Plasmon-Induced Water Splitting Using Metallic-Nanoparticle-Loaded Photocatalysts and Photoelectrodes.

DOI: 10.1002/cphc.201500761

Reviews

Plasmon-Induced Water Splitting Using MetallicNanoparticle-Loaded Photocatalysts and Photoelectrodes Kosei Ueno,[a] Tomoya Oshikiri,[a] and Hiroaki Misawa*[a, b]

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Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Reviews Visible- and near-infrared-light-driven water splitting, which splits water molecules to generate hydrogen and oxygen gases, is a significant subject in artificial photosynthesis with the goal of achieving a low-carbon society. In recent years, considerable attention has been paid to studies on the development of a plasmon-induced water-splitting system responding to visible light. In this review, we categorized water-splitting systems as gold-nanoparticle-loaded semiconductor photocatalytic particles system and metallic-nanoparticles-loaded

semiconductor photoelectrode systems, and introduce the latest studies according to these categories. Especially, we describe the studies that optimize a material or a structural design of metallic-nanoparticle-loaded semiconductor photoelectrodes and consider a whole water-splitting system, including a cathode design. Furthermore, we discuss important points when studying plasmon-induced water splitting, and we describe a methodology that enhances plasmon-induced water-splitting efficiency.

1. Introduction

ported. Their findings were as follows: 1) The water-splitting reaction using photocatalysts that respond to visible light is promoted by the electromagnetic field enhancement effect, as illustrated in Scheme 11 (sometimes called a plasmon-resonant energy-transfer mechanism), and 2) the most popular mecha-

To achieve a low-carbon society, efficient energy conversion of sunlight energy, which is renewable, is required. Artificial photosynthesis, which performs the generation of hydrogen[1] and the fixation of nitrogen[2] and carbon dioxide,[3] has received particular attention in relation to the storage and transportation of energy, even in solar energy conversion. The Honda–Fujishima effect, by which the photolysis of water advances by irradiating a titanium dioxide (TiO2) photoanode with ultraviolet light has been discovered in 1968;[4] subsequently, studies related to the semiconductor photocatalyst progressed.[5] However, ultraviolet light composes only approximately 5 % of sunlight energy that arrives at the surface of the Earth. The photolysis of water by visible and near-infrared light abundantly contained in the solar spectrum is indispensable for efficient light–energy conversion. Lengthening of the absorption wavelength has been attained. Semiconductor particles with a smaller band gap than that of TiO2 have been synthesized, and band engineering of a semiconductor has been used in the photolysis of water using conventional semiconductor photocatalytic particles.[6] However, in these systems, it is difficult to set up a response wavelength freely. Although the dye-sensitizing method was developed and even used in the water-splitting system,[7] gold (Au) nanoparticles and nanorods showing localized surface plasmon resonance, which is robust and able to control a response wavelength freely from a visible to a near infrared wavelength, have received recent attention.[8] Nanoparticles of noble metals, such as Au and silver (Ag), have shown localized surface plasmon resonances according to interactions with visible and near-infrared light. They exhibit an electromagnetic field enhancement effect in the vicinity of metallic nanoparticles, which reaches several orders of magnitude compared to incident light fields.[9] Many studies related to plasmon-enhanced water splitting have recently been re-

Scheme 1. Schematic illustrations of reaction scheme using gold nanoparticle-loaded semiconductor.

nism, the photolysis of water can be induced by plasmon-induced charge separation following an efficient electron transfer from metal to semiconductor when metallic nanoparticles are used as a sensitizer, as illustrated in Scheme 1 2 (sometimes discussed in relation to a hot electron-transfer mechanism). Here, we introduce research on the plasmon-induced watersplitting system, particularly in relation to the abovementioned two mechanisms, which have been actively studied in recent years. In this review, we categorize the water-splitting systems as schematically illustrated in Figure 1. First, the plasmon-induced water-splitting system can be roughly divided into two systems. One is the Au-nanoparticle-loaded semiconductor photo-

[a] Prof. Dr. K. Ueno, Prof. Dr. T. Oshikiri, Prof. Dr. H. Misawa Research Institute for Electronic Science Hokkaido University N21, W10, Kita-ku, 001-0021, Sapporo (Japan) E-mail: [email protected]

Figure 1. Categories of plasmon-induced water-splitting systems introduced in this review.

[b] Prof. Dr. H. Misawa Department of Applied Chemistry & Institute of Molecular Science National Chiao Tung University 1001 Ta Hsueh R., Hsinchu 30010 (Taiwan)

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catalytic particles system and the other is the metallic-nanoparticle-loaded semiconductor photoelectrode system. Furthermore, the metallic-nanoparticle-loaded semiconductor photo200


Reviews electrode system can be further divided into two categories by two-electrode system and three-electrode system. Section 2 describes studies on the Au-nanoparticle-loaded semiconductor photocatalytic particles system for hydrogen evolution and/or oxygen evolution. Section 3 introduces studies on the plasmon-induced water-splitting systems employing metallicnanostructures-loaded semiconductor photoelectrodes as working electrodes for the three-electrode-type of conventional photoelectrochemical measurement system. Especially, the studies that aimed to optimize photoelectrode design by controlling the semiconductor material and the structural design are described. Section 4 introduces the studies on the plasmon-induced water-splitting systems with two-electrode sys-

tems and discuss a design manual for not only photoelectrodes but also the whole water-splitting system. This review also describes the advantages and limitations of each system and discusses important issues related to plasmon-induced water splitting. It also discusses a methodology to improve plasmoninduced water-splitting efficiency.

2. Au-Nanoparticles-Loaded Semiconductor Photocatalytic Particle Systems In 2006, a study reported on a water-splitting system using photocatalytic particles loading Au nanoparticles as a co-catalyst. In the original research on Au nanoparticle catalysts and co-catalysts, in 1987, Haruta et al. famously and successfully demonstrated that small Au nanoclusters (e.g., a diameter of less than 2 nm) without showing localized surface plasmon resonance efficiently oxidized carbon monoxide .[10] Furthermore, Kamat et al. elucidated that the photocurrent value was enhanced by loading Au nanoparticles onto a TiO2 photoelectrode even with irradiation of ultraviolet light, although this finding was not water splitting.[11] Based on this enhancement of the photocurrent, it was considered that the recombination of electrons and holes in the conduction band and in the valence band of TiO2 could be suppressed because the electron was trapped with Au nanoparticles. This section introduces studies on the Au nanoparticles loaded semiconductor photocatalytic particles system for hydrogen evolution and/or oxygen evolution. Section 2.1 describes a research demonstrating a complete water-splitting system using photocatalytic semiconductor particles loaded with Au nanoparticles as a co-catalyst. Section 2.2 describes studies which hydrogen or oxygen evolution was induced as a half-reaction of water splitting using sacrificial reagents to suppress back-reaction of water splitting based on the recombination of the evolved hydrogen and oxygen gases and to explore the plasmonic effects on the water splitting at visible wavelengths.

Kosei Ueno is an associate professor at the Research Institute for Electronic Science at Hokkaido University, Japan. He received his Ph.D. in chemistry from Hokkaido University in 2004. From 2004 to 2006, he worked in Professor Hiroaki Misawa’s laboratory as a JSPS research fellow. He became an assistant professor at Hokkaido University in 2006 and was promoted to associate professor in 2008. He studies surface-plasmon-assisted nanolithography and surface-enhanced spectroscopy in the infrared wavelength region. Tomoya Oshikiri is an assistant professor at the Research Institute for Electronic Science at Hokkaido University, Japan. He received his Ph.D. in chemistry from Osaka University in 2008. From 2008 to 2012, he worked at Mitsubishi Rayon Co., Ltd. He became an assistant professor at Hokkaido University in 2012. He studies energy conversion via localized surface plasmon.

2.1. Au Nanoparticle-Loaded Photocatalytic Particles for Complete Water Splitting Kudo et al. conducted research employing Au nanoparticles as a co-catalyst for complete water splitting.[12] They explored the evolution efficiency of both hydrogen and oxygen from Aunanoparticle-loaded (diameter: 20 nm, plasmon resonance peaking at 520 nm) water-splitting photocatalysts, such as K4Nb6O17, Sr2Nb2O7, KTaO3, NaTaO3, and NaTaO3 doped with lanthanum (NaTaO3 :La, particle diameter: 300 nm–10 mm), with irradiation with ultraviolet and visible light. When the photocatalyst (0.3–0.5 g) with or without Au nanoparticles was dispersed in 350 mL of pure water and subsequently irradiated by a high-pressure mercury lamp, the water-splitting activities of niobate and tantalate photocatalysts for water splitting with Au nanoparticles were higher than those without Au nanoparticles, as shown in Table 1. It was concluded that the Au nanoparticles co-catalyst had an effect on water splitting. However, the back-reaction between evolved hydrogen and oxygen or

Hiroaki Misawa is a professor at the Research Institute for Electronic Science at Hokkaido University, Japan. He received his Ph.D. in Chemistry from the University of Tsukuba in 1984. After an assistant professorship at the University of Tsukuba, he joined the Microphotoconversion project (ERATO) of JST. He became an associate professor at the University of Tokushima in 1993 and was promoted to full professor in 1995. He moved to Hokkaido University as full professor in 2003. Since 2015, he has held an additional post as a chair professor at NCTU. He studies plasmonic chemistry.

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Reviews Research related to a water-splitting system employing photocatalytic nanoparticles loading Au nanoparticles as a co-catalyst has been reported in addition to the above research. However, many studies have reported improvement in the charge separation efficiency based on controlling a back-reaction or a back electron transfer reaction.[13]

Table 1. Hydrogen and oxygen evolution efficiency using Au-nanoparticle-loaded semiconductor photocatalysts.[12]

Photocatalyst

Amount of Au loaded [wt %]

Activity [mmol h¢1] H2 O2

K4Nb6O17

None 1 None 2 None 0.5 None 0.3 None 1

17 26 10 32 11 58 157 642 404 1950

Sr2Nb2O7 KTaO3 NaTaO3 NaTaO3 :La

5 13 0 14 0 25 60 224 187 880

2.2. Plasmon-Induced Water Oxidation or Reduction Using Au-Nanoparticle-Loaded Photocatalytic Particles In 2010, Garca et al. proposed a plasmon-induced hydrogen evolution or oxygen evolution systems that used TiO2 photocatalytic nanoparticles and loaded Au nanoparticles, showing a localized surface plasmon resonance.[14] In these systems, it was considered that the electron in the conduction band of TiO2 that transferred from the Au nanoparticle reduced protons or water, and holes might oxidize water molecules following plasmon excitation. The charge separation between Au nanoparticles and TiO2 has been reported in the photoelectric conversion system. One study has reported that water oxidation is efficiently induced by plasmon-induced charge separation; this content is introduced in Section 3.3. To use a TiO2 particle photocatalyst for a water-splitting system, Garca et al. conducted hydrogen and oxygen evolution as a half-reaction of water splitting using a sacrificial reagent.[14] First, in the case of hydrogen evolution, a mechanism was proposed in which electrons are injected into the conduction band of TiO2, which reduces protons or water to evolve hydrogen gas, and holes are quenched by electron donors, such as EDTA [2, 2’, 2’’, 2’’’-(ethane-1,2-diyldinitrilo)tetraacetic acid] and methanol, as schematically illustrated in Figure 3 a. Figure 3 b shows the irradiation time dependence of hydrogen evolution efficiency. In this experiment, methanol was employed as a sacrificial electron donor. Hydrogen evolution was confirmed based on the visible light irradiation, with wavelengths longer than 400 nm, and the existence of Au nanoparticles enhanced hydrogen evolution efficiency. In the case of oxygen evolution, a mechanism was proposed in which holes formed after plasmon-induced charge separation oxidized water molecules to evolve oxygen gas, and electrons injected in the conduction band of TiO2 reacted with electron acceptors, such as AgNO3 and (NH4)2Ce(NO3)6, as schematically illustrated in Figure 4 a. Figure 4 b shows the irradiation time dependence of oxygen evolution efficiency, indicating that the amount of evolved oxygen also increased even with an irradiation of visible light with a wavelength longer than 400 nm with the increase in the irradiation time, although the oxygen evolution efficiency under ultraviolet light irradiation conditions was higher than that under visible light irradiation conditions. In this study, the action spectrum of the evolved gas was not measured, and it is unknown whether the photolysis of water proceeded stoichiometrically to the consumed sacrificial reagent in the reaction.[14] Furthermore, there is a possibility that an interband transition of Au from d-bands to an sp-conduction band absorbing below a 500 nm wavelength of visible light also induced these reactions based on the charge

the photoreduction of the evolved oxygen simultaneously proceeded, even when Au nanoparticles were used as a co-catalyst, analogously to platinum, because the deactivation was detected with an increasing irradiation time. Figure 2 shows a bar graph of how much hydrogen and oxygen were consumed in the reaction between hydrogen and oxygen, as a model of the back-reaction of water splitting when platinum and Au nanoparticles were loaded on the Na-

Figure 2. Reaction rate between hydrogen and oxygen on platinum-loaded NaTaO3 :La and Au-loaded NaTaO3 :La in the dark. Initial pressure: H2 100 torr, O2 50 torr.[12]

TaO3 :La photocatalyst. It was clearly observed that a reaction between hydrogen and oxygen did not proceed easily compared with platinum nanoparticles when Au nanoparticles were used as a co-catalyst. This finding showed that Au nanoparticles worked as a co-catalyst for water splitting by employing a photocatalyst that responds to visible light, and it was difficult to advance a back-reaction as compared to platinum. Kudo et al. experimentally confirmed that the reduction of the evolved oxygen on Au nanoparticle caused the deactivation of water splitting. Most importantly, it is not clear from these experimental results whether the plasmon resonance was participating in the water splitting. To verify the participation of plasmon resonance, it is necessary to measure the action spectrum of water-splitting efficiency. ChemPhysChem 2016, 17, 199 – 215


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Figure 3. a) Schematic illustration of the proposed mechanism for H2 evolution with Au/TiO2 photocatalyst.[14] b) Volume of hydrogen evolved (VH2) during the photocatalytic runs (l > 400 nm) using catalysts with different gold loadings; TiO2 (&), Au (0.25 %)/TiO2 (*), Au (1.5 %)/TiO2 (~) and Au (2.2 %)/TiO2 ( ! ).[14]

Figure 4. a) Schematic illustration of the proposed mechanism for O2 evolution with Au/TiO2 photocatalyst.[14] b) Volume of oxygen (VO2) evolved during the photocatalytic runs with Au (1.5 wt %)/TiO2 and TiO2 under UV and visible light (l > 400 nm) using either AgNO3 or (NH4)2Ce(NO3)6 as sacrificial electron acceptors; *: Au/TiO2, UV, (NH4)2Ce(NO3)6. &: Au/TiO2, UV, AgNO3. !: Au/ TiO2, vis, (NH4)2Ce(NO3)6. ~: Au/TiO2, vis, AgNO3. *:TiO2, UV, (NH4)2Ce(NO3)6. [14] &: TiO2, UV, AgNO3. ~: TiO2, vis, AgNO3. !: TiO2, vis, (NH4)2Ce(NO3)6.

separation, analogously to plasmon-induced charge separation, because polychromatic light (l > 400 nm) was irradiated, as shown in Figures 3 and 4. However, the authors performed hydrogen and oxygen evolution and measured their apparent quantum yields using 560 nm monochromatic light around the plasmon resonant wavelength. Therefore, they suggested that plasmon resonance participated in these reactions. They also elucidated the water oxidation and reduction in a manner similar not only to TiO2, but also to cerium oxide.[15] They also employed not only Au, but also metal alloy such as Au–Pt as a plasmonic material.[16] In relation to Garcia’s studies, in 2013, Kominami et al. successfully confirmed hydrogen evolution as a half-reaction of water splitting in a 2-propanol aqueous solution by using Auand platinum-nanoparticle-loaded TiO2 particle photocatalysts, and they confirmed that plasmon resonance contributes to the improvement of hydrogen evolution efficiency.[17] The action spectrum of the quantum efficiency of hydrogen evolution was measured, and the spectrum was similar to the plasmon resonance spectrum. Therefore, the authors suggested that the plasmon resonance participated in the hydrogen evolution. It was considered that electrons injected in the conduction band of TiO2 migrated to the platinum co-catalyst and reduced protons or water to evolve hydrogen gas, and that holes were quenched by 2-propanol electron donors.[17] In the case of plasmon-induced charge separation using Au nanoparticleloaded TiO2, there is a possibility that the back electron-transfer reaction occurs simultaneously because the thickness of ChemPhysChem 2016, 17, 199 – 215


space charge layer depends on a TiO2 particle size. A relatively thicker TiO2 substrate with a high crystallinity is preferable for the charge separation.

3. Optimization of Materials and their Structure with Semiconductor Photoelectrodes for Plasmon-Induced Water Splitting Section 3 introduces the research that developed metallicnanoparticle-loaded semiconductor photoelectrodes for water splitting, studied by the three-electrode-type of conventional photoelectrochemical measurement system, which is now being studied by many researchers worldwide. Especially, the studies introduced in Section 3 focus on the fabrication and characterization of the photoelectrode, although it is difficult to call it a complete water-splitting system, unlike the conventional photocatalytic water-splitting system, because the threeelectrode system is used for the water-splitting reaction. Section 3.1 introduces research using metallic-nanostructures-loaded TiO2 photoelectrode, in which TiO2 was doped with nitrogen and/or fluorine (N–TiO2 and/or F–TiO2) as impurities, in order that the semiconductor photoelectrode absorbs visible light itself. Section 3.2 introduces metallic-nanoparticleloaded iron (III) oxide (a-Fe2O3) semiconductor photoelectro203


Reviews des. Because a-Fe2O3 absorbs visible light similarly to N-TiO2 and/or F-TiO2, the plasmon-resonant energy-transfer mechanism can be employed for water oxidation. However, it is difficult to induce complete water splitting because the conduction band energy of a-Fe2O3 is more positive than redox potential of hydrogen evolution. On the other hand, Section 3.3 describes studies using TiO2 single-crystal photoelectrodes that do not absorb visible light itself. In this case, Au nanoparticles were employed as a sensitization of visible light. Section 3.4 introduces research using a photoelectrode constituted of Aunanoparticle-loaded ZnO or TiO2 nanorods (nanowires). A larger electrode surface area is expected due to preparing the photoelectrode with semiconductor nanorods. Finally, Section 3.5 introduces studies related to a plasmon-induced water-splitting system using photonic nanostructured semiconductor photoelectrodes. In the reviewed studies, the synergetic effect of light confinement derived from plasmon resonance and a photonic crystal was clearly observed. 3.1. Metallic-Nanoparticle-Loaded TiO2 Photoelectrode Doped with Nitrogen and/or Fluorine Characteristic points of localized surface plasmon resonance are able to localize an electromagnetic field onto a nanometersized space, and an electromagnetic field enhancement effect is induced up to several orders of magnitude compared with an incident light field. In 2011, Cronin et al. fabricated a photoelectrode with a Au-nanoparticle-loaded photocatalyst that responded to an originally visible wavelength. They performed a photoelectrochemical measurement in a 1 m KOH aqueous solution under the conditions at which water oxidation tends to proceed.[18] In this experiment, a photoelectrode that contained a Au-nanoparticle-loaded photocatalyst was used for a working electrode, and a graphite electrode and an Ag/AgCl electrode were employed as counter and reference electrodes, respectively. When visible light was irradiated onto the photoelectrode, the enhancement of photocurrent was clearly observed. The photoelectrode that responded to the visible wavelength was prepared as follows. A titanium foil was oxidized to be applied 30 V in an ethylene glycol solution including 0.25 wt % NH4F and 2 wt % H2O for two hours as N- and FTiO2 was fabricated. Subsequently, Au nano-islands were formed on the photoelectrode by depositing a discontinuous Au thin film by vacuum evaporation. Thus, F- and N-TiO2 photoelectrodes with a discontinuous Au film was prepared. Figure 5 a shows the absorption spectra of TiO2 thin film prepared by the sol–gel method (general method), the TiO2 photoelectrode which was doped with fluorine and nitrogen as impurities, and the doped TiO2 photoelectrode which was loaded with Au nano-islands. The TiO2 photoelectrode doped with fluorine and nitrogen as impurities absorbed in the visible wavelength range, 400–800 nm, and when Au nano-islands were loaded, its absorption in the visible wavelength region increased due to localized surface plasmon resonance. Figure 5 b shows a current–time (I–t) curve with an irradiation wavelength of 633 nm for 22 s (0 V vs. Ag/AgCl, corresponding to + 0.83 V vs. Ag/AgCl under pH conditions of 14). When the Au ChemPhysChem 2016, 17, 199 – 215


Figure 5. a) Absorption spectrum of TiO2 with and without Au nanoparticles. b) I–t curve with and without Au nanoparticles.[18]

nano-islands were loaded onto the photoelectrode, the photocurrent value was enhanced by approximately 66 times compared to that in the absence of Au nano-islands. They also measured the photocurrent action spectrum. They suggested that the photocurrent generation was promoted by the plasmonically enhanced optical near field. The authors emphasized that the photocurrent measurement was directly associated with water splitting because hydrogen gas was quantitatively determined from the counter electrode. However, photoelectrochemical measurement with three-electrode systems determined the water oxidation reaction at the working electrode. In this case, the photoenergy conversion efficiency of splitting water is difficult to estimate quantitatively because of the additional bias applied to the counter electrode. In the case of the three-electrode system, we can only discuss water oxidation as a half-reaction of water splitting. It is natural that hydrogen gas is detected at the counter electrode when the oxidation reaction occurs at the working electrode because the reduction reaction that occurs at the counter electrode is the reverse reaction of that at the working electrode under electrochemical potential control by potentiostat. It should be noted that the reduction reactions at the counter electrode could not be specified because their relatively large overpotential induces additional reactions, such as the decomposition of electrolytes and oxygen reduction, ac204


Reviews companied by the hydrogen evolution reaction. Therefore, it is important to determine the quantitative analysis of oxygen gas from the working electrode using an oxygen isotope ratio. However, this paper is important because it represents the beginning of the trend in plasmon-induced water-splitting studies utilizing the plasmon-resonant energy-transfer mechanism. At the same time, Linic et al. prepared a photoelectrode that contained Ag nanocubes or Au-nanoparticle-loaded N-TiO2 as an impurity that responded to visible wavelength from 400 to 500 nm, and they performed photoelectrochemical measurements using three-electrode systems in 1 m KOH aqueous solution.[19] N-TiO2 was synthesized by heating 20 nm TiO2 particles in flowing gaseous NH3. An SEM image of the Ag nanocubes is shown in Figure 6 a. The average edge length was 118 nm, and the standard deviation of the size distribution was 25 nm. Importantly, the surface seems to be smooth, and the edge of the structure is sharp. The average diameter of the Au

nanoparticles used in this study was 24.5 nm, and the standard deviation was 4.5 nm.[19] Composite Ag/N-TiO2 or Au/N-TiO2 solutions which were prepared by mixing the Ag nanocube colloidal solution or Au nanosphere solution with the N-TiO2 in ethanol (5 % metal by weight) was dropped on an ITO substrate and dried to prepare the photoelectrode. The fabricated photoelectrode was used as a working electrode, and platinum wire and Hg/HgO electrodes were employed as counter and reference electrodes, respectively. Figure 6 b shows the extinction spectra of photoelectrodes composed of commercially available TiO2 nanoparticles (diameter: 20 nm), N-TiO2, and Ag/N-TiO2 or Au/N-TiO2. Although an extinction value of approximately 400–500 nm was observed with N-TiO2 electrodes, the extinction value was enhanced in the visible wavelength region, when Ag nanocubes or Au nanoparticles were loaded on the electrode. From the difference spectrum with and without a Ag or Au nanostructures (see the inset of Figure 6 b), a peak at approximately 420 nm and 530 nm was assigned by a plasmon resonance spectrum based on Ag nanocubes and Au nanoparticles, respectively. Additionally, the shoulder bands of the spectra can be considered due to the electromagnetic interaction between closely spaced metallic nanoparticles. Figure 7 shows an action spectrum of the photocurrent enhancement and the difference spectrum of Ag nanocubes, as

Figure 7. Photocurrent enhancement action spectrum and the difference spectrum of the photoelectrode with Ag nanocubes. (The data are shown in the inset of Figure 6 b).[19]

shown in the inset of Figure 6 b. The applied potential was set at + 0.3 V vs. Hg/HgO. Because the action spectrum corresponded to the difference spectrum of Ag nanocubes, they suggested that the photocurrent generation was induced based on the plasmon-resonant energy-transfer mechanism. Notably, the photocurrent value was enhanced with the Ag nanocubes, but it was very low with the Au nanoparticles. The Ag nanocubes had a higher electromagnetic field enhancement effect than the Au nanoparticles and a plasmon resonance of approximately 400–500 nm, which corresponded to a visible wavelength of N-TiO2. The authors also confirmed the

Figure 6. a) SEM image of Ag nanocubes. b) Extinction spectra of photoelectrodes composed of TiO2 nanoparticles, nitrogen-doped TiO2, and Agnanocube- and Au-nanoparticle-loaded nitrogen-doped TiO2. The inset shows the difference spectrum with and without Ag or Au nanostructures.[19]

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Reviews evolution of hydrogen gas from the counter electrode and oxygen gas from the working electrode with a stoichiometric ratio of 2:1.[19] Wang et al. fabricated a Au-nanoparticle-loaded TiO2 photoelectrode that responded to visible wavelength, as in Cronin’s and Linic’s studies, and performed a photoelectrochemical measurement in 1 m KOH aqueous solution with a three-electrode system.[20] The enhancement of the photocurrent was clearly observed. An advantage of this study is that Au nanoparticles were loaded on both sides of a TiO2 nanosheet fabricated on titanium film, and several TiO2 nanosheets were stacked and arranged on the titanium film to increase the electrode surface area for increasing the photocurrent value.[20] Although the authors measured the photocurrent action spectrum, the gas evolution was not measured. Parida et al. also fabricated a photoelectrode composed of Au-nanoparticleloaded TiO2 nanoparticles doped with sulfur and nitrogen to respond to visible wavelengths and confirmed photocurrent generation through irradiation with visible light.[21] As a characteristic point of this study, the incident photon-to-current efficiency (IPCE) action spectrum was measured and compared with the plasmon resonance spectrum.[21] The IPCE action spectrum was almost in accordance with the plasmon resonance spectrum. On the other hand, as a separate experiment, the authors also demonstrated the quantitative determination of hydrogen evolution as a half-reaction of water splitting using the Au-nanoparticle-loaded TiO2 particle aqueous suspension, including methanol as a sacrificial electron donor, as in similar experiments introduced in Section 2.2, with irradiation of visible light wavelength longer than 400 nm. The quantity of the evolved hydrogen linearly increased with the irradiation time and the difference in the hydrogen evolution efficiency with and without Au nanoparticles.[21]

Figure 8. Schematic illustrations of Au-nanoparticle-loaded hematite photoelectrodes. Au nanoparticles were embedded in the hematite film (a) and placed on the hematite platelets surface (b).[22]

electrode surface. They suggested that the photocurrent could be measured as a result of water oxidation by a plasmon-resonant energy-transfer mechanism, although they did not confirm the gas evolution. In the case of Au nanoparticles embedded in hematite film, they also suggested that photocurrent was not observed because the plasmon resonance became offresonant due to a longer spectrum shift, based on the higher refractive index. In addition, Thomann et al. prepared hematite photoelectrodes supporting Au nanoparticles coated by silica and performed a photoelectrochemical measurement in 0.2 m sodium acetate aqueous solution.[23] They successfully observed the enhancement of the photocurrent near the plasmon resonance wavelength. In this system, electromagnetic field enhancement effects based on plasmon resonance promoted the excitation efficiency of hematite without utilizing plasmon-induced charge separation because the Au nanoparticles were covered with silica. It is considered that the photocurrent was obtained based on the water oxidation although the evolved oxygen was not measured. Furthermore, Yang et al. fabricated a Au nanostructured array (base diameter: 300 nm, height: 300 nm, pitch: 700 nm) with a relatively larger area using nano-imprinting techniques and subsequently deposited hematite with a thickness of 90 nm to prepare the photoelectrode. They demonstrated the enhancement of the photocurrent by photoelectrochemical measurements.[24] Iandolo et al. also fabricated a hematite photoelectrode by placing Au nanodisks on the surface by a hole-mask colloidal lithography method, and they confirmed the enhancement of the photocurrent in the visible wavelength.[25] Pan et al. developed hematite photoelectrodes in which Au nanorods were deposited on the surface of hematite or embedded in hematite and used the photoelectrodes for the study of water oxidation by a photoelectrochemical measurement.[26] In contrast, Li et al. developed a photoelectrode in which a hematite nanorod array was arranged inside Au nanohole array, and they employed the photoelectrode for the photocurrent measurement in visible wavelength.[27] In this study, the ordered Au nanohole array was fabricated on the FTO substrate by a conventional nanosphere lithography technique.[27] The hematite nanorod array was grown on the bare FTO and

3.2. Plasmon-Enhanced Photocurrent Generation Based on Water Oxidation Using Hematite as a Semiconductor Photoelectrode Semiconductors other than TiO2, including iron(III) oxide (Fe2O3) and zinc oxide (ZnO), have been studied by many researchers. Warren et al. successfully fabricated Au-nanoparticleembedded or -loaded a-Fe2O3 (hematite) photoelectrodes. Hematite thin film and platelets were deposited onto fluorinedoped tin oxide (FTO) substrates, and the chemically synthesized Au nanoparticles were embedded in the hematite film, as schematically illustrated in Figure 8 a, or placed on the hematite platelets, as illustrated in Figure 8 b.[22] Because the band gap of hematite is approximately 2.2 eV, the hematite absorbs visible light. However, an external bias is necessary to evolve hydrogen because the conduction band energy of hematite is approximately 0.2 V positive compared to the redox potential of hydrogen. When Warren et al. performed a photoelectrochemical measurement using the fabricated Au-nanoparticleloaded hematite photoelectrode, a photocurrent derived from the interband transition of Au and plasmon excitation in the wavelength range from 400 to 600 nm was clearly observed only when Au nanoparticles were placed on a hematite photoChemPhysChem 2016, 17, 199 – 215


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Reviews Au hole array. The Au nanostructured substrate was heated at 100 8C in the aqueous solution of FeCl3 and NaNO3 with a relatively higher concentration. After being washed with deionized water and ethanol, the hematite nanorods were annealed in air at 650 8C for 20 min. Figure 9 a shows a schematic illustration of the Au nanohole array and the electrode in which the

was used as a working electrode, and platinum and saturated calomel electrode (SCE) were employed as counter and reference electrodes, respectively.[16] When visible and near-infrared light were irradiated onto the photoelectrode, an anodic photocurrent based on water oxidation was observed. Figure 10 a shows the extinction spectra of Au nanorods on Nb–TiO2 sub-

Figure 9. a) Schematic illustration of Au nanohole array and the electrode in which hematite nanorod array was arranged inside the Au nanohole array with a scheme. Scanning electron microscope images of the Au nanohole array (b) and the electrode in which the hematite nanorod array was arranged inside the Au nanohole array (c).[27]

hematite nanorod array was arranged inside the Au nanohole array. Figures 9 b,c show the SEM images of their surfaces. When photoelectrochemical measurements were performed by using the photoelectrode in which the hematite nanorod array was arranged inside the Au nanohole array, the enhancement of the photocurrent in the wavelength ranged from 560 nm to 650 nm, which is the longer-wavelength region of the hematite band edge, although the efficiency was relatively low. Yang et al. succeeded in the development of a photoelectrode that can utilize photons efficiently based on plasmon resonance. A silicon nanowire was coated by a metal, such as aluminum, showing a localized surface plasmon resonance, and was subsequently coated by hematite with a thickness of 50 nm.[28] Semiconductors, such as CuO,[29] GaP,[30] and MOS2,[31] have also been employed as a semiconductor photoelectrode material in addition to hematite and TiO2.

Figure 10. a) Extinction spectra of Au nanorods on Nb–TiO2 under non-polarized conditions (black curve) and polarized conditions [red (parallel to short axis) and blue (parallel to long axis) curves]. b) IPCE action spectra measured under non-polarized conditions (black squares) and polarized conditions [red circles (parallel to short axis) and blue triangles (parallel to long axis)]. The inset shows incident light intensity dependence of photocurrent density.[32]

3.3. Plasmon-Induced Water Oxidation Using Au Nanostructured TiO2 Single-Crystal Photoelectrode In 2010, our group observed photocurrent generation from a visible to a near-infrared wavelength, according to the efficient oxidation of water following plasmon-induced charge separation.[32] We fabricated Au-nanorod-loaded niobiumdoped TiO2 single-crystal photoelectrodes (0.05 wt % Nb-doped TiO2) using electron beam lithography and lift-off, and we performed photoelectrochemical measurements using three-electrode systems in an aqueous electrolyte solution (0.1 m KClO4 aq.).[32, 33] In this study, a Au-nanorod-loaded Nb-TiO2 substrate ChemPhysChem 2016, 17, 199 – 215


strate in the electrolyte solution. Under non-polarized conditions, plasmon resonance bands with two peaks derived from shorter and longer axes of Au nanorod were measured to be approximately 680 nm and 1000 nm, respectively. In contrast, under linear polarization conditions, plasmon resonance bands derived from both shorter and longer axes of Au nanorod 207


Reviews were separately obtained by controlling the incident polarization parallel to shorter and longer axis of the rod. In the IPCE action spectra shown in Figure 10 b, two peaks corresponding to the plasmon resonance band were observed under non-polarized conditions, and the IPCE action spectra were observed separately, corresponding to shorter and longer axes of the Au nanorods, analogous to plasmon resonance spectra under linear polarization conditions. Importantly, a photocurrent was observed even with an irradiation of near-infrared light with a wavelength of approximately 1000 nm without certain electron donors. The results suggested that water molecules are oxidized and oxygen gas is evolved at the Au-nanorod-loaded Nb–TiO2 working photoelectrode, whereas hydrogen gas is evolved at the platinum counter electrode. Most importantly, in the study of plasmon-induced water splitting, 1) only water or an aqueous electrolyte solution should be used for measurement, 2) it should be confirmed that photocurrent generation or gas evolution is based on the plasmon excitation according to measuring the action spectrum of not only the photocurrent but also gas evolution, as discussed previously, and 3) the evolved oxygen gas should be determined by quantitative analysis using gas chromatography/mass spectrometry using water enriched with oxygen-18 in order to distinguish the evolved oxygen from that in the air by measuring the oxygen isotope ratios. In this study, the photolysis of water was thought to be induced by the irradiation of visible and near-infrared light, because only aqueous electrolyte solution was employed, and the action spectrum was successfully measured. However, only the photocurrent was measured, and the evolved gas was not analyzed quantitatively.[32] We successfully determined that oxygen gas and hydrogen peroxide quantitatively evolved from Au-nanorod-loaded Nb– TiO2 photoelectrodes using the following method.[34, 35] Gas chromatography–mass spectrometry (GC–MS 2010-plus; Shimadzu) was used to quantify the amount of the evolved oxygen gas. Therefore, water containing 18O isotopes was used, and the amount of 34O2 was determined using GC–MS. The area of the gas chromatogram was used to calculate the gas concentration. The 34O2 gas chromatogram was used to quantitatively determine the oxygen evolution, which was then compared with the natural abundance ratio. Hydrogen peroxide was quantitatively determined by absorptiometry, using oxo[5,10,15,20-tetra(4-pyridyl)porphinato]titanium(IV) as an indicator of hydrogen peroxide. Figure 11 a shows a bar graph representing the rate of the evolved chemical species to the observed photocurrent value based on the water oxidation in each wavelength range from visible to near-infrared.[36] Not only oxygen, but also hydrogen peroxide that evolved as a result of two electronic oxidations of water molecules was confirmed, particularly in the longer-wavelength region. Twoelectron oxidization of water occurred competitively in the presence of near-infrared light. It is known that two-electron oxidation of water is easily induced kinetically compared to four-electron transfer reaction. It was speculated that hydrogen peroxide, under light irradiation, can decompose to O2 and a water molecule, and that such decomposition would be easy ChemPhysChem 2016, 17, 199 – 215


Figure 11. a) Oxygen and hydrogen peroxide evolution efficiency as a function of the observed photocurrent and its corresponding IPCE action spectrum.[34] b) Energy diagram of the plasmon-induced water oxidation.[32]

to induce through irradiation by shorter-wavelength ligh. Notably, water oxidation based on plasmon excitation proceeded almost stoichiometrically to the observed photocurrent value in every wavelength region. Furthermore, plasmon resonance may provide an efficient co-catalyst for water oxidation because water oxidation proceeded even with irradiation of nearinfrared light, whose energy almost corresponds to the redox potential of water at approximately 1.23 V. This finding does not explain why the non-linear optical phenomenon was induced by a plasmonically enhanced optical near field, because the photocurrent value linearly increased to the irradiating light intensity, as shown in the inset of Figure 10 b.[32] A possible mechanism for efficient water oxidation is as follows. As shown in Figure 11 b, the electron transfer from Au nanoparticles to the conduction band of Nb–TiO2 was induced by plasmon excitation, and the injected electrons reduced protons or water at the counter electrode to evolve hydrogen gas. The generated holes might be trapped in the surface states of TiO2 near the Au/TiO2/water interface because the plasmonically enhanced optical near field is localized to the restricted nanospace, and there is a possibility that the trapped multiple holes might oxidize water molecules efficiently.[36] 3.4. Au-Nanoparticle-Loaded ZnO Nanorod Photoelectrodes As mentioned in Section 3.2, in the case of metallic nanoparticle-loaded hematite electrodes, the plasmon-resonant energy208


Reviews transfer mechanism in which the absorption efficiency is enhanced by plasmon excitation occurs because hematites absorb visible light. The band gap of ZnO is known to be as high as approximately 3.4 eV, and the improvement of the absorption efficiency by a plasmon excitation is impossible because it does not interact with plasmon resonance in a visible wavelength. Therefore, it is necessary to take into account the plasmon-induced charge-separation mechanism described in Section 3.3. Importantly, it is advantageous that hydrogen evolution is induced without an external bias according to reducing protons or water molecules because the conduction band energy of ZnO is approximately 0 V vs. SHE. In this case, there is a possibility that a water-splitting reaction proceeds according to the plasmon-induced charge separation. Tsai et al. produced a plasmon-induced photocurrent generation system that responded to visible light according to loading chemically synthesized Au nanoparticles with an average diameter of 4.7 nm on ZnO nanorods, as shown in the SEM image in Figure 12 a.[37] When the photoelectrochemical measurement was performed in 0.5 m Na2SO4 aqueous solution, the anodic photocurrent was measured with an irradiation of AM1.5. They also measured the evolved hydrogen and oxygen gases from platinum counter and Au-nanoparticle/ZnO-nano-

rod working electrodes, respectively. Figure 12 b shows the irradiation time dependence of the quantity of the evolved hydrogen and oxygen, respectively. A stoichiometric ratio (1:2) of water splitting corresponding to the quantity of the evolved oxygen and hydrogen was clearly observed, and the gas evolution efficiency was improved when the Au nanoparticles were loaded. The authors discussed the plasmon-induced charge separation mechanism using the theory of Halas et al., who reported in 2011 the hot electron transfer from metal to semiconductors after plasmon excitation,[38] also reflecting on their Au nanoparticle-loaded ZnO nanorod photoelectrode system. Many plasmon-induced water-splitting systems that used TiO2 nanorods in addition to ZnO nanorods have also been reported.[39] Characteristics analogous to those of a ZnO nanorod were obtained. Jang et al. successfully fabricated a photoelectrode in which a ZnO nanowire array was arranged on a ZnO seed layer inside a nanohole array to fabricate a hole array pattern on the substrate using laser interference lithography. They employed the fabricated photoelectrode for the photocurrent measurement in an aqueous electrolyte solution.[40] Additionally, it was reported that a visible light response was given by the ZnO nanowire after coating it with ZnFe2O4 to reduce the band gap energy.[41] The ZnO nanowire itself was branched to increase the apparent optical absorption efficiency,[42] and the arrangement (design) and material of metallic nanoparticles were optimized to improve the efficiency of an optical absorption or a water-splitting reaction.[43] In contrast, Liu et al. demonstrated plasmon-induced photocurrent generation using a Au-nanoparticle-loaded ZnO nanorod photoelectrode sensitized by CdTe quantum dots irradiating near-infrared light, according to utilizing up-conversion luminescence from Er3 + /Yb3 + co-doped NaYF4 promoted by a plasmon-enhanced optical near-field and measured the evolution of hydrogen and oxygen gases.[44] In this system, plasmon resonance was employed only to induce up-conversion luminescence from Er3 + /Yb3 + co-doped NaYF4, and the luminescence excited the CdTe quantum dots. They proposed one possible mechanism that the electrons injected in the conduction band of a ZnO nanorod reduce protons or water in order to evolved hydrogen and that a hole at the valence band of the CdTe quantum dot oxidizes water in order to evolve oxygen. 3.5. Photonic Nanostructured Semiconductor Photoelectrode Loading Au Nanoparticles The localized surface plasmon resonance works as an optical antenna, showing an electromagnetic field enhancement effect to harvest photons and localizing the electromagnetic field to a nanometer-sized minute space. Micrometer- or nanometersized structures with a periodicity as small as a visible wavelength are known to have a function of light confinement, which is called a photonic crystal. In 2012, Wang et al. successfully fabricated a TiO2 photoelectrode with a photonic crystal structure supporting Au nanoparticles that showed a localized surface plasmon, and they developed a photocurrent genera-

Figure 12. a) SEM image of Au-nanoparticle-loaded ZnO nanorod photoelectrode surface. b) Irradiation time dependence of the quantity of the evolved hydrogen and oxygen from platinum counter electrode and Au nanoparticle-loaded ZnO nanorod photoelectrodes, respectively.[37]

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209


Reviews tion system using the photoelectrode for to obtain the synergetic effect of light confinement derived from plasmon resonance and the photonic crystal.[45] A TiO2 nanotube photonic crystal was fabricated by a twostep anodic oxidation method on titanium film. Figure 13 a

Figure 13. a) SEM image of the fabricated TiO2 nanotube photonic crystal. The inset figures show SEM images of an oblique view 158 (lower left) and a cross-sectional view (upper right). b) SEM image of Au nanoparticle-loaded TiO2 nanotube photonic crystals.[45]

shows a SEM image of the fabricated TiO2 nanotube photonic crystal. The lower left inset of Figure 13 a shows the SEM image of an oblique view at 158, and the upper right inset shows an SEM image of a cross-sectional view of the photonic crystal. It was confirmed that the nanohole array with a diameter of approximately 200 nm was formed at a depth of 2 mm and that a photonic stop band existed near 468–744 nm. The Au nanocrystals were subsequently deposited onto the TiO2 nanotube photonic crystals by a simple photocatalytic reduction method as follows. The TiO2 nanotube photonic crystal was soaked in aqueous HAuCl4 solution and was then irradiated with a 300 W Xe lamp to reduce the absorbed Au3 + to Au by photocatalysis as a reverse reaction of water oxidation. Au nanocrystals with an average size of 20 nm whose plasmon resonant wavelength was 556 nm were formed. In this study, Au nanocrystals with a size of 40 nm were also prepared according to changing HAuCl4 concentration. Figure 13 b shows SEM image of Au nanocrystals loaded TiO2 nanotube photonic crystal. Figure 14 a shows the IPCE action spectrum when the photoelectrochemical measurement was performed in 1 m KOH aqueous solution. First, a photocurrent was not observed in the visible wavelength region using a TiO2 nanotube photonic crystal photoelectrode without Au nanoparticles. A photocurrent derived from plasmon excitation was observed when a Au-nanoparticle-loaded TiO2-nanotube or a TiO2-nanotube photonic crystal was employed. Furthermore, the IPCE action spectrum shows a red-shift from 556 nm to 590 nm when larger Au nanoparticles were supported on TiO2. Notably, the photocurrent value obtained by the photoelectrode containing the Au-nanoparticle-loaded TiO2 nanotube photonic crystal shows a higher value than the loaded TiO2 nanotube that does not have a photonic stop band in the plasmon wavelength. This finding indicates that a synergetic effect of light confinement derived from plasmon resonance and the photonic crystal was obtained. They also performed a quantitative determiChemPhysChem 2016, 17, 199 – 215


Figure 14. a) IPCE action spectra; TiO2 nanotube photonic crystal without Au nanoparticles (black), Au nanoparticle-loaded TiO2 nanotube (red) or TiO2 nanotube photonic crystal [ocher (556 nm plasmon) and blue (590 nm plasmon)]. (b) Irradiation time dependence of the quantity of the evolved hydrogen and oxygen corresponding to the consumed electrons in this watersplitting reaction, estimated by the photocurrent measurement.[45]

nation of the evolved hydrogen and oxygen from counter and working electrodes. The quantity of the evolved hydrogen and oxygen showed a stoichiometric ratio (2:1), as shown in Figure 14 b, and a stoichiometric evolution of gases to the observed photocurrent was demonstrated. Importantly, they have also successfully demonstrated a spectrum shift in the IPCE action spectrum with a difference in particle size. One advantage of a plasmon-induced water-splitting system is that it is able to control the responding wavelength by particle size or shape. It is known that the near-field enhancement is highly associated with the plasmon dephasing time (T2), especially in the case of the Au spherical nanoparticle, because the shape is same.[46] It was reported that Au spherical nanoparticle whose plasmon-resonant energy is around 2.2 eV (564 nm) shows longer T2 because there is not only an interband damping due to an interband transition of gold in a shorter-wavelength region but also a radiative decay based on light scattering in a longer-wavelength region.[46, 47] When the Au nanoparticle is placed on the TiO2 substrate, smaller nanoparticles show longer T2 because the reflective index of TiO2 is relatively higher than that of glass or water if 210


Reviews only the resonant wavelength is considered. However, smaller particles less than 20 nm increase the probability that oscillating electrons scatter from the surface of the metal nanoparticle because of the larger surface-to-volume ratio.[48] Although it is still unclear whether the stronger electromagnetic field enhancement correlates with the plasmon-induced water splitting directly, at least the particle sizes around 20 nm were considered to show stronger electromagnetic field enhancement effects in the case of Au spherical nanoparticles.[46] Jang et al. employed a photoelectrode composed of Aunanoparticle-loaded inverse opal photonic crystals made of TiO2 as the working electrode for the photocurrent measurement in aqueous electrolyte solutions.[49] First, a closely packed opal structure made of polystyrene beads (diameter: 130, 350, and 600 nm) was fabricated on a FTO substrate, and then the inverse opal structure was fabricated by a sol–gel method using the opal structure as a template. Furthermore, Au nanoparticles were supported on the inverse opal nanostructures. A photoelectrochemical measurement was performed in an aqueous electrolyte solution including Na2S and Na2SO3. Photocurrent was observed with an irradiation of visible light based on plasmon-induced charge separation and subsequent oxidation of S2¢ and SO32¢. Importantly, the enhancement of photocurrent was clearly observed when 350 nm-sized polystyrene beads were used as a template because the photonic stop band of TiO2 inversed the opal structure, in accordance with the plasmon resonance band. The synergetic effect of light confinement derived from photonic crystal and plasmon resonance was obtained. Similarly, Kang et al. employed a photoelectrode composed of inverse photonic crystal supported by Au nanoparticles as a photoelectrode for hydrogen and oxygen evolutions and demonstrated the synergetic effect.[50] Zhang et al. also fabricated Au-nanoparticle-loaded Mo:BiVO4 inverse photonic crystals and elucidated the enhancement of both hydrogen and oxygen evolutions.[51] Fan et al. fabricated Au-nanoparticleloaded TiO2 nanostructures using a butterfly-wing architecture as a template and proposed their application as a photocatalyst for water splitting.[52]

a semiconducting material, a co-catalyst for hydrogen evolution, and ohmic contact between the semiconductor and cocatalyst. Section 4 introduces advanced water-splitting systems using Au nanorods capped with TiO2 and Au nanostructured strontium titanate single-crystal substrates, and it describes the importance of the system design not only in photoelectrodes or photoanodes but also in the whole water-splitting system. Section 4.1 introduces a study constructing a complete watersplitting system using Au nanorods capped by TiO2 with cocatalysts for both hydrogen and oxygen evolution. Section 4.2 introduces a study on a plasmon-induced complete watersplitting system using two sides of same strontium titanate substrate. The studies take the whole water-splitting system into consideration. 4.1. Complete Water-Splitting System Using Au Nanorod A study by Moskovits et al. published in 2013 is a plasmon-induced water-splitting study that considers the design of a whole water-splitting system.[53] They produced an efficient water-splitting device in which TiO2 was deposited at the tip of the upper portion of a Au nanorod, and subsequently a platinum co-catalyst was supported on TiO2 for hydrogen evolution. Furthermore, a cobalt co-catalyst for water oxidation was loaded onto the lower side of Au nanorod, as schematically illustrated in Figure 15 a. Vertically oriented gold nanorod arrays were electrodeposited in a porous anodic aluminum oxide template with a diameter of 90 nm to 100 nm. TiO2 with a thickness of 20 nm were electron-beam deposited onto the upper portion of the gold nanorods. After the cobalt/borate was electrochemically deposited onto the TiO2-decorated gold nanorods, electron-beam deposition of 2 nm of platinum was performed to form the Pt nanoparticles on top of the TiO2.[53, 54] Figure 15 b shows the SEM image of the developed water-splitting system using Au nanorods. Moskovits et al. performed water-splitting experiments in a 1 m potassium borate electrolyte (pH 9.6) aqueous solution with irradiation by visible light with wavelengths longer than 410 nm. Figure 16 a shows an irradiation time dependence of the quantity of the evolved hydrogen gas. The quantity of the evolved hydrogen gas linearly increased with the irradiation time, and it was shown that the system had high stability when the experiment was performed with many cycles of six hours each. The external quantum efficiency (EQE) of hydrogen evolution was estimated to be 0.1 % (l > 410 nm), and the efficiency was high, such that the EQE obtained was 0.25 % under irradiation of AM1.5 (3 sun, 300 mW cm¢2 in this study).[53] Figure 16 b shows a bar graph of the quantity of the evolved hydrogen gas in an arbitrarily irradiated wavelength region. It is notable that the evolution efficiency was higher under irradiation with a wavelength longer than 600 nm compared with irradiation from 350 to 520 nm wavelengths. Plasmon-induced water splitting proceeded, and furthermore, a stoichiometric ratio between hydrogen and oxygen (2:1) was confirmed. It was concluded that the system took into account not only the design of the photoanode but also the system design of

4. Plasmon-Induced Complete Water-Splitting System To develop a plasmon-induced complete water-splitting system, it is necessary to take into account not only the optimization of the photoelectrode design, as described in Section 3, but also the reduction reaction in which hydrogen evolution occurs, although the reduction is a dark reaction. To construct a complete water-splitting system, a three-electrode system is not suitable because a photoelectrochemical measurement with a three-electrode system determines only the redox reactions at the working electrode through controlling the electrode potential; that is, it determines only water oxidation as a half-reaction of water splitting, as mentioned above. To construct a complete water-splitting system, the design manual in a whole water-splitting system should be explored, such as the electrode potential of the conduction band of ChemPhysChem 2016, 17, 199 – 215


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Figure 16. a) Irradiation time dependence of the quantity of the evolved hydrogen gas measured with 11 cycles for six hours each. b) Quantity of the evolved hydrogen gas in an arbitrary irradiated wavelength region using a bar graph.[53]

4.2. Plasmon-Induced Water-Splitting System Using Two Sides of the Same Strontium Titanate Substrate Figure 15. a) Schematic illustration of the plasmon-induced water-splitting system using Au nanorod proposed by Moskovits et al. b) SEM images of the developed water-splitting system using Au nanorod. The inset shows the magnified SEM image around the upper and lower sides of the Au nanorod.[53]

There are many difficulties, such as preventing a reverse reaction or the isolation of hydrogen gas, in a water-splitting system using photocatalytic particles. Therefore, the separation of anodic and cathodic chambers is important. The cathodic chamber should be placed on the backside of the anodic chamber to save the light-irradiating area because hydrogen evolution is a dark reaction, and it is not necessary to irradiate the cathode. Our group has successfully utilized the front and back of the same strontium titanate single-crystal substrate as an anode and a cathode for oxygen and hydrogen evolution.[55] A schematic illustration of the plasmon-induced watersplitting system proposed in this study is shown in Figure 17.[55] Au nano-islands were fabricated on 0.05 wt % niobium-doped strontium titanate single-crystal substrate (Nb– SrTiO3) by sputtering and annealing methods, and a platinum board as a co-catalyst for hydrogen evolution was stuck onto the backside of the Nb–SrTiO3 substrate via an indium–gallium

the whole water-splitting system. However, there is approximately 1.1 eV in platinum and TiO2 (even anatase) as a Schottky barrier, and the Schottky barrier was considered to be the cause of the reduction in the hydrogen evolution efficiency. Therefore, an ohmic contact is thought to be necessary between a platinum co-catalyst and TiO2. Additionally, the isolation of hydrogen gas is required because hydrogen and oxygen evolution sites are not separated in this system. In Section 4.2, improvements to these drawbacks are discussed.

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Figure 17. Schematic illustration of the plasmon-induced water-splitting system using two sides of the same strontium titanate substrate.[55]

alloy to obtain ohmic contact. The anodic and cathodic side chambers were separated by a Nb–SrTiO3 substrate loaded with Au nano-islands. The volume of the anodic and cathodic side chamber were 230 mL and 600 mL, respectively. A conventional salt bridge containing 2 wt % agar with an inner diameter of 0.2 mm was employed to maintain the charge balance between these chambers. It was speculated as a possible mechanism that plasmon-induced charge separation is induced following electron transfer from the Au nano-island to the conduction band of Nb–SrTiO3, and the injected electrons migrate to the surface of platinum to reduce protons for hydrogen evolution. The holes formed in Au nano-island oxidize hydroxyl ions or water molecules were speculated to evolve oxygen gas. Therefore, hydrogen gas and oxygen gas can be separated from two sides of the same Nb– SrTiO3 substrate. In this study, pH regulation was used to control chemical bias. When the visible light from 450 nm to 850 nm was irradiated on the Au nano-islands, the quantity of the evolved hydrogen and oxygen gases increased linearly with the irradiation time with a stoichiometric ratio of 2:1, as shown in Figure 18 a.[55] In addition, the authors reported that the efficiency falls to approximately half when there is no ohmic contact with a platinum co-catalyst.[56] From the action spectrum of the evolved hydrogen gas, as shown in Figure 18 b, it can clearly be seen that plasmon-induced water splitting was induced because the action spectrum corresponds to the plasmon resonance spectrum. In this system, the back-reaction did not proceed because the anode and cathode chambers were separated. Ohmic contact was established between Nb–SrTiO3 and the platinum cocatalyst. Therefore, it was possible to optimize the co-catalyst for hydrogen evolution. The co-catalyst’s effects on the watersplitting efficiency were elucidated according to deposition of ruthenium or rhodium and their oxide on the platinum board with a thickness of 3 or 4 nm by sputtering.[57] Figure 19 shows the water-splitting efficiency with each co-catalyst represented by a bar. It is known that platinum is the most efficient co-catalyst for hydrogen evolution because the Gibbs free energy of hydrogen adsorption is lowest on the platinum surface. In this study, the water-splitting efficiency may have been enhanced with each co-catalyst when the surface area of the co-catalyst was relatively large because the surface was discontinuous beChemPhysChem 2016, 17, 199 – 215


Figure 18. a) Irradiation time dependence of the quantity of the evolved hydrogen and oxygen gases. b) Action spectrum of the evolved hydrogen gas using a bar graph and plasmon resonance spectrum of Au nano-islands.[55]

Figure 19. Water-splitting efficiency with each co-catalyst represented by a bar graph; H2 (patterned filled bars) and O2 evolution (open bars).[57]

cause of the sputtered thin film.[57] Furthermore, a synergetic effect was observed for not only a lower Gibbs free energy of adsorption of hydrogen on platinum but also a higher co-catalyst function actualized by a valence state of rhodium, although the efficiency was particularly high in rhodium. It was concluded that the system accounts for not only the design of photoanodes but also the system design in the whole water-splitting system, analogously to 4.1. The hydrogen evolution co-catalyst was employed, ohmic contact was used between the semiconductor substrate and the co-catalyst, and 213


Reviews anodic and cathodic chambers were separated. The problem in this water-splitting system is that the capacity of each of its chambers is very small because a single crystal is used. Therefore, the salt bridge that connects the chambers must also be thin. That is, it is not easy to maintain the charge balance between the two chambers in the design of the cell. Although collapse of the charge balance might be ignored in the case of a higher concentration of protons or hydroxyl ions even if protons or hydroxyl ions react somewhat, it is necessary to keep a charge balance under strictly neutral conditions.

and semiconductor substrate. Almost all studies introduced in this review were employing three-electrode systems. Research using two-electrode systems, which are plasmon-induced complete water-splitting systems is expected to increase in future.

Acknowledgements The authors acknowledge funding from the Ministry of Education, Culture, Sports, Science, and Technology of Japan: KAKENHI Grant-in-Aid for Scientific Research (Nos. 23225006, 15H00856, 15H01073, and 15K04589) and the Nanotechnology Platform (Hokkaido University).

5. Conclusions Gold nanoparticles have been employed as a co-catalyst to suppress the back-reaction of water splitting and as an electron trap site to suppress the recombination of electron and hole pairs in photocatalytic water-splitting systems. Since 2010, gold nanoparticles showing a localized surface plasmon resonance have been utilized in water-splitting systems as a reaction site initiated by plasmon-induced charge separation and an enhanced optical field, thereby enhancing the excitation efficiency of a photocatalyst responding to visible light. Almost all studies reported so far have focused on the optimization of a material or a structural design of metallic nanoparticleloaded semiconductor photoelectrodes or photoanodes to oxidize water. However, as described in section 4, the system design of the whole water-splitting system is very important, including the optimization of cathode material or design and of its co-catalyst, in which hydrogen evolution occurs as a dark reaction. One section of this review reflected on the abovementioned research, and studies of the latest plasmon-induced water-splitting systems were introduced. It is necessary to clarify the mechanism in detail to improve not only the optimization of electrodes and the system design but also the water-splitting efficiency. Currently, many researchers have demonstrated that plasmon-induced charge separation is initiated by a hot electron transfer from metallic nanoparticles to the conduction band of a semiconductor.[58, 59] However, it is not easy to explain the mechanism of how water molecules are oxidized, even by the irradiation of near-infrared light via four electronic transitions by only a hot electron transfer or the formation of hot holes. Further study is necessary to explain the mechanism; for example, the formed hot holes may be trapped on the surface states of the semiconductor material before the hole migrates to the Fermi level. Studies that pursue electron transfer, a back-electron transfer process, the dynamics of the formation and annihilation of hot electrons, and a change in the Fermi level of gold nanoparticles by transient absorption spectroscopy will be increasingly important in the near future. Moreover, the height of the Schottky barrier and the thickness of the space charge layer between gold nanoparticles and the semiconductor are also important. Because the efficiency of charge separation may increase (because a back-electron transfer reaction is suppressed) when optimizing the Schottky barrier, there is a possibility that the water-splitting efficiency could improve extraordinarily by controlling the interfacial structure between metallic nanoparticles ChemPhysChem 2016, 17, 199 – 215


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Manuscript received: September 3, 2015 Final Article published: November 23, 2015

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Micro- and nanostructures of photoelectrodes for solar-driven water splitting.

Photoelectrodes based upon Mo:BiVO4 inverse opals for photoelectrochemical water splitting.

Plasmon-enhanced water splitting on TiO2-passivated GaP photocatalysts.

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Nanoscale Effects in Water Splitting Photocatalysis.

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