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Development of Plasmonic Cu2O/Cu Composite Arrays as Visible- and Near-Infrared-Light-Driven Plasmonic Photocatalysts Kosuke Sugawa, Natsumi Tsunenari, Hideyuki Takeda, Saki Fujiwara, Tsuyoshi Akiyama, Jotaro Honda, Shuto Igari, Kyo Tokuda, Naoto Takeshima, Yasuhiro Watanuki, Satoshi Tsukahara, Kouichi Takase, Tetsuo Umegaki, Yoshiyuki Kojima, Nobuyuki Nishimiya, Nobuko Fukuda, Yasuyuki Kusaka, Hirobumi Ushijima, and Joe Otsuki Langmuir, Just Accepted Manuscript • Publication Date (Web): 19 May 2017 Downloaded from http://pubs.acs.org on May 20, 2017
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Development of Plasmonic Cu2O/Cu Composite Arrays as Visible- and NearInfrared-Light-Driven Plasmonic Photocatalysts
Kosuke Sugawa,*,† Natsumi Tsunenari,† Hideyuki Takeda,† Saki Fujiwara,† Tsuyoshi Akiyama,‡ Jotaro Honda,† Shuto Igari,† Kyo Tokuda,† Naoto Takeshima,† Yasuhiro Watanuki,† Satoshi Tsukahara,† Kouichi Takase,§ Tetsuo Umegaki,† Yoshiyuki Kojima,† Nobuyuki Nishimiya,† Nobuko Fukuda, Yasuyuki Kusaka, Hirobumi Ushijima and Joe Otsuki†
†
Department of Materials and Applied Chemistry, College of Science Technology, Nihon University,
Chiyoda, Tokyo 101-8308, Japan ‡
Department of Materials Science, School of Engineering, The University of Shiga Prefecture, Hikone,
Shiga 522-8533, Japan §
Department of Physics, College of Science and Technology, Nihon University, Chiyoda, Tokyo 101-
8308, Japan Flexible
Electronics Research Center (FLEC), National Institute of Advanced Industrial Science and
Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan
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ABSTRACT We describe efficient visible- and near-infrared (Vis/NIR) light-driven photocatalytic properties of hybrids of Cu2O and plasmonic Cu arrays. The Cu2O/Cu arrays were prepared simply by allowing to stand a Cu half-shell array in oxygen atmosphere for 3 h, which were prepared by depositing Cu on two-dimensional colloidal crystals with diameters of 543 or 224 nm. The localized surface plasmon resonances (LSPRs) of the arrays were strongly excited at 866 and 626 nm, respectively at which the imaginary part of the dielectric function of Cu is small. The rate of photodegradation of methyl orange were 27 and 84 times faster, respectively, than that with a Cu2O/non-plasmonic Cu plate. The photocatalytic activity was demonstrated to be dominated by Cu LSPR excitation. These results showed that the inexpensive Cu2O/Cu arrays can be excellent Vis/NIR-light-driven photocatalysts based on efficient excitation of Cu LSPR.
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1. INTRODUCTION Recent reports have demonstrated that the loading or doping of metal nanoparticles that are several tens of nanometers in size, which generate localized surface plasmon resonance (LSPR) in semiconductor photocatalysts dramatically enhances photocatalytic activity for the production of clean energy and environmental remediation.1-17 Although the mechanism is still under debate, three major mechanisms are being put forward to account for the LSPR-based enhancement of photocatalytic activities. Expanding the photocatalytically active wavelength range and/or improving charge separation efficiency through a direct electron transfer (DET)18-20 and/or an effect of LSPR-mediated local electromagnetic fields (LEMF)3,21 have been reported to dramatically improve the photocatalytic activity of hybrids of a plasmonic metal nanoparticle and a semiconductor. More recently, plasmoninduced resonance energy transfer (PIRET) has been suggested to be an important mechanism behind the improvement of photocatalysis.3,22 The TiO2, the most widely used material as photocatalyst, is a wide band gap (B.G.) semiconductor (3.2 eV) and thus can work only by irradiation of UV light which constitutes only 5% of solar energy. However, TiO2 loaded with Au nanoparticles can be made into active catalyst by the LSPR excitation within the visible region, which accounts for approximately 45% of solar energy.15 On the other hand, a few efforts have been made in utilizing near-infrared (NIR) light, which is also abundant in the solar spectrum.23-27 In addition, while most semiconductor photocatalysts are inexpensive and abundant metal oxides (TiO2, ZnO, and Cu2O), nanoparticles that have frequently been used as plasmonic materials have consisted of expensive Au and Ag.1-10,12-16 3 ACS Paragon Plus Environment
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Therefore, for the development of practical photocatalysts, the utilization of inexpensive plasmonic metals that can harvest visible and NIR light is required. Cu is significantly less expensive than Au and Ag and is easily recycled. Moreover, the LSPR generated from Cu nanostructures has the potential to be tuned over a wide range of wavelengths from the visible to the NIR region by adjusting the morphologies and the sizes of the Cu nanostructures.28,29 Therefore, the development of plasmonic photocatalysts based on Cu nanostructures is a very important challenge from the point of view of both fundamental science and practical applications. Cu is oxidized spontaneously in the atmosphere, giving rise to cuprous oxide (Cu2O),30 which acts as a photocatalyst in various beneficial reactions such as the decomposition of water, reduction of CO2, and degradation of organic pollutants under irradiation with short-wavelength visible light owing to its narrower B.G. (ca. 2.0 eV) compared to that of TiO2.31-33 Therefore, plasmonic Cu nanostructures have a tremendous potential for cheaper solar-light (visible-NIR)-driven plasmonic photocatalysts consisting of plasmonic Cu/Cu2O composites. We have noted that the Cu/Cu2O composite could be prepared taking advantage of the spontaneous oxidation of Cu and without any complicated processes as with the case of Au(Ag) nanostructure/Cu2O composites.3-5,7-10,13 Although plasmonic photocatalysts consisting of metal oxides (TiO2, ZnO, and Cu2O) and spherical Cu nanoparticles have been developed recently, details regarding the effects of Cu LSPR on photocatalytic activity are still unclear.34-40 In addition, Cu nanoparticles used in these photocatalysts have exhibited LSPR at 560-575 nm34-40 where the imaginary part (2) of the dielectric function of Cu 4 ACS Paragon Plus Environment
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is fairly large owing to the strong electronic interband transition.41 Consequently, the LSPR of spherical Cu nanoparticles suffers from substantial damping.42,43 Previously, we had fabricated plasmonic Cu half-shell (CuHS) arrays by utilizing two-dimensional (2D) silica colloidal crystals as a template. In addition, we have demonstrated that the stronger local electromagnetic fields generated by efficiently exciting the Cu LSPR at wavelengths longer than ca. 600 nm corresponding to lower 2 values drastically enhanced fluorescence signals44 and the photoelectric conversion efficiency45 of adjacent porphyrin molecules, as compared with the weak enhancements when the LSPR was generated at wavelengths shorter than 600 nm. Therefore, for the development of Cu LSPR-based plasmonic photocatalysts, the wavelength of Cu LSPR should be tuned to regions of lower 2, which is from the longer red visible region to the NIR region. In this study, we developed photocatalysts based on plasmonic Cu nanostructures. Specifically, the following three important goals have been achieved. First, composite structures consisting of a small amount of Cu2O and plasmonic Cu arrayed structures, which can tune the LSPR wavelength, were fabricated simply through the spontaneous oxidation of the surface of the plasmonic Cu arrays. Second, we demonstrated that these composite arrays exhibited excellent photocatalytic activity owing to the efficient excitation of Cu LSPR by visible and NIR light irradiation, as compared with those shown by non-plasmonic composite films. Third, we have clarified that the reactive species are the same as those for a previously-reported Au nanoparticle/Cu2O-based plasmonic photocatalyst.13
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2. EXPERIMENTAL SECTION 2.1. Materials. Milli-Q-grade water (resistivity = 18.2 MΩ·cm) was used to prepare all aqueous solutions. Aqueous NH3 (28%), tetraethyl orthosilicate (TEOS), 1-butanol, 2-propanol (isopropanol: IPA), and aqueous H2O2 (34.5%) were obtained from Kanto Chemical, Japan. Absolute ethanol (99.5 vol.%), methyl orange (MO), and sodium oxalate (Na2C2O4) were obtained from Wako Pure Chemical, Japan. 3Mercaptopropyltriethoxysilane (MPTS) was obtained from Tokyo Kasei, Japan. p-Benzoquinone (PBQ) were obtained from Kishida Chemical, Japan. All materials were used without further purification.
2.2. Syntheses of Colloidal Solutions of Silica Particles Colloidal solutions of silica particles with diameters of 224 ± 18 and 543 ± 17 nm (mean ± standard deviation (SD)) were prepared. The particles with a diameter of 224 nm were synthesized according to a modified version of a previously reported procedure.46 Aqueous NH3 (0.89 mL) and Milli-Q water (6.4 mL) were added to absolute ethanol (20.9 mL), and the mixture was stirred for 10 min at room temperature. TEOS (1.0 mL) was added quickly to the solution, and the mixture was stirred for 3 h. The resultant colloidal solution of silica particles was centrifuged at 10,000 rpm for 3 min and then redispersed twice in an equivalent amount of ethanol. It was then centrifuged at 10,000 rpm for 3 min and redispersed twice in an equivalent amount of 1-butanol. The silica particles with a diameter of 543 6 ACS Paragon Plus Environment
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nm were synthesized using a seed-mediated growth method.47 A mixture solution of aqueous NH3 (1.0 mL) and absolute ethanol (10 mL) was stirred for 10 min. After the additions of TEOS (0.1 mL) and absolute ethanol (0.4 mL), the solution was stirred for an additional 2 h, resulting in the formation of silica seeds. Then, NH3 (2.6 mL), absolute ethanol (6.3 mL), and TEOS (1.0 mL) were added to the seed solution, and the resulting mixture was stirred for 12 h. Finally, the colloidal butanol solution of silica particles was obtained by centrifuging and redispersing the resultant solution as described above.
2.3. Fabrication of Cu2O/Cu Regular Arrayed Substrates The fabrication process of the Cu2O/plasmonic Cu arrayed structures is shown in Scheme 1. The Cu arrays were fabricated using a modified version of our previously reported procedure.44,45 Glass plates (0.9 × 2.3 cm) were treated with a mixed solution of aqueous NH3/aqueous H2O2 (1/1 v/v) at 100 °C for 2 h and then washed with Milli-Q water to produce a hydrophilic surface. A small amount of each colloidal butanol solution of the silica particles was softly added dropwise to water in a Petri dish, resulting in the formation of 2D colloidal crystals of the silica particles on the water surfaces.48 Afterwards, the crystals were transferred onto the glass surface through Marangoni flow by placing the edge of glass plate in contact with the surface and the prepared samples were annealed at 500 °C for 1 h to physically strengthen the colloidal crystals. The fabrication of Cu-deposited substrates is illustrated as the first step in scheme 1. A mixed solution of MPTS (1 mL) and Milli-Q water (1 mL) in IPA (40 mL) was refluxed for 1 h at 100 °C. 7 ACS Paragon Plus Environment
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After the solution was cooled, the sample substrates were immersed in the solution for 12 h to expose thiol groups to the silica surface as binders for the Cu film, washed with ethanol and dried in air at 75 °C for 1 h.49 A layer of Cu with a thickness of 50 nm was thermally deposited under high vacuum (6.7 × 10-7 Torr) using a TMP vacuum evaporator (VE-2012, Shinkuu Device Co., Ltd.) onto the surface of the 2D colloidal crystals of the surface modified silica particles, resulting in the formation of CuHS arrays. Finally, for the reproducible formation of uniform Cu2O as a photocatalytic material on the Cu surface, the CuHS arrays were immersed in glacial acetic acid for 1 min to remove unintended Cu2O and then left to stand under oxygen atmosphere at 30 °C (humidity = 0%) for 180 min (second step in Scheme 1). The resultant substrates were composed of the plasmonic arrays with the Cu2O/Cu composites (denoted hereafter as Cu2O/CuHS(d), where d is the mean diameter of the silica particles in nanometers). A planar Cu plate with a mirror surface (CuP) was prepared via thermal deposition of Cu with a thickness of 50 nm onto a glass substrate modified with MPTS. Then, the surface of CuP was oxidized in the same manner as that described above, resulting in the formation of a non-plasmonic planar Cu plate with Cu2O/Cu composite layers as a reference substrate.
2.4. Evaluation of Photocatalytic Activity The photocatalytic activities of the as-prepared Cu2O/CuHS arrays and Cu2O/CuP were evaluated through the degradation of MO under irradiation by a xenon lamp (100 W, LAX-103, Asahi Spectra). 8 ACS Paragon Plus Environment
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The range of irradiation wavelengths was controlled by combining various optical filters. For the photocatalytic tests, the sample substrates were immersed in 2 mL of an aqueous solution of MO (1.2 × 10-2 mM) at room temperature. Prior to irradiation, the solution in which the substrates were immersed was stirred at ca. 250 rpm in the dark for 30 min to ensure the adsorption-desorption equilibrium of MO molecules between the sample surface and solution. Then, the solution was irradiated using the xenon lamp under magnetic stirring at ca. 250 rpm. At each irradiation time interval (30 min), an aliquot of the solution (0.2 mL) was taken and diluted by 10 times and then its absorption spectrum was measured using a UV-vis spectrophotometer. Note that the photocatalytic tests were performed immediately (within 1 min) after the oxidation treatment of the sample substrates as a precaution to avoid further oxidation of the sample substrates in air.
2.5. Investigation of Reactive Species The major reactive species in the photocatalytic reactions driven by exposing the Cu2O/CuHS(543) and the Cu2O/CuHS(224) to 700-1000 nm and 420-1000 nm light, respectively, were determined by using respective scavengers for potential reactive species. A solar simulator (PEC-L01, Peccell Technologies, Inc.) was used as a light source. The irradiation wavelength ranges (700-1000 nm and 420-1000 nm) were controlled by combining an infrared (IR) cut filter and long-pass filter of 700 and 420 nm. Na2C2O4 was used as a hole (h+) scavenger, PBQ was used as a superoxide radical anion (·O2−) scavenger, and IPA was used as a hydroxyl radical (·OH) scavenger. The final concentrations of 9 ACS Paragon Plus Environment
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Na2C2O4, PBQ, and IPA in the MO solutions were respectively adjusted to be 0.25, 0.25, and 10 mM. Changes in the absorption spectrum of MO in the presence of the scavengers were measured after light irradiation for 3 h. For the photocatalytic evaluation of the Cu2O/CuHS(224), the experiment involving the addition of PBQ was performed by irradiating with 520-1000 nm light because of the unexpected degradation of PBQ due to irradiation with 420-520 nm light. The photocatalytic test was also performed using an MO solution that was bubbled with N2 gas for 2 h to remove oxygen from the system.
2.6. Measurements UV-vis spectral measurements were carried out using a JASCO V-630 spectrophotometer. Reflectance spectra were measured using a JASCO MV-3250 portable spectrophotometer. Field emission scanning electron microscopy (FE-SEM) observations were carried out using a HITACHI S-4500 microscope. Surface analyses of Cu2O/CuHS(d) were performed by X-ray photoelectron spectroscopy (XPS) using an ESCA-3400 electron spectrometer (Shimadzu Co., Japan), with a base pressure of
