresponded to the LSPR band, which is indicated by the solid line in Fig. 4. Therefore, water splitting proceeded based on the plasmon-induced charge separation between Au-Nis and SrTiO3.
Here, we discuss the possible mechanism of the plasmon-induced water splitting system. The plasmonically enhanced optical near-field promotes the interband or intraband transition of gold, and the excited electron can be transferred to the conduction band of SrTiO3. The electrons injected into the Nb-SrTiO3 reduce protons at the Pt board surface to evolve H2, and the holes may oxidize hydroxyl ions and water molecules as a result of the evolution of O2 at the gold nanostructured SrTiO3 surface. The pH dependence clearly demonstrated that H2 and O2 evolve even at pH 3 and 6.8, respectively. The irradiation time dependence of H2 and O2 evolution with a pH combination of 3 and 6.8 is shown in Fig. 5. Linear H2 and O2 evolution were observed even at an irradiation time of 48 h, indicating the high stability of the system. Water splitting was induced by a chemical bias of approximately 230 mV, which corresponds to the difference between these pH values. In this study, water splitting was not confirmed at a pH combination of 4 and 13. This result indicates that the number of protons adsorbed to the surface of the platinum co-catalyst is critical for the plasmon-induced water splitting system, and thus, H2 evolution is the rate-limiting process in this reaction.
Plasmon-based ammonia synthesis system using visible light irradiation
As an analog of the plasmon-induced water splitting system, Au-Nis were formed on the Nb-SrTiO3 single-crystal substrate for use in an ammonia (NH3) synthesis system. Au-Nis were prepared on the Nb-SrTiO3 substrate by sputtering and annealing. Ruthenium (Ru) was used as a co-catalyst for NH3 evolution. Ru nanoparticles were deposited by an electron beam deposition system on the backside of the Nb-SrTiO3 substrate. Figure 6(a) shows the plasmon-based NH3 synthesis system using an Au-Nis/Nb-SrTiO3/Ru photoelectrode. In this system, the Au-Nis side of the photoelectrode functioned as the anode (oxidation), whereas the Ru side functioned as the cathode (reduction). The chemical bias between the anode-side and cathode-side chambers was controlled by pH. Ethanol (10 vol %) was added to the anode-side chambers containing water as a sacrificial electron donor. An aqueous potassium hydroxide/ethanol solution with a pH of 13 was added to the anode-side chamber, and water vapor saturated nitrogen (N2) gas was fed into an aqueous hydrochloric acid solution (pH 2) vessel in the cathode-side chamber to supply protons.
Figure 6(b) shows the irradiation time dependence of the quantity of evolved ammonia based on N2 fixation in the cathode-side chamber. In this experiment, a spectrally filtered light with wavelengths from 550 nm to 800 nm (0.2 W/cm2) was used to irradiate the Au-Nis from the optical window of the anode-side chamber. The quantity of the evolved NH3 increased linearly with light irradiation time, demonstrating that NH3 evolution proceeds due to N2 fixation under visible-light irradiation. By contrast, NH3 evolution was not detected in the absence of irradiation or Au-Nis. Furthermore, in the anode-side chamber, both acetaldehyde from the oxidation of ethanol and O2 from the oxidation of hydroxyl ions were detected and quantified. In the cathode-side chamber, H2 was also simultaneously evolved because the redox potential of H2 evolution (U0(H+/H2)) is comparable to that of NH3 evolution (U0(N2/NH3)).
Figure 7(a) shows a histogram of the action spectrum of the apparent quantum efficiency of NH3 evolution for several wavelength regions. In the 550–800 nm region, the apparent quantum efficiency of NH3 has a value approximately corresponding to the LSPR band, which is indicated by a solid line in Fig. 7(a), clearly indicating that plasmon-based NH3 synthesis due to N2 fixation occurred. By contrast, in the 410–550 nm region, the quantum efficiency was higher than the spectrum for Au-Nis. This indicates that NH3 was evolved not only by LSPR excitation but also by direct excitation of the interband transition of gold. An energy diagram of the plasmon-based NH3 synthesis system is shown in Fig. 7(b). The plasmonically enhanced optical near-field excites an electron in gold, which can be transferred to the conduction band of SrTiO3 and electrochemically reduce N2 at the surface of the Ru nanoparticles loaded in the SrTiO3, resulting in the evolution of NH3. The holes might oxidize ethanol or hydroxyl ions.
In this study, we have demonstrated that a single-crystal substrate of TiO2 or SrTiO3 loaded with gold nanoparticles having an optical antenna function can be used to develop a photoelectric conversion system or an artificial photosynthesis system responding to visible and near-infrared light. To enhance energy conversion efficiency of the photovoltaic generation or artificial photosynthesis, practical use of the unused solar light energy of the longer wavelength region is indispensable. This study provides a means of accessing this region. The results of this study suggest that localized surface plasmon resonance can contribute not only to the physical process of light excitation of a chemical substance but also to the efficient propagation of chemical reactions, such as plasmon-induced charge separation, creating new opportunities for the field of “plasmonic chemistry” in photochemical research. Plasmonic chemistry will contribute to the construction of efficient solar light energy conversion systems.
A collection of invited papers based on presentations at the XXVth IUPAC Symposium on Photochemistry, Bordeaux, France, July 13 – 18, 2014.
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 (s) (no. 23225006) and the Innovative Areas “Artificial Photosynthesis (AnApple)” (no. 25107501) grant from the Japan Society for the Promotion of Science (JSPS), the Nanotechnology Platform (Hokkaido University), and the Low-Carbon Research Network of Japan.
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