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Carrier-selective Blocking Layer Synergistically Improves the Plasmonic Enhancement Effect Tokuhisa Kawawaki, Tatsuo Nakagawa, Masanori Sakamoto, and Toshiharu Teranishi J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 05 May 2019 Downloaded from http://pubs.acs.org on May 5, 2019

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Journal of the American Chemical Society

Carrier-selective Blocking Layer Synergistically Improves the Plasmonic Enhancement Effect Tokuhisa Kawawaki,† Tatsuo Nakagawa,‡ Masanori Sakamoto,*,† Toshiharu Teranishi*,† †Institute for Chemical Research, Kyoto University, Gokasho, Uji 611-0011, Japan ‡Optical Instruments Division, Unisoku Co., Ltd. Kasugano 2-4-3, Hirakata, Osaka 573-0131, Japan. Supporting Information Placeholder ABSTRACT: Plasmonic enhancement is a versatile and convenient way to enhance the conversion efficiency of various photo-energy conversion systems, such as photocatalysts and solar cells. We refine a plasmonic enhancement system by focusing on a carrier blocking layer (between a plasmonic metal and a photoactive layer), which is commonly used to prevent a major quenching channel in a plasmonic enhancement system. The hydrogen evolution reaction (HER) activity is enhanced by 33 times from the introduction of a carrier-selective blocking layer (CSBL) in Ag-CdS nanoparticles. The Ag2S layer, a typical example of a CSBL, synergistically improves the plasmonic enhancement effect of Ag on the photocatalytic HER activity of CdS by both the selective blocking of photo-excited electrons and the effective transfer of holes, which extends the lifetime of the active species (electrons in the conduction band) in the semiconductor photocatalyst (CdS) to accelerate the photocatalytic HER. We propose a new strategy for a further improvement of plasmonic enhancement systems.

Recently, the plasmonic enhancement effect has attracted much attention in the field of photo-energy conversion.1-14 Integration of plasmonic materials in solar energy conversion systems is a versatile and convenient way to enhance the energy conversion efficiency.1-3, 15-21 The use of plasmonic materials is among the limited number of ways to improve the intrinsic performance of energy conversion systems by directly enhancing their intrinsic natures. Therefore, the plasmonic enhancement effect has been widely applied in the research field of photocatalysts. In particular, improvement of the efficiency of the photo-induced hydrogen evolution reaction (HER) using photocatalysts is a typical

application of the plasmonic enhancement.3, 10, 15-18, 21 The enhancement is based on strong photon absorption by the localized surface plasmon resonance (LSPR) of plasmonic materials. Collected photons by plasmonic materials are converted to a strong oscillating electric field (i.e., optical near-field) and far-field scattering light.5-6, 22-23 The strong optical electric field (i.e., exciton-plasmon coupling) promotes a charge separation efficiency in a photocatalyst (Figure 1a).24-26 In addition, the far-field scattering light increases the effective light path length and enhances light absorption by a photocatalyst.22, 27 How the plasmonic enhancement works is based on a balance between the positive effects (i.e., optical near-field, far-field scattering light) and the negative effects (i.e., unfavorable carrier transfer to plasmonic metal materials; Figure 1b). The drained carrier in the plasmonic materials cannot contribute to the photoenergy conversion.28-29 Therefore, suppression of carrier transfer to the plasmonic materials is a key issue in the plasmonic enhancement effect. Introduction of an insulator layer, such as SiO2,1, 17, 30 Al2O331 and polymers,32 on plasmonic materials has been a typical method to suppress this negative effect on plasmonic enhancement in the last decade. Although the above-mentioned insulator layers suppress both electron and hole transfer (Figure 1c), the necessary carriers should remain in plasmon enhanced photocatalysts, and unnecessary carriers should transfer to the blocking layer to enhance the spatial charge separation lifetime, which improves the photocatalytic activities through the prevention of carrier recombination.33-37 For further improvement of the plasmonic enhancement effect, we propose a carrier-selective blocking layer (CSBL) as a new type of carrier blocking layer. A typical CSBL suppresses only the necessary carrier transfer to plasmonic materials and accepts the unnecessary carriers for spatial charge separation (Figure 1d).

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Figure 1. Schematic illustration of (a) a mechanism of carrier excitation (black arrow) in a photoactive layer (semiconductor, yellow box) by plasmonic NP (metal, red sphere) based on the plasmonic enhancement effect. Carrier loss processes (b) by the back-carrier transfer from semiconductor to plasmonic NP (blue arrow) at the metal/semiconductor interface and (c) by the carrier recombination in semiconductor (blue arrow) in a conventional metal/insulator (grey box)/semiconductor system. (d) Efficient charge separation (red arrow) in the present metal/carrier-selective blocking layer (CSBL, green box)/semiconductor system. red shifted (see Figure S9). Therefore, this strong absorption in the visible region originated from LSPR of Ag metal. From these results, we concluded that the metallization of the Ag2S tip to Ag metal led to the formation of Ag-Ag2S-CdS NPs.

Figure 2. Schematic illustration of the photo-induced hydrogen evolution reaction (HER) mechanism in Ag-Ag2S-CdS (metal/CSBL/semiconductor) NPs. To demonstrate the effect of CSBL on the plasmonic enhancement system, we synthesized the Ag-Ag2S-CdS (plasmonic material/CSBL/photoactive layer) nanoparticles (NPs). The Ag2S layer, which selectively blocks the electron transfer from the conduction band (CB) of CdS and withdraws the holes in CdS to its valence band (VB), worked as a CSBL for the plasmonic enhancement of photo-induced HER of CdS (Figure 2). Timeresolved transient absorption (TA) spectroscopy revealed the surprisingly long-lived electron in the CB of the CdS phase of AgAg2S-CdS NPs in comparison with that of CdS NPs. The prolonged charge separation lifetime by the introduction of CSBL achieved further enhancement of photocatalytic HER activity of Ag-Ag2SCdS NPs. Control over the carrier transportation in heterostructured materials including plasmonic metal leads to the further improvement of the energy conversion efficiencies of photoactive devices. The Ag-Ag2S-CdS NPs were synthesized by a two-step fabrication method. First, the Ag2S-CdS NPs (Figure 3b) were synthesized by a seeded growth method from the Ag2S seed NPs (Figure 3a and S1, diameter = 8.4  1.1 nm). The transmission electron microscopy (TEM) images showed that a Ag2S-CdS NP was composed of rodlike CdS (34.2 ± 9.7 × 7.6 ± 1.4 nm, Figure S2) with a Ag2S tip. Their X-ray diffraction (XRD) pattern assigned the two crystal phases to monoclinic Ag2S and hexagonal CdS. Next, the Ag-Ag2S-CdS NPs were synthesized by a chemical extraction method from the Ag2S-CdS NPs. The shape of the CdS phases in the Ag-Ag2S-CdS NPs was maintained after the extraction of S2– ions from the Ag2S-CdS NPs (Figure 3c and S3). The different contrast in the Ag2S tips from the TEM images indicated the partial conversion of Ag2S into Ag metal (see Figure S4). Approximately 92% of the synthesized NPs had the Ag2S layers with a thickness of 3.2 ± 1.6 nm. Owing to the lattice mismatch of Ag2S-CdS interface being smaller than that of the Ag-CdS interface, Ag metal was thought to be generated at the opposite side of CdS. Excess conversion from Ag2S to Ag metal cause desorption of Ag-Ag2S tip from CdS NPs (see Figure S5). From the XRD pattern of AgAg2S-CdS NPs, the peak assigned to cubic silver appeared at approximately 38 degrees (Figure 3d). Furthermore, the absorption peak of the Ag-Ag2S-CdS NPs at 410 nm (Figure 3e) increased when the amount of TOP increased from 1:12 to 1:90 (molar ratio of Ag:TOP, see Figure S8). As the refractive index of the surrounding solvent increases, the peak wavelength was linearly

Figure 3. TEM images of (a) Ag2S seed NPs, (b) Ag2S-CdS NPs, and (c) Ag-Ag2S-CdS NPs. (d) XRD patterns and (e) UV-Vis absorption spectra of Ag2S seed NPs, Ag2S-CdS NPs, and AgAg2S-CdS NPs Next, transient absorption (TA) measurements were carried out to clarify the photo-induced carrier dynamics of the CdS, Ag-CdS, and Ag-Ag2S-CdS NPs.38 Figure 4a and c show the TA spectra of the CdS NPs and the Ag-Ag2S-CdS NPs upon excitation of the CdS phases by a 460-nm laser pulse. The TA spectra of the CdS NPs and the Ag-Ag2S-CdS NPs showed a bleaching feature owing to the state filling of CdS (Figure 4b). The recovery rate of the bleaching feature of CdS NPs (kr) was well-fitted to a double exponential growth function and was estimated to be 8.8 ± 0.6 and 96 ± 7 ns. The fast and slow components (kr1 and kr2, respectively) should correspond to the trapped-hole mediated charge recombination following the hole trapping and/or Auger mediated hole trapping (Figure 4b).39-41 The recovery rate (kr) of the bleaching feature of CdS was decreased by attaching Ag NPs on CdS NPs owing to the electron transfer from the CB of CdS to the Ag phases (see Figure S11). The kinetic profile of the bleaching feature of Ag-Ag2S-CdS NPs is shown in Figure 4d. The kinetic profile was fitted well to the triple exponential growth function and kr values were estimated to be 5.9 ± 0.3, 68 ± 3, and 845 ± 40 ns. The decrease of kr1 and kr2 may reflect the different surface conditions (see the S. I. for details). The slowest recovery component, which is only observed when introducing the CSBL layer, should be a charge recombination between CdS and Ag2S, as discussed in Figures 1d and 2. Contrary to the CdS NPs, the kr values for Ag-Ag2S-CdS NPs were dramatically prolonged. These results clearly demonstrated that the selective blocking of electron transfer from CdS to Ag phases by a Ag2S layer dramatically enhanced the electron lifetime in the CB of CdS, which is favorable for photo-induced HER.


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Figure 4. (a) TA spectra of CdS NPs upon 460-nm laser excitation and (b) kinetic profile at 510 nm. (c) TA spectra of Ag-Ag2S-CdS NPs upon 460-nm laser excitation and (d) kinetic profile at 510 nm. The red lines show the best fit. The photocatalytic activities of three kinds of NPs for HER were then investigated. The amount of evolved hydrogen by the water reduction reaction was measured by custom-built gas chromatography system. Figure 5a shows the amount of evolved hydrogen versus light irradiation time. The Ag NPs and Ag2S NPs exhibited no HER activities. In the case of the CdS NPs, the amount of evolved hydrogen maintained until 1 h of light irradiation, but then gradually saturated after 1 h (Figure 5a, orange square). Such a saturation in the photocatalytic activity was caused by the selfdissolution of CdS owing to accumulated holes. However, in the case of the Ag-Ag2S-CdS NPs, the photocatalytic activity was maintained without deterioration even after 12 h of light irradiation (Figure 5a, blue sphere, Figure S13), because the extraction of holes from CdS to Ag2S prevented the self-dissolution of CdS. In fact, the shape of the Ag-Ag2S-CdS NPs was almost maintained after 5 h of light irradiation (Figure S6). The hydrogen evolution rate of the Ag-Ag2S-CdS NPs under light irradiation at 400 nm was improved approximately 33 times higher than that of the Ag-CdS NPs. In the case of Ag-CdS NPs, as the electrons in the CB of CdS were rapidly transferred to the Ag metal phase, their HER activity was quite low in comparison with the CdS NPs. The enhancement of the photocatalytic activity of HER by introduction of the CSBL was remarkable. The TA measurement indicated that the electrons in the CB of CdS react with water to produce hydrogen in the AgAg2S-CdS NPs and the holes transferred from the CdS to the Ag2S layers react with sacrificial reagents (Figure 2). Therefore, we concluded that the spatial hole transfer from the CdS to the Ag2S CSBL is responsible for the enhancement of the photocatalytic activity of the Ag-Ag2S-CdS NPs. To clarify the contribution of the plasmonic enhancement effect on the increase of the hydrogen evolution rate, the wavelength dependence of the hydrogen evolution rate was investigated. Under irradiation of monochromatic light from 350 to 500 nm, the hydrogen evolution rates of the Ag-Ag2S-CdS NPs and the CdS NPs were measured. An enhancement factor of the hydrogen evolution rate (EF H2 evolution) was calculated by dividing the hydrogen evolution rate of Ag-Ag2S-CdS NPs by that of CdS NPs (see the S. I. for details). An enhancement factor of the absorbance (EF absorbance) was also calculated by dividing the absorbance of Ag-Ag2S-CdS NPs by that of CdS NPs (see the S. I. for details). Wavelength dependence of the EF H2 evolution (red symbol) and

the EF absorbance (blue line) is shown in Figure 5b. The maximum values of the EF H2 evolution and the EF absorbance were obtained at around 400 nm, which was coincident with the LSPR peak of Ag. These unique characteristics of wavelength dependence of the enhancement strongly suggested that the photocatalytic activity of the CdS phases was increased by the plasmonic enhancement effect of the Ag phases in the Ag-Ag2S-CdS NPs. Therefore, it was concluded that the enhancement of the photocatalytic activity of the Ag-Ag2S-CdS NPs as compared with the CdS NPs is caused by not only spatial hole transfer but also a plasmonic enhancement effect. The HER activity depending on the thickness of Ag2S layer clearly indicated that the optimized structure could further improve the activity (see the S.I.).A synergistic effect of long-lived charge separation and plasmonic enhancement on the photocatalytic HER significantly increased the performance of CdS photocatalysts in Ag-Ag2S-CdS NPs. In conclusion, we have demonstrated that a CSBL synergistically improved the plasmonic enhancement effect on the photocatalytic activity of HER. The TA measurement revealed that the selective carrier blocking by CSBL added a new positive effect on the plasmonic enhancement system: the prolongation of carrier (CB electrons) lifetime in semiconductor photocatalysts (CdS). The present work proposes a new strategy for further improvement of a plasmonic enhancement system, which will impact the various photo-energy conversion systems, including photocatalytic reaction and solar cells.

Figure 5. (a) Time-dependent photocatalytic HER activities of the Ag-Ag2S-CdS NPs, Ag-CdS NPs and CdS NPs in aqueous solution (OD:0.2 at 300 nm) containing with 0.1 M Na2SO3 under white light (λ > 350 nm, 1000 mW cm-2). (b) Wavelength dependence of enhancement factor for the hydrogen evolution (EF H2 evolution) and the absorbance (EF absorbance) of Ag-Ag2S-CdS NPs.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental details, TEM images, XRD measurement, UV-vis measurement, Transient absorption measurement, calculation methods of enhancement factor

AUTHOR INFORMATION Corresponding Author *[email protected], *[email protected]

ORCID Tokuhisa Kawawaki: 0000-0003-3282-8964 Masanori Sakamoto: 0000-0001-5018-5590 Toshiharu Teranishi: 0000-0002-5818-8865

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT


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This work was supported by JSPS KAKENHI (Grant No. JP16H06520, Coordination Asymmetry) (T.T.) and (Grant No. JP17H05257, Photosynergetics) (M.S.) and a JSPS Research Fellow (Grant No. 16J04179) (T. K.) and JST SENTAN (T. N.).

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