Plasmon-Enhanced Photoluminescence and Photocatalytic Activities

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Plasmon-Enhanced Photoluminescence and Photocatalytic Activities of Visible-Light-Responsive ZnS-AgInS Solid Solution Nanoparticles 2

Takuya Takahashi, Akihiko Kudo, Susumu Kuwabata, Akira Ishikawa, Hajime Ishihara, Yasuyuki Tsuboi, and Tsukasa Torimoto J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 13 Nov 2012 Downloaded from http://pubs.acs.org on November 17, 2012

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The Journal of Physical Chemistry

Plasmon-Enhanced Photoluminescence and Photocatalytic Activities of Visible-Light-Responsive ZnS-AgInS2 Solid Solution Nanoparticles Takuya Takahashi,1 Akihiko Kudo,2 Susumu Kuwabata,3 Akira Ishikawa4 Hajime Ishihara5, Yasuyuki Tsuboi,6 and Tsukasa Torimoto,*1

1

2

Faculty of Science, Tokyo University of Science, Shinjuku-ku, Tokyo 162-8601, Japan 3

4

Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan

Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan

Interdisciplinary Graduate School of Medicine and Engineering, The University of Yamanashi, Takeda 4-3-11, Kofu 400-8511, Japan 5

Graduate School of Engineering, Osaka Prefecture University, Gakuencho 1-1, Nakaku 599-8531, Sakai, Osaka, Japan 6

Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan

RECEIVED DATE

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CORRESPONDING AUTHOR FOOTNOTE Tsukasa Torimoto Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan e-mail: [email protected]

ABSTRACT:

Semiconductor-metal nanocomposite materials composed of ZnS-AgInS2 solid

solution (ZAIS) nanoparticles and SiO2-coated Au particles were prepared, the particle distance between ZAIS and Au being precisely adjusted in nanometer scale by changing the thickness of the SiO2 coating layer on Au core particles. The SiO2 layer also acted as an insulator layer to prevent direct electron transfer from photoexcited ZAIS to Au particles. The photoluminescent (PL) and photocatalytic properties of ZAIS particles were modulated by the locally-intensified electric field produced by photoexcitation of the localized surface plasmon resonance (LSPR) peak of Au particles. The PL intensity was enhanced with an increase in the distance between ZAIS and Au particles (dZAISAu)

up to ca. 21 nm, due to the enhancement of photoexcitation of ZAIS particles by the LSPR-induced

electric field, but further increase in dZAIS-Au inversely decreased the PL intensity. The photocatalytic H2 evolution rate with ZAIS particles immobilized at the appropriate distance from Au core particles was tunable by controlling the chemical composition of ZAIS. The plasmonic enhancement of photocatalytic activity was increased with an increase in the overlapping between the absorption properties of ZAIS and the LSPR peak of Au, the maximum enhancement factor obtained being ca. 2 for particles of AgInS2.


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TOC graphic

KEYWORDS: Surface plasmon resonance, Gold particle, Photocatalyst, Photoluminescence, Semiconductor nanoparticle, Chalcopyrite



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1. Introduction Photoexcitation of a localized surface plasmon resonance (LSPR) peak of metal nanoparticles, such as Au, Ag and Cu, produces an intensified electric field near the metal particles, the intensity of which is ~105-times larger than that of incident light.1-7 The physicochemical properties of chromophores, such as organic dyes and semiconductor nanoparticles, are modulated if they are located in the LSPR-induced electric field. These phenomena have attracted much attention for various applications,4,7-15 such as surface enhanced Raman scattering (SERS), photochemical reactions, photoluminescence enhancement, and photovoltaic devices. For example, it has been reported16-24 that the photoluminescence (PL) intensity of semiconductor nanoparticles in the proximity of Au or Ag particles was greatly enhanced due to the LSPR-induced electric field, while non-radiative energy transfer predominantly occurred from photoexcited semiconductor nanoparticles to metal when semiconductor nanoparticles were located close to metal particles, resulting in quenching of photoexcited semiconductor nanoparticles. Furthermore, Misawa and co-workers have reported that Au nanorod arrays on a TiO2 single crystal produced an anodic photocurrent with near-infrared light irradiation, accompanied by O2 evolution by the decomposition of water.14 Watanabe and co-workers reported that photodegradation of organic dyes in the presence of O2 molecules was enhanced by using TiO2 photocatalyst films deposited on SiO2-coated Ag nanoparticles due to LSPR-induced electric fields (plasmonic photocatalyst).25 Ohtani and co-workers reported26 that photoexcitation of the LSPR peak of Au particles in Au-deposited TiO2 photocatalysts caused injection of electrons in Au particles to the conduction band of TiO2 particles, that is, photosensitization of TiO2 particles with Au particles, resulting in photocatalytic oxidation of organic compounds in the presence of O2 molecules. In our previous paper, we reported27 that photocatalytic activity of CdS particles was enhanced by the immobilization on the surface of SiO2-coated Au particles, the optimum distance between CdS and Au particles for the enhancement being increased with an increase in the size of Au particles. Since the overlapping between optical properties of chromophores and metal nanoparticles seems to be necessary for the effective utilization of LSPR-induced electric fields, the size- and composition- dependent 4 Environment ACS Paragon Plus

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absorption properties of semiconductor nanoparticles are suitable for the plasmonic applications combined with metal nanostructures. On the other hand, size-quantized semiconductor nanoparticles, less than ca. 10 nm in size, have been exploited in the fabrication of solar-light energy conversion systems28-33 and the development of new photoluminescence materials.34-36 So far, binary semiconductors, such as CdSe, CdTe, PbS and InP, have been intensively investigated. Though high-quality particles could be prepared by solution phase synthetic procedures, these binary compounds contain highly toxic elements, and low toxicity of particles has thus been desired for practical applications. Recently, we successfully prepared nanoparticles of a I-III-VI group chalcopyrite semiconductor of AgInS2 and its solid solution with ZnS in hot organic solutions.37,38 The resulting nanoparticles of ZnS-AgInS2 solid solution (ZAIS) exhibited optical properties depending on the chemical composition of particles. The absorption onset of ZAIS particles was blue-shifted from ca. 700 to 500 nm with an increase in the content of ZnS in the solid solution, accompanied by a blue shift of photoluminescence peak wavelength from ca. 780 to 540 nm. These tunable optical properties of ZAIS are attractive features for plasmonic applications because the LSPR peaks are located in a wide wavelength range from visible to near-infrared regions. However, to the best of our knowledge, little is known about the influence of the LSPR-induced electric field in the vicinity of metal particles on the photoactivities of I-III-VI group chalcopyrite semiconductor nanoparticles. In this study, we prepared nanocomposite materials composed of ZAIS nanoparticles and Au particles, in which the particle distance between ZAIS and Au was precisely adjusted in nanometer scale by changing the thickness of the SiO2 coating layer on Au core particles, and then investigated their photoluminescent and photocatalytic properties. Enhancement of PL intensity was observed by the immobilization of ZAIS particles near Au particles, the degree being dependent on both the distance between ZAIS and Au particles and the composition of ZAIS particles. The photocatalytic H2 evolution rate with ZAIS particles was tunable by controlling the chemical composition of ZAIS. The plasmonic



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enhancement of photocatalytic activity was increased with an increase in the overlapping between the absorption properties of ZAIS and the LSPR peak of Au.

2. Experimental 2.1 Materials A cationic polymer of poly(diallyldimethylammonium) (PDDA) chloride was obtained from Aldrich. Thiol compounds of sodium 2-mercaptoethane sulfonate (MES) and 3-mercaptopropyl trimethoxysilane (MPTS) were supplied by Tokyo Chemical Industry. Other reagents were supplied by Kishida Reagents Chemicals. Aqueous solutions were prepared with purified water just before use by a Millipore Milli-Q system.

2.2 Preparation of silica-coated gold nanoparticles Silica-coated gold nanoparticles (Au@SiO2) were prepared by our previously reported method with a slight modification.39 Au nanoparticles having an average diameter of 15 nm were prepared by the citrate reduction method40 and used as core particles. After surface modification of the Au particles with 3-aminopropyltrimethoxy silane, the particles were uniformly covered with an amorphous SiO2 layer by hydrolysis of tetramethoxy silane (TMOS). The SiO2 shell thickness in the resulting Au@SiO2 particles could be controlled by varying the amount of TMOS added.

2.3 Preparation of ZnS-AgInS2 solid solution (ZAIS) nanoparticles ZAIS nanoparticles were prepared by thermal decomposition of a precursor powder (50 mg) of (AgIn)xZn2(1-x)(S2CN(C2H5)2)4 in oleylamine (OLA) (3.0 cm3) at 180 ºC by the same procedure as that reported in our previous paper.37 After large particles had been removed from the thus-obtained oleylamine suspension by centrifugation, the as-prepared ZAIS nanoparticles were separated from the supernatant by addition of methanol, and then the precipitates were dissolved in chloroform. The prepared particles are denoted as ZAIS(x) with the x value of (AgIn)xZn2(1-x)(S2CN(C2H5)2)4 precursors 6 Environment ACS Paragon Plus

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used in the preparation. The content of ZnS in the thus-obtained ZAIS(x) particles decreased with an increase in the x value.37 The concentration of thus-obtained OLA-modified ZAIS(x) particles in solution was determined by X-ray fluorescence spectroscopy (Rigaku, EDXL-300). The size distribution of the particles was obtained using a Hitachi H7650 transmission electron microscope (TEM) with an acceleration voltage at 100 kV. The obtained average particle size was ca. 4.4 nm, regardless of the chemical composition of the particles. The modifier on the ZAIS surface was changed from OLA to 2-mercaptoethane sulfonate or 3-mercaptopropyl trimethoxysilane, if necessary.

2.4 Layer-by-layer immobilization of MES-ZAIS and Au@SiO2 particles on quartz substrates The surface of ZAIS was modified with 2-mercaptoethane sulfonate (MES) according to our previously reported procedure,41 in order to dissolve the particles in aqueous solutions. The resulting MES-modified ZAIS(x) (MES-ZAIS(x)) particles had negative surface charges. Multilayer films composed of Au and ZAIS(x) particles were fabricated using the electrostatic layerby-layer deposition technique. The procedure for preparation of multilayer films is schematically illustrated in Fig. 1a. The quartz substrate was pre-coated with a cationic polymer of poly(diallyldimethylammonium) (PDDA) chloride by immersion in an aqueous solution of 5.0 g dm-3 PDDA (pH 9.0) for 20 min. The thus-obtained substrate was alternately dipped in an aqueous solution containing Au@SiO2 (1.3 × 1015 particles dm-3) (pH 6.8) for 60 min and in an aqueous solution containing 5.0 g dm-3 PDDA (pH 9.0) for 20 min. After each deposition, the substrate was washed with water and dried in nitrogen gas flow. A series of these operations was repeated twice, and then finally MES-ZAIS(x) particles were layer-by-layer-accumulated as the topmost layer by immersing the substrates in an aqueous solution containing MES-ZAIS(x) particles (3.3 × 1018 particles dm-3). The thus-obtained multilayer film was denoted as MES-ZAIS(x)/Au@SiO2. The amount of Au particles immobilized on quartz substrates was determined from the extinction spectra of the substrates by using the extinction coefficient of Au particles of 4.8 × 108 dm3 mol(particle)-1 cm-1 at 530 nm. The distance between ZAIS particles and Au core particles was varied by changing the SiO2 shell thickness on the 7 Environment ACS Paragon Plus


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Au core and was assumed to be equal to the thickness of the SiO2 shell. For comparison, a ZAIS monoparticle layer was deposited on the quartz substrate, denoted as MES-ZAIS(x)/quartz, in the same way except for the immobilization of Au@SiO2. UV-visible extinction and absorption spectra were obtained with an Agilent Technology 8453A spectrophotometer. Photoluminescence measurements were performed using a fluorescence microscope (Olympus, BX51) equipped with a photonic multi-channel analyzer (HAMAMATSU, PMA11). The light irradiation for the photoexcitation and the collection of the emitted photons were carried out through a ×40 objective lens having a numerical aperture of 0.75 in a direction perpendicular to the film surface. PL lifetime measurements were conducted by using a streak camera (Hamamatsu Photonics, C4334) as a photodetector at 532 nm excitation (LOTIS TII Ltd., 532 nm, pulse width ~ 10 ns).

2.5 Preparation of a plasmonic photocatalyst composed of ZAIS and Au@SiO2 particles Nanocomposite photocatalysts composed of Au@SiO2 particles and ZAIS(x) having various compositions were prepared by our previously reported method with a slight modification (Fig. 1b).27 OLA-modified ZAIS particles were dissolved in toluene to give a transparent solution containing 1.2 × 1018 particles dm-3 and then a 150-mm3 portion of 0.10 mol dm-3 MPTS toluene solution was added to the ZAIS(x) solution (25 cm3) to change the surface modifier on ZAIS from OLA to MPTS. The thusobtained MPTS-modified ZAIS(x) (MPTS-ZAIS(x)) particles were accumulated on Au@SiO2 particles. The calculated amount of Au@SiO2 particles suspended in 2-propanol was added to a 25-cm3 portion of MPTS-ZAIS toluene solution (ZAIS particle concentration: 1.2 × 1018 particles dm-3), followed by refluxing the solution for 30 min, where trimethoxysilyl groups of MPTS on ZAIS react strongly with hydroxyl groups on Au@SiO2 particles to give Si(SiO2 shell)-O-Si(MPTS) bond, as reported in our previous papers.27,42 Hydrolysis of the remaining trimethoxysilyl groups was performed by dropwise addition of 10 cm3 of water to the refluxed solution, resulting in the formation of a thin SiO2 layer on the surface of ZAIS(x) particles (ZAIS(x)@SiO2) that were immobilized on Au@SiO2 particles. Thus8 Environment ACS Paragon Plus

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obtained ZAIS(x)@SiO2-deposited Au@SiO2 particles had a semiconductor/insulator/metal structure and are denoted here as ZAIS(x)@SiO2/Au@SiO2. After the suspension had been subjected to centrifugation, the obtained precipitates were washed several times with methanol and then suspended in a 2-propanol solution. For comparison, SiO2-coated ZAIS(x) particles (ZAIS(x)@SiO2) were prepared without addition of Au@SiO2. Photocatalytic hydrogen (H2) evolution was performed by irradiation of light from a Xe lamp (λ > 350 nm) with light intensity of ca. 500 mW cm-2. Each photocatalyst containing 5 nmol(particle) of ZAIS particles was suspended in an aqueous solution (5 cm3) containing 2-propanol (50 vol%) and irradiated under an argon atmosphere at a room temperature with vigorous magnetic stirring.

3. Results and discussion 3.1 Photoluminescence enhancement of ZAIS particles immobilized on Au@SiO2 particulate films Figure 2a shows TEM images of Au@SiO2 particles with SiO2 shell thickness of 14 or 26 nm. Spherical Au@SiO2 particles were observed without formation of large aggregates. The surface of the Au core (average diameter: 15 nm) was uniformly covered with the SiO2 layer, the thickness of which was enlarged with an increase in the amount of TMOS added. Thus-obtained Au@SiO2 particles having anionic surface charges were electrostatically layer-by-layer-deposited with a PDDA polymer layer as a cationic spacer layer on quartz substrates. As shown in Fig. 2b, dense immobilization of Au@SiO2 could be attained, regardless of the SiO2 shell thickness. The replacement of surface modifier from OLA to MES did not vary the optical properties of ZAIS(x) particles. Being in good agreement with our previous paper,37 as shown in Fig. S1, with a decrease in the x value in the precursor used, the absorption spectra of MES-ZAIS(x) were blue-shifted due to the increase in the ZnS content in ZAIS solid solution. The absorption onset (λonset) of particles was varied from 700 to 540 nm with a decrease in x from 1.0 to 0.7. Furthermore, regardless of the kind of surface modifier, ZAIS(x) particles exhibited a broad PL spectra originating from the donor-acceptor pair



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recombination of photogenerated charge carriers, in which peak wavelength was blue-shifted from ca. 800 to 630 nm with a decrease in the x value, as reported previously.37,38 The ZAIS modified with MES had negative surface charges and could be electrostatically immobilized as the topmost layer on Au@SiO2 on quartz substrates with a PDDA polymer layer. Figure 3a shows pictures of the obtained multilayer films of MES-ZAIS(0.9)/Au@SiO2 with different SiO2 shell thicknesses as well as the MES-ZAIS(0.9) monolayer immobilized on a quartz substrate. The MES-ZAIS(0.9)/Au@SiO2 films were transparent with a faded pink color due to the SPR peak of immobilized Au@SiO2 particle layers. With irradiation of UV light, each film exhibited photoluminescence of red light, but its intensity was remarkably different depending on the kind of Au@SiO2. Figure 3b shows the extinction spectra of MES-ZAIS(0.9)/quartz and MESZAIS(0.9)/Au@SiO2 with SiO2 shell thickness of 9.6 and 21 nm. The MES-ZAIS(0.9)/quartz exhibited a broad absorption spectrum with the absorption onset of ca. 600 nm, being similar to that of MESZAIS(0.9) dissolved in the solution (Fig. S1c). This suggested that the nanoparticles could be immobilized on the substrate without considerable coalescence between particles. On the other hand, the extinction spectra of MES-ZAIS(0.9)/Au@SiO2 exhibited intense LSPR peak at 530 nm, while the broad absorption of ZAIS particles became unclear. The peak intensity of LSPR became lower for the composite films prepared with Au@SiO2 having thicker SiO2 shell, due to the decrease in the amount of Au particles immobilized from 6.8 × 10-14 to 4.1 × 10-14 mol(particle)/cm2 with a decrease in the SiO2 shell thickness from 9.6 to 21 nm (Fig. S2b). The photoluminescence spectra of the films exhibited broad emission with a peak wavelength at around 680 nm, as shown in Fig. 3c, being similar to that of MES-ZAIS(0.9) particles dissolved in the solution (Fig. S1d). The PL intensity was larger for ZAIS particles immobilized on Au@SiO2 films with SiO2 shell thickness of 21 nm rather than for the MESZAIS(0.9) film immobilized on a quartz substrate, while ZAIS particles on the particle layer of Au@SiO2 (shell thickness of 9.6 nm) exhibited the weaker PL intensity than that of ZAIS film only. The amount of ZAIS particles in MES-ZAIS(0.9)/quartz was determined to ca. 1.3 × 10-12 mol(particle)/cm2 from the absorption spectra of the substrates by using the absorption coefficient of 10 Environment ACS Paragon Plus

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ZAIS(0.9) particles of 2.2× 106 dm3 mol(particle)-1 cm-1 at 300 nm. Furthermore the density of immobilized ZAIS particles was not varied even if Au@SiO2 particle films were used as substrates regardless of the SiO2 shell thickness (Fig. S2a). These results indicated that the change in PL intensity of MES-ZAIS(0.9)/Au@SiO2 was not due to the variation of the amount of ZAIS particles immobilized but due to the change in interparticle distance between Au and ZAIS. It is interesting to clarify the dependence of PL intensity of ZAIS on interparticle spacing between Au and ZAIS. Figure 4 shows the relationship between the enhancement factor of PL and thickness of the SiO2 shell on the Au core. The enhancement factor (fenhance) was calculated as the ratio of PL intensity of ZAIS on Au@SiO2 to that of only the ZAIS particles immobilized on the quartz substrate (MESZAIS(0.9)/SiO2). Since the surface-modifier layer of MES on ZAIS was very thin (ca. 0.5 nm), it is reasonable to assume that the distance between the Au core and ZAIS particles was roughly equal to the thickness of the SiO2 shell layer. The value of fenhance increased with an increase in SiO2 shell thickness and then became larger than unity at SiO2 thickness > 15 nm. The optimum value of fenhance (1.7) was obtained at SiO2 thickness of 21 nm. Further increase in SiO2 shell thickness resulted in a decrease in fenhance. As mentioned before, since the amount of ZAIS particles immobilized on the composite films was almost constant to 1.3 × 10-12 particles/cm2 and the density of Au@SiO2 particles immobilized simply decrease with an increase in their SiO2 shell thickness as shown in Fig. S2, the enhancement of PL intensity (Fig. 4) could not be explained by the changes in the particle number of Au or ZAIS in the composite films. Figure 5a shows the PL excitation (PLE) spectra of the composite films. The PLE spectrum of MESZAIS(0.9) nanoparticles immobilized on a quartz substrate agreed well with the absorption spectrum of ZAIS in solution, indicating that the PL simply originated from the photoexcitation of ZAIS particles in the film. On the other hand, the PLE spectrum of MES-ZAIS(0.9)/Au@SiO2 gave a similar onset wavelength at around 600 nm, but the intensity was much larger than that of the ZAIS film only at each wavelength. Figure 5b shows the wavelength dependence of fenhance in PLE spectra, calculated from the results shown in Fig. 5a. The profile of the spectrum was roughly in agreement with the shape of the 11 Environment ACS Paragon Plus


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LSPR peak of Au@SiO2 dispersed in the solution, in which the peak appeared at around 550 nm in the profile of the wavelength dependence of fenhance, being red-shifted from that of LSPR peak (530 nm) of Au@SiO2. The shift of LSPR peak was probably due to the increase in the dielectric constant surrounding the medium in the vicinity of Au particles with the immobilization of ZAIS particles on the surface of Au@SiO2 particle layer.43 Therefore it was suggested that photoexcitation of the surface plasmon of Au particles, which produced a locally enhanced electric field near the Au particles, played an important role in the increase in photoexcitation probability of ZAIS particles. The observed change in PL intensity in Fig. 4 was explained on the basis of the energetic interactions between ZAIS and Au particles. It has been reported that Förster-type energy transfer from semiconductor nanoparticles to Au particles easily occurred when semiconductor nanoparticles were located in the close vicinity of metal particles, resulting in the quenching of semiconductor particles.17,44 This was true of the case of SiO2 shell thickness smaller than ca. 15 nm; that is, photoexcited ZAIS particles were quenched by the energy transfer from photoexcited ZAIS to Au particles, that is, fenhance < 1. On the other hand, it is well known that photoexcitation of LSPR peaks of Au or Ag particles can produce a locally enhanced electric field in the proximity of metal nanoparticles, resulting in effective photoexcitation of chromophores such as organic dyes or semiconductor nanoparticles located in the LSPR-induced electric field.44 Therefore, since the probability of energy transfer drastically declined with an increase in dZAIS-Au, the photoexcitation of ZAIS caused by the LSPR-induced local electric field became remarkable, resulting in fenhance more than 1 with dZAIS-Au larger than ca. 15 nm. Furthermore, since the LSPR-induced local electric field in the vicinity of Au particles is known to decay with increase in distance from the metal surface, it is reasonable that further increase in dZAIS-Au from the optimum value of dZAIS-Au= ca. 21 nm resulted in a decrease of fenhance. The degree of PL enhancement of MES-ZAIS(x)/Au@SiO2 films varied depending on the chemical composition of ZAIS particles, even if the SiO2 shell thickness of Au@SiO2 was constant to 21 nm. Figure 6b shows the PL spectra of various kinds of MES-ZAIS(x)/Au@SiO2 films. PL peak of the composite films was blue-shifted with a decrease in the absorption onset (λonset) of MES-ZAIS(x) used 12 Environment ACS Paragon Plus

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(Fig. 6a), that is, a decrease in the x value, being similar to PL spectra of MES-ZAIS(x) in solution. The PL enhancement factor of MES-ZAIS(x) on the Au@SiO2 particle layers was obtained as a function of the x value, as shown in Fig.6c. With an increase in the x value from 0.7 to 1.0, that is, a red shift of λonset from 540 to 700 nm, fenhance monotonously increased from ca. 1.1 to 1.9. As mentioned above, it was thought that the PL enhancement of ZAIS particles on the Au@SiO2 layer originated from the increase in photoexcitation probability of ZAIS due to the LSPR-induced local electric field around Au particles. Therefore, the results shown in Fig. 6c indicated that photoexcitation of ZAIS particles with an LSPR-induced electric field became more remarkable in the case of considerable superimposition appearing between the absorption spectra of ZAIS and the LSPR peak at around 530 nm of Au particles. It should be noted that the obtained enhancement factor was relatively small in comparison to those previously reported for semiconductor nanoparticles, 3-10,18,19,45 probably because efficient energy transfer from ZAIS to Au particles and/or re-absorption of emitted photons occurred due to the overlapping between broad PL peak of ZAIS and the LSPR peak of Au particles. In order to investigate the origin of the PL enhancement, the PL lifetimes were measured for the composite films containing MES-ZAIS(x) with different x value. Figure 7 shows the representative PL decay curves for MES-ZAIS(0.9)/quartz and MES-ZAIS(0.9)/Au@SiO2 (SiO2 thickness of 21 nm). The PL decay curves were well fitted by the double-exponential decay function (eq. 1) with the fitting parameters listed in Table 1.

I(t) = A0 exp(-t / τ0) + A1 exp(-t / τ1)

--- (1)

where τ0 and τ1 represent the lifetimes of PL emission, and A0 and A1 are the amplitudes corresponding to the lifetimes. Average lifetime of PL () was calculated using the equation, = ΣAiτi2/ΣAiτi. The estimated parameters for PL decay curves of the composite films are summarized in Table 1. Regardless of the kind of ZAIS(x) used, the relatively long lifetimes (sub-microseconds) of PL were observed, being attributed to the donor-acceptor pair recombination. The average lifetime was slightly increased 13 Environment ACS Paragon Plus


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with an increase in the x value. It should be mentioned that the lifetimes of individual components and their contributions were roughly constant for each x value, regardless of the presence of Au@SiO2 particle layers, except for the case of x= 0.7. Though the larger fenhance was observed for the cases of x= 0.9 and 1.0 in Fig. 6c, the results indicated that the radiative recombination rate of photogenerated electrons and holes was not enhanced by the surface plasmon of Au particles. Considering relatively low PL quantum yields (ca.10~30%) of MES-ZAIS(x) dispersed in the solution, this was probably because non-radiative recombination processes of photogenerated charge carriers, which was not influenced by the surface plasmon, greatly controlled the PL lifetimes. Therefore we can conclude that the PL intensity was mainly enhanced by the increase in excitation probability of ZAIS particles due to the LSPR-induced electric field.

3.2 Plasmon-enhanced photocatalytic activity of ZAIS-Au nanocomposite particles The LSPR-induced electric field around Au particles could enhance the PL intensity of ZAIS immobilized in the vicinity of Au particles as aforementioned. This is probably due to modulation of the photoexcitation probability of ZAIS particles with the LSPR-induced electric field, which is also expected to increase the photocatalytic activity of ZAIS particles. Thus, we prepared nanocomposite photocatalysts by immobilization of ZAIS nanoparticles on Au@SiO2 with an almost optimum SiO2 shell thickness, ca. 18 nm. Hydrolyzing trimethoxysilyl groups of MPTS-ZAIS in the presence of Au@SiO2 resulted in surface coating of ZAIS with a thin SiO2 layer and then the immobilization of ZAIS on Au@SiO2 particles via Si-O-Si bond (ZAIS@SiO2/Au@SiO2 particles). Figure 8a shows TEM images of thus-obtained nanocomposite particles. By comparing TEM images of Au@SiO2 only (Fig. 2a), it was found that the nanoparticles of ZAIS were densely immobilized on the surface of Au@SiO2. The size of the Au core and the SiO2 shell thickness seemed to be unchanged after immobilization of ZAIS(0.8)@SiO2 particles. Figure 8b shows diffuse reflectance spectra of ZAIS(0.8)@SiO2/Au@SiO2 particles with different ratios of Au to ZAIS particles. With an increase in the content of Au particles in the composite particles, the 14 Environment ACS Paragon Plus

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LSPR peak of Au particles at 500~600 nm became remarkable. As shown in the inset of Fig. 8b, by subtracting the spectrum i from the spectrum iv, the LSPR peak of Au in the composite photocatalysts was clearly recognized at the wavelength of 550-560 nm. It should be noted that the LSPR peak of ZAIS(0.8)@SiO2/Au@SiO2 particles was slightly red-shifted from that of Au@SiO2 dispersed in a solution, 530 nm. This indicated that the surface of Au@SiO2 was coated by ZAIS particles, resulting in an increase in the dielectric constant surrounding the medium in the vicinity of Au particles to induce the red shift of the LSPR peak.43 Figure 9a shows the time course of H2 evolution by irradiation to suspensions of ZAIS(0.8)@SiO2/Au@SiO2 having an SiO2 shell layer of 18 nm in thickness. For comparison, results for ZAIS(0.8)@SiO2 particles are also shown. Though a small induction period was observed below 30 min of irradiation, an almost linear increase in the amount of H2 was observed with prolonged irradiation, regardless of the kind of photocatalysts, indicating that ZAIS particles worked as a stable photocatalyst under the experimental conditions used in this study. However, the H2 evolution rate (R(H2)), calculated from the slope of the linear portion in time-course curves, became larger in the case of using ZAIS(0.8)@SiO2/Au@SiO2 than that in the case of ZAIS(0.8)@SiO2 only. The thick SiO2 layer (18 nm in thickness) was uniformly coated on the Au core particle as shown in TEM images (Fig. 8a), and then it was not expected that the direct electron transfer occurred from the photoexcited ZAIS particles to the Au core; that is, the Au core particles could not act as a co-catalyst for H2 evolution. Furthermore, it should be noted that the simple particle mixture of ZAIS(0.8)@SiO2 and Au@SiO2 did not cause any enhancement or retardation of H2 evolution rate (not shown), indicating that enhancement of photocatalytic activity required the immobilization of ZAIS photocatalysts at an appropriate distance from Au particles. Action spectra of photocatalytic H2 evolution are shown in Fig. 9b. In both cases, H2 evolution was observed with irradiation of monochromatic light with a wavelength less than ca. 550 nm, and the apparent quantum yield (Φapp) increased with a decrease in the wavelength of irradiation light. ZAIS particles prepared at x= 0.8 had an absorption onset at around 570 nm, being in agreement with the 15 Environment ACS Paragon Plus


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onset wavelength of photocatalytic reaction. This result indicated that the photoexcitation of ZAIS particles via a single-photon process induced the H2 evolution and then no reaction proceeded even if the LSPR peak of Au particles was photoexcited by photons having energy lower than the energy gap of the ZAIS particles used. Figure 9c shows the wavelength-dependent enhancement factor (fenhance) of the photocatalytic reaction, which was calculated as the ratio of Φapp of ZAIS(0.8)@SiO2/Au@SiO2 to that of ZAIS(0.8)@SiO2. The spectral profile of fenhance roughly corresponded to the LSPR peak observed in the extinction spectrum of Au@SiO2 in the composite films (the inset of Fig. 8b), being similar to the case of PL enhancement as shown in Fig. 5b. These results indicated that photoexcitation of the LSPR peak of Au@SiO2 played an important role in the enhancement of R(H2) of ZAIS particles. It should be noted that irradiation of monochromatic light with a wavelength less than 500 nm caused little enhancement of the photocatalytic hydrogen evolution, being in good agreement with the fact that the intensity of the LSPR peak became considerably small in the region of wavelengths less than ca. 500 nm.43 It is also interesting to investigate the composition-dependent photocatalytic activity of ZAIS(x) nanoparticles. Figure 10 shows R(H2) of ZAIS(x)@SiO2/Au@SiO2 composites and ZAIS(x)@SiO2 nanoparticles having various x values as a function of the absorption onset wavelength of ZAIS(x) used. The thickness of the SiO2 shell on the Au core was constant at 18 nm. Volcano-type dependence of R(H2) was observed regardless of the presence of Au particles, that is, with a blue shift in λonset from 700 to 540 nm (with a decrease in x of ZAIS particles from 1.0 to 0.7), R(H2) was enlarged, though further blue shift of λonset from 540 nm reduced the value of R(H2). Similar behavior was observed for the photocatalytic H2 evolution with use of bulk ZAIS particles as reported in our previous paper.46 The dependency of the H2 evolution reaction on the composition would mainly be due to the change in the band structure of ZAIS particles. The photocatalytic activity was increased with a negative shift of the conduction band edge as the content of ZnS in ZAIS particles increased (that is, the x value decreased). However, the photocatalytic activity for H2 evolution was lessened as λonset became smaller than 540 nm


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(the value of x being smaller than 0.7) because the number of photons having energy larger than the energy gap of the ZAIS(x) particles decreased in the irradiation light from the Xe lamp. The immobilization of ZAIS particles on Au particles increased R(H2), especially for the cases using ZAIS(x) particles with λonset larger than 570 nm, that is, particles with x larger than 0.8. The enhancement factor for R(H2) was calculated for each kind of ZAIS(x) particles. Figure 10b shows the fenhance value plotted as a function of λonset of ZAIS particles used. The value of fenhance monotonously increased with the red shift of λonset of ZAIS particles and then exceeded unity with λonset longer than 570 nm. Optimum fenhance was ca. 2.0 with use of ZAIS particles having x= 1.0. This behavior agreed well with that obtained for PL enhancement (Fig. 6c). As aforementioned in the enhancement of PL intensity of ZAIS, it was indicated that the absorption properties of ZAIS particles needed to effectively overlap with the LSPR peak of Au particles to obtain a larger value of fenhance. Since the LSPR peak appeared at ca. 550-560 nm for ZAIS(x)@SiO2/Au@SiO2 as shown in Fig. 8b, it was reasonable that the ZAIS(x) particles with x larger than 0.8, of which λonset was longer than ca. 570 nm, were effectively photoexcited with the LSPR-induced electric field, the degree being enlarged with an increase in the value of x. It should be noted that the composite particles of ZAIS(0.5)@SiO2/Au@SiO2 exhibited lower photocatalytic activity than that of the corresponding ZAIS(0.5)@SiO2 only. The λonset of ZAIS particles was ca. 500 nm and then the small overlapping between the absorption spectrum of ZAIS and the LSPR peak of Au caused little excitation of ZAIS(0.5) with LSPR-induced electric field, while the addition of Au@SiO2 to the reaction system increased the degree of light scattering of the composite photocatalysts, resulting in decrease in the number of photons absorbed in ZAIS particles. It is noteworthy that the maximum fenhance of ca. 2.0 obtained for ZAIS(1.0) in the present study was larger than that previously reported for the case of CdS-Au nanocomposite photocatalysts,27 in which the immobilization of CdS nanoparticles on Au@SiO2 caused ca. 1.5 times higher R(H2) than that obtained for CdS only. Since λonset of CdS particles, ca. 520 nm, was much shorter than that of ZAIS(1.0), the lower degree of overlapping between the absorption spectrum of CdS and the LSPR peak of Au resulted in the smaller enhancement factor than that observed for ZAIS(1.0) in the present study. 17 Environment ACS Paragon Plus


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Consequently, we can conclude that the photocatalytic activity of semiconductor nanoparticles was effectively enhanced when their absorption property was more remarkably overlapped with the LSPR peak of Au particles to excite semiconductor nanoparticles with LSPR-induced local electric field.

4. Conclusion We have successfully demonstrated that the photochemical properties of nanoparticles of a ZnSAgInS2 solid solution (ZAIS) could be modulated by combination with Au nanoparticles, the degree being dependent on both the distance between ZAIS and Au nanoparticles and the chemical composition of ZAIS nanoparticles. The PL intensity was most effectively enhanced at the optimum distance (ca. 21 nm) between ZAIS and Au particles, due to the enhancement of photoexcitation of ZAIS particles by the LSPR-induced electric field. We have also found that ZAIS nanoparticles worked as a visible-light-driven photocatalyst and that their photocatalytic H2 evolution rate was enhanced by immobilization of ZAIS particles at the appropriate distance from Au core particles. The plasmonic enhancement of photocatalytic activity was tuned by the precise control of the overlapping between the absorption properties of semiconductor nanoparticles and the LSPR peak of nanostructured metal particles. These findings will be important for the construction of plasmon-assisted efficient energy conversion systems in the visible and near IR wavelength regions from solar light to electrical or chemical energy, such as photocatalysts and quantum dot solar cells. Studies along this line are now in progress.

ACKNOWLEDGMENTS

This work was supported by a Grant-in-Aid for Scientific Research on

Priority Areas “Strong Photon-Molecule Coupling Fields (No. 470)”, a Funding Program for Next Generation World-Leading Researchers from the Japan Society for the Promotion of Science, and Special Coordination Funds for Promoting Science and Technology from the Ministry of Education, Culture, Sports, Science and Technology of Japan.


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SUPPORTING INFORMATION AVAILABLE Absorption and PL spectra of OLA-ZAIS(x) and MES-ZAIS(x) particles dissolved in solutions are shown in Fig. S1. The amounts of Au@SiO2 particles and ZAIS particles immobilized on substrates are shown in Fig. S2. This information is available free of charge via the Internet at http://pubs.acs.org.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1 Double-exponential fitting results for PL decay profiles of MES-ZAIS(x)/quartz and MESZAIS(x)/Au@SiO2 (SiO2 shell: 21 nm) films containing ZAIS(x) particles with different x values. x value

sample

τ0 / μs

τ1 / μs

A0

A1

/ μs

0.7

ZAIS(a)

0.029

0.276

0.54

0.037

0.344

0.7

ZAIS+Au(b)

0.169

0.675

0.08

0.031

0.476

0.9

ZAIS(a)

0.179

0.787

0.088

0.056

0.627

0.9

ZAIS+Au(b)

0.174

0.748

0.094

0.060

0.595

1.0

ZAIS(a)

0.132

0.865

0.067

0.021

0.625

1.0

ZAIS+Au(b)

0.186

0.882

0.078

0.031

0.641

(a) MES-ZAIS(x)/quartz films. (b) MES-ZAIS(x)/Au@SiO2 films.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figures

(a) Au@SiO2

PDDA quartz

PDDA

Au@SiO2

MES‐ZAIS

Au@SiO2 PDDA MES‐ZAIS

MES‐ZAIS(x)/Au@SiO2

(b)

SiO2

SiO2 Au

ZAIS MPTS‐ZAIS

Au

hydrolysis

Au@SiO2

ZAIS(x)@SiO2/Au@SiO2

Fig. 1. Schematic illustrations of (a) layer-by-layer immobilization of MES-ZAIS and Au@SiO2 particles on quartz substrates and (b) preparation of a plasmonic photocatalyst composed of ZAIS and Au@SiO2 particles.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)


SiO2 shell thickness 14 nm 26 nm

50 nm

(b)

50 nm

SiO2 shell thickness 14 nm 26 nm 80 nm 80.00

0 nm

300 nm

300 nm

Fig. 2. (a) TEM images of Au@SiO2 particles with different SiO2 shell thicknesses. (b) AFM images of layer-by-layer-deposited Au@SiO2 with one deposition cycle.



(a) Under room light i

ii

iii

i

ii

iii

Under UV

quartz

Au@SiO2

0.04

(b) Absorbance

0.03

ii

0.02

iii

0.01

x2 i 0

300

400

500

600

700

800

Wavelength / nm PL Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 34

(c)

iii i ii 500

600

700

800

900

Wavelength / nm Fig. 3. (a) Pictures of ZAIS-immobilized substrates under room light or UV light (360 nm). (b) Absorption and (c) PL spectra of ZAIS-immobilized substrates. The samples used were MESZAIS(0.9)/quartz (i) and MES-ZAIS(0.9)/Au@SiO2 with SiO2 shell thicknesses of 9.6 (ii) and 21 nm (iii). PL spectra were measured with excitation light of wavelength at 540 nm. 26 Environment ACS Paragon Plus

Page 27 of 34

2 1.5

fenhance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 0.5 0 0

5

10

15

20

25

30

35

SiO2 shell thickness / nm

Fig. 4. Relationship between fenhance for PL intensity of MES-ZAIS(0.9) in the composite films and thickness of the SiO2 shell on the Au core.



PL Intensity / a.u.

(a)

Absorbance / a.u.

350 400 450 500 550 600 650 700

Wavelength / nm 4

(b) fenhance

3 2 1 0

400

450

500

550

600

Extinction / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 34

650

Wave length / nm

Fig. 5. (a) PL excitation spectra of MES-ZAIS(0.9)/quartz (open circles) and MES-ZAIS(0.9)/Au@SiO2 with SiO2 shell thickness of 21 nm (solid circles). The absorption spectrum of MES-ZAIS(0.9) used is also shown (solid line). (b) Wavelength dependence of fenhance for PL intensity (solid circles). The extinction spectrum of Au@SiO2 dispersed in water is also shown (solid line).


(a)

0.9

1.0

0.7 0.8

400

500

600

700

800

PL Intensity (normalized)

Wavelength / nm 0.7

0.8 0.9

600

1.0

700

(b)

800

900

Wavelength / nm 2.2 1.0

(c)

0.9

1.8

fenhance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Absorbance (normalized)

Page 29 of 34

0.8

1.4

0.7 1 0.6 450

500

550



600

onset

650

700

/ nm

Fig. 6. (a) Absorption of original MES-ZAIS(x) particles dissolved in water. An arrow indicates the LSPR peak wavelength of Au@SiO2 used. (b) PL spectra of MES-ZAIS(x)/Au@SiO2 films with SiO2 shell thickness of 21 nm. (c) The degree of PL enhancement of MES-ZAIS(x)/Au@SiO2 films as a function of λonset of MES-ZAIS(x) particles used. The values of x are indicated in the figures. PL spectra were measured with excitation light of wavelength at 540 nm.


(a)

10103 3

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D

10102 2

10101 1

0

10

0

0

1

1

2

2

3

3

Time / s

4

4

5

5

Time / μs Emission Intensity Emission intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Emission Intensity Emission intensity / a.u.


(b) C

10103

3

10102

2

10101

1

0

10

0

0

1

1

2

3

2 Time / 3s

4

4

5

5

Time / μs

Fig. 7. PL decay profiles of (a) MES-ZAIS(0.9)/quartz and (b) MES-ZAIS(0.9)/Au@SiO2 (SiO2 shell: 21 nm) films. The experimental decay curves are fitted by double-exponential decay curves (red lines) with the parameters listed in Table 1.


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(a)

ZAIS

Au core

50 nm

20 nm

SiO2 shell

(b) K.M Function / a.u.

K-M Function (normalized)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


iv

400 500 600 700 800

Wavelength / nm

iii ii i 400

500

600

700

800

Wavelength / nm

Fig. 8. (a) TEM images of ZAIS(0.8)@SiO2/Au@SiO2 particles. (b) Diffuse reflectance spectra of ZAIS(0.8)@SiO2/Au@SiO2 particles with ratios of Au to ZAIS particles of 0 (i), 0.52 × 10-4 (ii), 1.0 × 10-4 (iii), and 3.1 × 10-4 (iv). (inset in panel b) The extinction spectrum of Au@SiO2 in the composite photocatalysts obtained by subtracting the spectrum i from iv.


2

Page 32 of 34

(a)

30 20 10 0

0

100

200

300

400

Irradiation time / min 0.2

0.1



APP

/%

(b)

0 400

450

500

550

600

Absorbance / a.u

650

Wavelength / nm (c)

2

fenhance

1.5 1 0.5 0

Extinction / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Amount of H / mol


400 450 500 550 600 650 700

Wavelength / nm

Fig. 9. (a) Time courses of H2 evolution by photocatalysts of ZAIS(0.8)@SiO2 (open circles) and ZAIS(0.8)@SiO2/Au@SiO2 particles (solid circles). The SiO2 layer on Au core was 18 nm in thickness. The ratio of Au to ZAIS particle was 5.2 × 10-5. The experiments were performed by irradiation of light from a 300-W Xe lamp (λ > 350 nm). (b) Action spectra of photocatalytic H2 evolution of ZAIS(0.8)@SiO2 (open circles) and ZAIS(0.8)@SiO2/Au@SiO2 particles (solid circles). (c) Wavelength dependence of fenhance obtained from the results shown in panel b (solid circles). The extinction spectrum of Au@SiO2 contained in the composite particles (the inset of Fig. 8b) is also indicated as a solid line. 32 Environment ACS Paragon Plus

Page 33 of 34


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



10

R(H2) / mol h-1

(a)

0.7 0.6 0.5

1

0.8

0.9

0.1

0.01 450

500

550

600

1.0

650

700

onset / nm

(b)

1.0

0.9

2 0.8

fenhence

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 34

0.6

1 0.7

0.5

0 450

500

550

600

650

700

onset / nm

Fig. 10. (a) Photocatalytic H2 evolution rate with use of ZAIS(x)@SiO2 (open circles) and ZAIS(x)@SiO2/Au@SiO2 particles (solid circles) as a function of λonset of ZAIS particles used. (b) Relationship between fenhance for photocatalytic activity of ZAIS(x)@SiO2/Au@SiO2 particles and λonset of the ZAIS(x) particles used. The experiments were performed by irradiation of light from a 300-W Xe lamp (λ > 350 nm). The values of x are indicated in the figures. The ratio of Au to ZAIS(x) particle was 5.2 × 10-5.


Plasmon-Enhanced Photoluminescence and Photocatalytic Activities

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