Plasmon-induced Charge Separation Research Articles

(Invited) Site-Selective Introduction of MnO2 Co-Catalyst to Gold Nanocubes for Enhanced Plasmonic Photocatalysis

Plasmon-induced charge separation (PICS) is known to take place at the interface between a nanoparticle exhibiting localized surface plasmon resonance (LSPR) and a semiconductor such as titanium oxide.1 PICS has been applied to photovoltaics, photocatalysis, and so on.2 For energy harvesting based on plasmonic photocatalysis, it is important to optimize geometrical arrangements of the plasmonic nanoparticles, semiconductor, and co-catalyst if any, to enhance the charge separation efficiency and minimize charge recombination probability.Since LSPR confines light to a nanoscale region smaller than visible light wavelength, PICS allows one to fabricate plasmonic materials in nanoscale beyond the diffraction limit.3-7 We have reported fabrication of wide variety of plasmonic nanostructures via site-selective reactions such as oxidative dissolution of silver and oxidative deposition of lead oxide.In the present work, gold nanocubes were adsorbed onto a thin film of titanium oxide to obtain a plasmonic photocatalyst. Then, lead oxide was deposited on the top or bottom part of the gold nanocube via the site-selective oxidation of lead ions based on PICS. Lead ions in the deposited lead oxide were replaced with manganese ions by galvanic replacement and the lead oxide was transformed to manganese oxide. The manganese oxide moieties on the gold nanocubes serve as co-catalyst of the plasmonic photocatalyst.We examined photocatalytic currents for ethanol oxidation by PICS. As a result, the photocurrents were enhanced by introducing manganese oxide to the top part of the gold nanocubes. In marked contrast, the photocurrents were suppressed when the manganese oxide was introduced to the bottom part of the nanocubes. In the latter case, charge recombination occurs between the holes injected to manganese oxide and the electrons injected to titanium oxide. On the other hand, the charge recombination is suppressed in the former case, because manganese oxide is not directly contact with titanium oxide. Thus we found that the position of a co-catalyst greatly affect the activity of plasmonic photocatalysts.8 1. Y. Tian and T. Tatsuma, J. Am. Chem. Soc., 127, 7632 (2005).2. T. Tatsuma, H. Nishi, and T. Ishida, Chem. Sci., 8, 3325 (2017) [review].3. I. Tanabe and T. Tatsuma, Nano Lett., 12, 5418 (2012).4. K. Saito, I. Tanabe, and T. Tatsuma, J. Phys. Chem. Lett., 7, 4363 (2016).5. H. Nishi, M. Sakamoto, and T. Tatsuma, Chem. Commun., 54, 11741 (2018).6. K. Saito and T. Tatsuma, Nano Lett., 18, 3209 (2018).7. T. Tatsuma and H. Nishi, Nanoscale Horiz., 5, 597 (2020) [review].8. K. Kim, H. Nishi, and T. Tatsuma, J. Chem. Phys., 157, 111101 (2022).

Excellent plasmon-induced photoelectrochemical properties of WO3/Au nanosheets with uniform dispersion of dense Au nanoparticles fabricated by SILAR method

Constructing noble metal-tungsten oxide heterostructure is an effective strategy for extending visible light absorption to longer wavelength owing to the plasmonic effect, thus endowing them with more potential applications in photo(electro)catalysis. Herein, a facile successive ionic layer adsorption and reaction (SILAR) method was developed to fabricate Au NPs decorated WO3 nanosheets (WO3-NSs) electrode for elucidating the mechanism of plasmon-induced charge separation (PICS) and enhancing PICS efficiency. Various characterizations suggested that dense Au NPs with small particle size (<20 nm) were highly dispersed on WO3-NSs by SILAR. The as-prepared WO3/Au electrodes exhibited strong and broad absorption in the visible light region of 500–700 nm, while pristine WO3 electrode had very weak absorption in this region. Anodic photocurrent responses and negative photopotential shifts were observed for WO3/Au electrode, demonstrating that PICS process indeed occurs at the Au-WO3 interface. Beneficial from the good dispersion and high loading amount of Au NPs, as well as formation of large interface between Au NPs and WO3-NSs, the as-prepared WO3/Au electrode achieved satisfactory PICS efficiency. This study paves a new way for the deposition of well-dispersed Au NPs on WO3 photoanodes with excellent PICS-based photoelectrochemical performance.

Observation of Charge Separation Enhancement in Plasmonic Photocatalysts under Coupling Conditions.

Surface plasmon resonance-induced charge separation plays key roles in plasmon-related applications, especially in photocatalysis and photovoltaics. Plasmon coupling nanostructures exhibit extraordinary behaviors in hybrid states, phonon scattering, and ultrafast plasmon dephasing, but plasmon-induced charge separation in these materials remains unknown. Here, we design Schottky-free Au nanoparticle (NP)/NiO/Au nanoparticles-on-a-mirror plasmonic photocatalysts to support plasmon-induced interfacial hole transfer, evidenced by surface photovoltage microscopy at the single-particle level. In particular, we observe a nonlinear increase in charge density and photocatalytic performance with an increase in excitation intensity in plasmonic photocatalysts containing hot spots as a result of varying the geometry. Such charge separation increased the internal quantum efficiency by 14 times at 600 nm in catalytic reactions as compared to that of the Au NP/NiO without a coupling effect. These observations provide an improved understanding of charge transfer management and utilization by geometric engineering and interface electronic structure for plasmonic photocatalysis.

Metallic On-Chip Light Concentrators Fabricated by In Situ Plasmonic Etching Technique

One-dimensional tapered metallic nanostructures are highly interesting for nanophotonic applications because of their plasmonic waveguiding and field-focusing properties. Here, we developed an in situ etching technique for unique tapered crystallized silver nanowire fabrication. Under the focused laser spot, plasmon-induced charge separation of chemically synthesized nanowires is excited, which triggers the uniaxial etching of silver nanowires along the radial direction with decreasing rate, forming tapered structures several micrometers long and with diameter attenuating from hundreds to tens of nanometers. These tapered metallic nanowires have smooth surfaces showing excellent performance for plasmonic waveguiding, and can be good candidates for nanocircuits and remote-excitation sources.

Engineering Heterostructures of Layered Double Hydroxides and Metal Nanoparticles for Plasmon-Enhanced Catalysis

Artificially designed heterostructures formed by close conjunctions of plasmonic metal nanoparticles (PNPs) and non-plasmonic (2D) lamellar nanostructures are receiving extensive interest. The synergistic interactions of the nanounits induce the manifestation of localized surface plasmon resonance (LSPR) in plasmonic metals in the specific environment of the 2D-light absorbing matrix, impacting their potential in plasmon enhanced catalysis. Specifically, layered double hydroxides (LDH) with the advantages of their unique 2D-layered structure, tuned optical absorption, ease of preparation, composition diversity, and high surface area, have emerged as very promising candidates for obtaining versatile and robust catalysts. In this review, we cover the available PNPs/LDH heterostructures, from the most used noble-metals plasmonic of Au and Ag to the novel non-noble-metals plasmonic of Cu and Ni, mainly focusing on their synthesis strategies toward establishing a synergistic response in the coupled nanounits and relevant applications in plasmonic catalysis. First, the structure–properties relationship in LDH, establishing the desirable features of the 2D-layered matrix facilitating photocatalysis, is shortly described. Then, we address the recent research interests toward fabrication strategies for PNPs/support heterostructures as plasmonic catalysts. Next, we highlight the synthesis strategies for available PNPs/LDH heterostructures, how these are entangled with characteristics that enable the manifestation of the plasmon-induced charge separation effect (PICS), co-catalytic effect, or nanoantenna effect in plasmonic catalysis with applications in energy related and environmental photocatalysis. Finally, some perspectives on the challenges and future directions of PNPs/LDHs heterostructures to improve their performance as plasmonic catalysts are discussed.

Time-dependent measurement of plasmon-induced charge separation on a gold nanoparticle/TiO2 interface by electrostatic force microscopy

Plasmon-induced charge separation (PICS) is an efficient way to use the hot carriers generated by localized surface plasmon resonance. Although the ultrafast dynamics of hot carrier generation and annihilation itself are well understood, the slow dynamics of PICS are not, despite their importance for the use of hot carriers in chemical reactions. In this work, we directly observed the slow dynamics of PICS on an Au nanoparticle (NP)/TiO2 interface by using electrostatic force microscopy with time-resolved measurements obtained by sideband signal of frequency shift. The change in contact potential difference induced by PICS had a bias voltage dependence, indicating that the number of holes in the Au NPs ([{mathrm{h}}_{mathrm{AuNP}}^{+}]) accumulated by laser irradiation depended on bias voltage. The decay constant for the annihilation of the separated charge on the Au NPs at the Au NP/TiO2 interface was directly determined to be ca. 150 ms, and the annihilation process was discussed in a simple model based on the transient Schottky barrier.

Site-selective introduction of MnO2 co-catalyst onto gold nanocubes via plasmon-induced charge separation and galvanic replacement for enhanced photocatalysis.

For energy harvesting with plasmonic photocatalysis, it is important to optimize geometrical arrangements of plasmonic nanomaterials, electron (or hole) acceptors, and co-catalysts so as to improve the charge separation efficiency and suppress charge recombination. Here, we employ a photocatalytic system with Au nanocubes on TiO2 and introduce MnO2 as an oxidation co-catalyst onto the nanocubes via site-selective oxidation based on plasmon-induced charge separation (PICS). However, it has been known that PbO2 is the only material that can be deposited onto Au nanomaterials through PICS with sufficient site-selectivity. Here we addressed this issue by introducing an indirect approach for MnO2 deposition via site-selective PbO2 deposition and subsequent galvanic replacement of PbO2 with MnO2. The indirect approach gave nanostructures with MnO2 introduced at around the top part, bottom part, or entire surface of the Au nanocubes on a TiO2 electrode. The activity of those plasmonic photocatalysts was strongly dependent on the location of MnO2. The key to improving the activity is to separate MnO2 from TiO2 to prevent recombination of the positive charges in MnO2 with the negative ones in TiO2.

(Invited) Photoelectrochemical Fabrication of Chiral Plasmonic Nanostructures By Circularly Polarized Light

Chiral plasmonic nanostructures attracts attention because they are potentially applicable to optical materials such as enantioselective sensors and metamaterials, as well as photoelectrochemical devices. Chiral nanostructures are often prepared by electron beam lithography or synthesis based on DNA templates. We have recently developed a photoelectrochemical method, in which handedness of the chiral nanostructure can be controlled by right- or left- circularly polarized light.The photoelectrochemical method is based on plasmon-induced charge separation (PICS),1,2 in which electrons are injected from a plasmonic metal nanoparticle to a semiconductor such as titania in direct contact. In PICS, anodic reactions often occur at the resonance sites of the plasmonic nanoparticle, at which electron oscillation is localized.3,4 Energetic electron-hole pairs generate at the resonance site, and holes are used for the local anodic reaction, probably via trap sites. On the basis of the mechanisms, we have demonstrated site-selective etching of silver nanoparticles and site-selective deposition of lead oxide on gold nanoparticles.Under right-circularly polarized light (CPL), distribution of the resonance sites could be the mirror image of that under left-CPL.5 Therefore, we performed site-selective deposition of lead oxide on gold nanocuboids on titania under right- or left-CPL.6 As a result, lead oxide was deposited on the gold nanocuboids in a chiral geometry. The nanostructures thus obtained exhibited circular dichroism (CD), and the CD spectrum obtained for the structure prepared under right-CPL was opposite to that obtained for the structure prepared under left-CPL. Reversible switching of the handedness of the chiral plasmonic nanostructures can also be possible.7 This method also allows us to fabricate spiral nanostructures. 1. Y. Tian and T. Tatsuma, J. Am. Chem. Soc., 127, 7632 (2005). 2. T. Tatsuma, H. Nishi, and T. Ishida, Chem. Sci., 8, 3325 (2017) [review]. 3. I. Tanabe and T. Tatsuma, Nano Lett., 12, 5418 (2012). 4. T. Tatsuma and H. Nishi, Nanoscale Horiz., 5, 597 (2020) [review]. 5. S. Hashiyada, T. Narushima, and H. Okamoto, J. Phys. Chem. C, 118, 22229 (2014). 6. K. Saito and T. Tatsuma, Nano Lett., 18, 3209 (2018). 7. K. Morisawa, T. Ishida, and T. Tatsuma, ACS Nano, 14, 3603 (2020).

Surface-Functionalized Au-Pd Nanorods with Enhanced Photothermal Conversion and Catalytic Performance.

Bimetallic nanostructures comprising plasmonic and catalytic components have recently emerged as a promising approach to generate a new type of photo-enhanced nanoreactors. Most designs however concentrate on plasmon-induced charge separation, leaving photo-generated heat as a side product. This work presents a photoreactor based on Au-Pd nanorods with an optimized photothermal conversion, which aims to effectively utilize the photo-generated heat to increase the rate of Pd-catalyzed reactions. Dumbbell-shaped Au nanorods were fabricated via a seed-mediated growth method using binary surfactants. Pd clusters were selectively grown at the tips of the Au nanorods, using the zeta potential as a new synthetic parameter to indicate the surfactant remaining on the nanorod surface. The photothermal conversion of the Au-Pd nanorods was improved with a thin layer of polydopamine (PDA) or TiO2. As a result, a 60% higher temperature increment of the dispersion compared to that for bare Au rods at the same light intensity and particle density could be achieved. The catalytic performance of the coated particles was then tested using the reduction of 4-nitrophenol as the model reaction. Under light, the PDA-coated Au-Pd nanorods exhibited an improved catalytic activity, increasing the reaction rate by a factor 3. An analysis of the activation energy confirmed the photoheating effect to be the dominant mechanism accelerating the reaction. Thus, the increased photothermal heating is responsible for the reaction acceleration. Interestingly, the same analysis shows a roughly 10% higher reaction rate for particles under illumination compared to under dark heating, possibly implying a crucial role of localized heat gradients at the particle surface. Finally, the coating thickness was identified as an essential parameter determining the photothermal conversion efficiency and the reaction acceleration.

Plasmon-induced charge separation by Ag nanoparticles between titanium dioxide and MWCNTs for natural sunlight-driven photocatalysis

Plasmon-induced charge separation by Ag nanoparticles between titanium dioxide and MWCNTs for natural sunlight-driven photocatalysis

Carrier depletion mediated exciton-surface plasmon coupling at the mesoporous TiO2/Ag interface

We systematically investigated the effect of the change in the carrier depletion width in the TiO2 on the exciton-plasmon coupling at the TiO2/Ag nanoparticle (NP) interface, excluding complications associated with the optical and electrochemical screening of Ag NPs. We found that Ag NP-induced interfacial traps are coupled with the energy band bending in the TiO2 under illumination, suppressing plasmon-induced interfacial charge separation at the TiO2/Ag NP interface. Under illumination with solar simulator including UV, reduction in the depletion width of the TiO2 facilitated back-transfer of the excited electrons in the TiO2 to Ag NP-induced interfacial localized states, while green light irradiation enhanced the photocurrent by plasmonic-induced charge separation. Surface potential change depending on light irradiation is discussed relating to exciton-plasmon coupling at the TiO2/Ag NP interface combined with Mott-Schottky analysis. For the first time, we distinguished the optical and electrochemical screening by the plasmonic nanostructure from carrier depletion-induced band bending at the TiO2/Ag NP interface. We demonstrated the significant effect of the localized interfacial states and the energy band bending on the exciton-plasmon coupling.

Emerging of Ag particles on ZnO nanowire arrays for blue-ray hologram storage

Noble-metal/metal-oxide-semiconductor nanostructures as an important material platform have been applied in massive data storage. ZnO exhibits excellent optical modulation ability. However, plasmon induced charge separation effect in Ag/ZnO systems is very weak due to the low chemical activity on surface of the oxide. Herein, we prepare ZnO nanowire arrays via the hydrothermal method, and measure their absorption spectra, photoluminescence spectra and electron paramagnetic resonance, proving the existence of oxygen defects in ZnO. Accordingly, an idea of “electron reverse transfer” is proposed such that blue-ray (403.4 nm) induces reduction of Ag+ ions through the excitation of ZnO. Rod-like and spherical silver nanoparticles emerge on the surface and in the gap of ZnO nanowire arrays, respectively, after the visible light stimulus. It is found that nanowire density, oxygen defects and surface roughness are dependent on hydrothermal time. The optimized diffraction efficiency of 0.08% is obtained for reconstructing hologram in the nanocomposite film. This work provides a bright way for construction of ZnO-based optoelectronic integrated devices.

Preparation of silver-nanoparticle-loaded C60-ethylenediamine adduct microparticles and their application to photoelectric conversion

We report here new types of suprastructure products consisting of C60-ethylenediamine microparticles and silver nanoparticles that were loaded onto the surfaces of the microparticles by electrostatic adsorption. The suprastructure products could be deposited on the surface of an indium tin oxide electrode by electrostatic deposition. These modified electrodes generated cathodic photocurrents in the presence of ascorbic acid and oxygen. The photocurrent generation behavior implies an ensemble of the electron-transfer processes of silver-nanoparticle-mediated electron transfer acting in the cathodic direction and plasmon-induced charge separation acting in the anodic direction.

Patterning of transition metal dichalcogenides catalyzed by surface plasmons with atomic precision

Summary Plasmon-induced charge transfer has drawn considerable interests and spurred rapid developments of photovoltaics and photocatalysis. Various plasmonic metal/transition metal dichalcogenide (TMD) heterostructures have been developed to promote charge separation. For plasmon-induced catalysis, the transformation of TMDs is rare, and the erosion of TMDs has not yet been reported. Here, we report the etching of two-dimensional TMDs by planar surface plasmon polaritons (SPPs). We found that SPPs can etch various TMDs into desired layers and lateral size through controlling the light power and incident direction. In aqueous media, the strong plasmonic coupling at Au/MoS2 interface generates holes in the valence band of MoS2 that can weaken its interlayer interaction. As a synergistic effect, the plasmonic hot electrons produce oxidizing H2O2 to dissolve MoS2 from the top layers. Our results provide a new perspective for understanding the plasmonic coupling at metal-TMD interfaces and a reliable route toward fabricating well-defined TMDs.

One-pot synthesis of visible-light-responsive titanium oxide photocatalyst with embedded silver nanoparticles

A composite powder of titanium oxide and silver nanoparticles was obtained by using a combination of the sol–gel process and ultraviolet light induced reduction of silver ion via one-pot synthesis. After calcination, silver nanoparticles were embedded in an anatase crystalline titanium oxide matrix, as revealed by transmission electron microscopy. Silver nanoparticles with sizes ranging from a few nanometers to several tens of nanometers were observed in the titanium oxide matrix. The crystalline titanium oxide-silver nanoparticle composite powder decomposed the methylene blue dye upon visible light irradiation. The decomposition rate of the dye using the crystalline titanium oxide-silver nanoparticle composite powders was considerably faster than that in the case of the reference titanium oxide powder without silver nanoparticle. Thus, plasmon-induced charge separation from the silver nanoparticle to the conduction band of titanium oxide expected to occur, and holes on silver nanoparticles or induced active oxygen decomposed methylene blue in the solution or adsorbed on titanium oxide.

Plasmon-enhanced lateral photovoltaic effect observed in Ag-ZnO core–shell nanoparticles

The lateral photovoltaic effect (LPE) is widely used in sensitive position detectors. Discovering comprehensive mechanisms and continuously improving their sensitivity are the ongoing goals in this field. However, the limited absorption and rapid recombination are two major challenges in the traditional LPE. Here, surface plasmon-based approaches have been used to boost the energy conversion efficiency, and the Ag-ZnO core–shell nanoparticles (NPs) with enhanced LPE are prepared on the Si substrate through atomic layer deposition. Owing to the enhanced light absorption, prolonged hot electron generation, and plasmon-induced charge separation, this system exhibits good LPE performance with a maximum sensitivity of 122.1 mV/mm at 980 nm, which is about seven times larger than that observed in conventional Ag/Si (5.03 mV/mm) and ZnO/Si (76.13 mV/mm). Besides, its sensitivity can increase up to 114.7% by using rapid thermal annealing to change the shape and size of active plasmon Ag NPs. Furthermore, the surface-enhanced Raman scattering spectra and finite-difference time-domain electric field simulation prove the occurrence of localized surfaced plasmon resonance excitation. This infrared sensitive shell-isolated structure has great potential for applications in high sensitivity and stability photoelectric devices.

Plasmon-induced charge separation based on a nanocomposite containing MoO2under visible light irradiation

Plasmon-induced charge separation (PICS) was achieved with compound nanomaterials containing MoO2, which has a plasmon resonance wavelength close to gold.

Visualization of nano-localized and delocalized oxidation sites for plasmon-induced charge separation.

Oxidation reaction sites for plasmon-induced charge separation at Au nanocubes on TiO2 were visualized on the basis of deposition and dissolution reactions. For Pb2+ oxidation, PbO2 was deposited selectively at resonance sites of the nanocube, while oxidation polymerization of pyrrole to polypyrrole and oxidative dissolution of Au took place over the entire nanocube surface. The localized and delocalized reaction sites are explained in terms of a relationship between oxidation potentials of the electron donors and potentials of the entire nanocube and localized holes.

Approaches to Improvement of Stabilities and Efficiencies of Photoenergy Conversion Devices Based on Plasmon Induced Charge Separation

It is known that metal nanoparticles exhibit localized surface plasmon resonance (LSPR) with the resonant light irradiation. When the resonant light irradiates to a plasmonic metal nanoparticle, light energy can be localized in nanospaces around the surface of the nanoparticle. Hence, LSPR is expected to be an innovative technology in order to utilize photoenergy with high efficiency. Furthermore, a plasmonic metal nanoparticle combined with a semiconductor exhibit plasmon induced charge separation (PICS) [1]. When the resonant light irradiates to a plasmonic metal nanoparticle on an n-type semiconductor, electrons in the metal nanoparticle transfer to the semiconductor. As a result, oxidative and reductive reactions occur on the surfaces of the metal nanoparticle and the semiconductor, respectively. Therefore, PICS can be applied to various photoenergy conversion devices, such as photocatalysts and phorovoltaic cells. However, conventional PICS system has some problems to be solved. For example, the oxidation power generated at the metal nanoparticles promotes oxidative dissolution of the metal nanoparticles. Furthermore, the detail mechanism of PICS has been still unclear. In addition, the efficiencies of charge separation have not achieved high enough.In order to overcome these problems, the chemical and thermal stabilities of plasmonic metal nanoparticles, such as silver nanospheres, gold nanorods (AuNRs), and silver nanoplates (AgPLs), were improved by introducing an Al2O3 nanomask and/or a TiO2 thin film [2-4]. Accordingly, these plasmonic metal nanoparticles has come to be applied to PICS system even in a severe condition.We also investigated the positive charge generated by PICS on the surface of gold nanoparticles (AuNPs) deposited on a TiO2 substrate [5,6]. As a result, oxidative photopolymerization based on PICS were observed site-selectively in nanospaces on the surface of AuNP. From the results of the nanoscale photopolymerization of phenol, pyrrole, and 3,4-ethylenedioxythiophene (EDOT), we proposed a possible mechanism of generation of oxidation power on the metal nanoparticles based on PICS (Fig. 1).Furthermore, we engaged to develop PICS system consisting of plasmonic metal nanoparticles and a p-type semiconductor (p-type PICS). Because the p-type PICS can be expected to exhibit charge separation opposite direction to conventional PICS, which employ an n-type semiconductor. As a result, the system overcome the problem of oxidative dissolution of the metal nanoparticles based on PICS. It is also expected to realize the charge separation with high efficiency.These technologies should progress the field of photoenergy conversion systems based on PICS with high efficiency. Y. Tian, T. Tatsuma, J. Am. Chem. Soc., 127, 7632 (2015). Y. Takahashi, T. Tatsuma, Nanoscale, 2, 1494 (2010).Y. Takahashi, N. Miyahara, S. Yamada, Anal. Sci., 29, 101 (2013).Y. Takahashi, K. Suga, T. Ishida, S. Yamada, Anal. Sci., 32, 275 (2016).Y. Takahashi, Y. Furukawa, T. Ishida, S. Yamada, Nanoscale, 8, 8520 (2016).Y. Takahashi, Y. Sota, T. Ishida, Y. Furukawa, S. Yamada, J. Phys. Chem. C, 124, 4202 (2020). Figure 1

Stability Improvement of Copper Nanoparticles for All-Solid-State Photovoltaic Cells Based on Plasmon Induced Charge Separation with p-Type Semiconductors

[Introduction] When the resonant light irradiates to a metal nanoparticle, enhanced electric fields on the surface of the particle are generated based on the collective oscillation of the free electrons. This phenomenon is called localized surface plasman resonance (LSPR). Furthermore, when the resonant light irradiates to a metal nanoparticle combined with an n-type semiconductor, the electron is transferred from the metal nanoparticle to the semiconductor. This phenomenon, which is called plasmon-induced charge separation (PICS), can be applied to photocatalysts and solar cells[1]. The most popular plasmonic metal nanoparticles consist of gold or silver (AuNPs and AgNPs), because they are highly stable for thermal and chemical treatments. Furthermore, their resonant wavelengths are generally in visible to near-infrared region. On the other hand, copper nanoparticles (CuNPs) are easily oxidized in the ambient air resulting in disappearance of the properties based on LSPR. Therefore, CuNPs have not been actively studied even their resonant wavelength exhibits in similar region to AuNPs. Even more importantly, an n-type semiconductor, such as TiO2 and ZnO2, was mostly employed for combination with metal nanoparticles in PICS system so far. In the case of n-type PICS, reverse electron transfer might cause the low efficiency of PICS, because metal nanoparticles can function as an electron acceptor. Furthermore, positive charge generated in metal nanoparticles often results in their oxidative dissolving. To solve these problems, our group are focusing PICS between a metal nanoparticle and a p-type semiconductor, because p-type PICS was expected to occur charge separation opposite direction. In this study, we developed a PICS system consisting of CuNPs, which are less expensive than AuNPs and AgNPs, and a p-type semiconductor. Furthermore, we improved the stabilities of CuNPs by introducing an Al2O3 nanomask[2] in order to realize simple all-solid-state photovoltaic cells based on p-type PICS without any electron or hole transport materials. [Experimental] An Al2O3 nanomask was prepared by dip-coating method from a precursor solution containing a block copolymer, ethanol, tetrahydrofuran, NH3, and AlCl3·6H2O on an indium tin oxide (ITO)-coated glass substrate, followed by annealing at 500 oC for 1 h as reported before[2]. Then, CuNPs were electrodeposited through an Al2O3 nanomask on the ITO electrode. A NiO film was coated on the electrode by a spray pyrolysis method from an aqueous solution containing Ni(NO3)2 and LiNO3 at 350 °C, and then annealed at 500 °C for 2 h. As a counter electrode, gold film was sputtered on the NiO layer resulting in the all-solid-state ITO/Al2O3 nanomask/CuNP/NiO/Au cell. For a control experiment, an ITO/Al2O3 nanomask/NiO/Au cell (without CuNPs) was also prepared. The photoelectrochemical measurements were performed by irradiating to the cell with a solar simulator (100 mW · cm− 2) equipped with a UV (<400 nm) sharp cut filter. [Results and Discussion] It was reported that the thermal stabilities of AuNPs and AgNPs could be improved by introducing an Al2O3 nanomask, because the nanomask can prevent the nanoparticles from coalescing[2]. In a similar way, we tried to improve the thermal stability of CuNPs. The average size of CuNPs without an Al2O3 nanomask increased from ~83 to ~112 nm after annealing at 350 °C for 5 min (Fig. 1(a)). In contrast, the average sizes of CuNPs with the nanomask before and after annealing were almost same (from ~53 to ~56 nm) (Fig. 1(b)). Accordingly, we could improve the thermal stabilities of CuNPs. As a result, overcoating of the NiO film at 350 °C on the CuNPs became available. We confirmed the plasmonic absorption based on CuNPs was observed even after covered with the NiO film. Then photoelectrochemical measurements of the ITO/Al2O3 nanomask/CuNP/NiO/Au cell was carried out. We observed photocurrents and photovoltages of the cell in the opposite direction to those of the all-solid state cells (ITO/Al2O3nanomask/AuNP or AgNP/TiO2/In) based on n-type PICS which were reported before[3]. The photovoltage of the prepared cell (with CuNPs) exhibited much more negative than that of the control cell (without CuNPs). These results suggested that the p-type PICS at the interface between CuNP/NiO can be observed. In summary, we succeeded improvement of the thermal and chemical stabilities of CuNPs. Due to the high stable CuNPs, the all-solid-state ITO/Al2O3 nanomask/CuNP/NiO/Au cell could be fabricated. Photoelectrochemical measurements suggested that p-type PICS could proceed at the interface between CuNP/NiO. It will lead to designs of cupper-based plasmonic cells.[1] Y. Tian, T. Tatsuma, J. Am. Chem. Soc. 2015, 127, 7632.[2] Y. Takahashi, T. Tatsuma, Nanoscale 2010, 2, 1494.[3] Y. Takahashi, T. Tatsuma, Appl. Phys. Lett. 2011, 99, 182110. Figure 1