Pt

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Letter pubs.acs.org/JPCL

Two-Dimensional Au-Nanoprism/Reduced Graphene Oxide/PtNanoframe as Plasmonic Photocatalysts with Multiplasmon Modes Boosting Hot Electron Transfer for Hydrogen Generation Zaizhu Lou, Mamoru Fujitsuka, and Tetsuro Majima* The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan S Supporting Information *

ABSTRACT: Two-dimensional Au-nanoprism/reduced graphene oxide (rGO)/Ptnanoframe was synthesized as plasmonic photocatalyt, exhibiting activity of photocatalytic hydrogen generation greater than those of Au-nanorod/rGO/Ptnanoframe and metallic plasmonic photocatalyst Pt−Au. The single-particle plasmonic photoluminescence study demonstrated that Au-nanorod has only a longitudinal plasmon resonance mode for hot electron transfer to rGO, while Aunanoprism has in-plane dipole and multipole surface plasmon resonance modes for hot electron transfer, leading to highly efficient charge separation for hydrogen generation.

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catalysts22−24 have been found, while there is no report on metal/rGO/metal plasmonic photocatalysts. It is known that nonplasmonic metals generally exhibit high catalytic activity, while strong plasmonic metals show low efficiency in catalysis.25 When rGO is composited with plasmonic metal as light antenna and nonplasmonic metal as catalytic sites, hot electrons generated on plasmonic metals transfer to catalytic metals through rGO, leading to hydrogen generation. Among various metals, two-dimensional Au TNP with strong SPR26 and Pt nanoframe (NF) with high catalytic activity27 are preferable for the plasmonic photocatalyst Au-TNP/rGO/PtNF. In the present work, two-dimensional Au-TNP and Pt-NF synthesized by chemical growth and chemical etching, respectively, were coupled with GO to prepare two-dimensional Au-TNP/GO/Pt-NF. After GO was reduced to rGO by NaBH4, the two-dimensional structure Au-TNP/rGO/Pt-NF was obtained as plasmonic photocatalyts for hydrogen generation. Under visible and near-infrared (NIR) light irradiation, the hydrogen generation rate of Au-TNP/rGO/ Pt-NF was 2.5- and 10-fold greater than those of Au-TNP/rGO and Pt-NF/rGO, respectively. Compared to that of Au nanorod (NR) and nanosphere (NS) in Au/rGO/Pt-NF, Au-TNP/ rGO/Pt-NF exhibited 2- and 6-fold higher photocatalytic hydrogen generation, respectively. In addition, Au-TNP/rGO/ Pt-NF exhibited photocatalytic efficiency that was 1.5-fold higher than that of metallic plasmonic photocatalyst Pt-edged Au-TNP. The single-particle plasmonic PL study demonstrated that Au-NR has longitudinal SPR (LSPR) mode for hot electron transfer from Au to rGO, while Au-TNP has in-plan

lasmonic photocatalysis, based on surface plasmon resonance (SPR) of noble metal nanoparticles, is attractive for its applications in water splitting,1−3 environmental treatment,4,5 and organic photosynthesis6,7 under visible light irradiation. In most widely reported plasmonic photocatalysts composed with metal nanoparticle and semiconductor, SPRinduced hot electrons transfer from metal to semiconductor, leading to hydrogen generation.8,9 However, fast recombination of photoinduced electron−hole pairs on metal hinders hot electron transfer to the semiconductor, resulting in low efficiency of plasmonic photocatalysis.10 Recently, bimetallic plasmonic photocatalysts Pt−Au nanorods (NR) and triangular nanoprisms (TNP) have been developed to facilitate electron transfer between different metals, leading to efficient charge separation.10−12 Single-particle plasmonic photoluminescence (PL) study has demonstrated that SPR-induced hot electrons transfer from Au to other metals (Pt, Pd), leading to efficient photocatalytic hydrogen generation.10−13 Although such bimetallic plasmonic photocatalysts show quantum yield higher than those of traditional plasmonic photocatalysts composed with metal/semiconductor, charge recombination is the main decay process for SPR-induced hot electrons.10,12 How to achieve fast transfer of hot electrons and efficient charge separation is crucial for improving the efficiency of plasmonic photocatalysis. Graphene, as a two-dimensional material with unique optical and electrical properties, has been used in various fields such as optical devices,14 catalysis,15 and energy storage.16,17 Recently, reduced graphene oxide (rGO) with high conductivity has been used to improve the charge separation of photocatalysts, leading to high photocatalytic efficiency.18−20 For example, BiVO4/rGO/metal sulfides were synthesized as Z-scheme photocatalyts for full water splitting to generate hydrogen and oxygen.21 To date, various metal/rGO/semiconductor photo© XXXX American Chemical Society

Received: December 28, 2016 Accepted: February 3, 2017 Published: February 3, 2017 844

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The Journal of Physical Chemistry Letters multipolar (MSPR) and dipolar SPR (DSPR) modes for hot electron transfer, leading to highly efficient hydrogen generation. Two-dimensional Au triangular nanoprisms (TNP) with 140 nm length and 6 nm thickness (Figure S1a) were synthesized by the method reported in our previous work.10,11 Pt nanoframes (NF) (Figure S1c) were synthesized by the chemical etching of Pt-edged Au-TNP (Figure S1b) using HAuCl4 (0.5 mM) in cetyltrimethylammonium bromide (CTAB) (0.1 M) solution. Visible and NIR extinction spectra (Figure S1d) of Au-TNP, Pt-edged Au-TNP, and Pt-NF further confirmed the formation of Pt-NF. The detailed synthetic process of Pt-NF was monitored by the real-time visible−NIR extinction spectra (Figure S2). Au-TNP solution (26.4 mg L−1, 5 mL) was mixed with GO solution (0.04 mg L−1, 10 mL) under constant stirring for 30 min. Due to the electrostatic adsorption between positive charges (CTAB+) of Au-TNP and negative charge (−COO−) of GO, a composite Au-TNP/GO was formed by strong interaction between GO layers and (111) facets of Au-TNP (Figure 1a). The same process is followed for

Figure 2. Visible−NIR extinction spectra of Au-TNP, Pt-NF/GO, AuTNP/GO, and Au-TNP/GO/Pt-NF (a) and Pt-NF/rGO, AuTNP/rGO, and Au-TNP/rGO/Pt-NF (b). Hydrogen generation using Pt-NF/rGO, Au-TNP/rGO, and Au-TNP/rGO/Pt-NF as the photocatalyst under visible−NIR light irradiation (>420 nm) (c). Photocatalytic hydrogen generation rate for Au-TNP/rGO/Pt-NF, Au-TNP/rGO, Pt-edged Au-TNP, Pt-covered Au-TNP, and pure AuTNP (d).

Au-TNP and Pt-NF were loaded on the surface of GO, twodimensional Au-TNP/GO/Pt-NF showed the extinction spectrum, similar to Au-TNP/GO. When GO was reduced to be rGO, light absorption of Pt-NF/rGO became strong because of absorption of rGO (Figure 2b). Both DSPR and MSPR bands of Au-TNP loaded on rGO showed a slight blue shift, which is consistent with finite-difference time-domain simulation (Figures S4 and S5). Compared to Au-TNP/rGO, the DSPR band of Au-TNP/rGO/Pt-NF has a slight red shift attributed to the loading of Pt. To clarify the photocatalytic activities of three different structures, photocatalytic hydrogen generation over Au-TNP/rGO, Pt-NF/rGO, and Au-TNP/ rGO/Pt-NF as photocatalyst was measured in methanol/water (1:3 v/v) solution under visible−NIR light irradiation (>420 nm). The 1.53 μmol of hydrogen produced by Au-TNP/rGO/ Pt-NF for 3 h (Figure 2c) was 2.5- and 10-fold greater than that produced by Au-TNP/rGO (0.61 μmol) and Pt-NF/rGO (0.13 μmol), respectively. These results suggest that electrons for photocatalytic hydrogen generation are mainly from SPR excited Au-TNP, rather than excited rGO. The apparent quantum efficiency (AQE) of Au-TNP/rGO/Pt-NF for hydrogen generation under monochromatic light irradiation was measured and calculated as shown in Figure S7a, and the action spectra of AQE were in agreement with absorption spectra of Au-TNP/rGO/Pt-NF, demonstrating the SPR hot electrons for hydrogen generation. In Au-TNP/rGO/Pt-NF, Pt-NF collects electrons and acts as cocatalyst for hydrogen generation. Compared to the bimetallic plasmonic photocatalysts Pt−Au reported in our previous work,10 hydrogen generation rate (0.52 μmol h−1) of Au-TNP/rGO/Pt-NF was 1.5 and 5.5 folds larger than those of Pt-edged Au TNPs (0.37 μmol h−1 ) and Pt-covered Au TNPs (0.09 μmol h−1 ), respectively. Considering the actual amount of Pt−Au used in photocatalysis, the rate of Au-TNP/rGO/Pt-NF (0.12 mg) was

Figure 1. TEM images of Au-TNP/GO (a), Pt-NF/GO (b), AuTNP/GO/Pt-NF (c), and Au-TNP/rGO/Pt-NF (d). Scale bar: 200 nm.

Pt-NF/GO (Figure 1b). A mixed solution containing the same number of Pt-NF and Au-TNP particles was used to prepare composite Au-TNP/GO/Pt-NF (Figure 1c). Different nanostructures of NF and TNP make it easy to identify positions of Pt-NF and Au-TNP on the surface of GO. After GO was reduced to rGO by NaBH4 (0.1 M, 0.1 mL), the twodimensional structure Au-TNP/rGO/Pt-NF was obtained with curly layers of rGO (Figure 1d). Nanostructures Au-TNP/rGO and Pt-NF/rGO are shown in the TEM images of Figure S3. In the visible−NIR extinction spectrum of Au-TNP (Figure 2a), two distinct bands around 1190 and 730 nm were observed and were assigned to in-plane DSPR and MSPR modes, respectively.10 When Au-TNP was loaded on the surface of GO, both DSPR and MSPR bands showed a slight red shift attributed to the difference of dielectric constant between GO and water. No band was observed in the extinction spectra of Pt-NF/GO, implying no SPR absorption of Pt-NF. When both 845

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absorption of Au-NS. Compared with Au-NR and Au-NS, 2D Au-TNP with larger contact surface area has strong interaction with two-dimensional rGO to facilitate hot electron transfer from Au to rGO, leading to efficient hydrogen generation. Photocurrent and electrochemical impedance spectra (EIS) of Au-TNP/rGO and Au-NR/rGO were measured as shown in Figure S8, which further demonstrated the high charge separation in Au-TNP/rGO. Single-particle photoluminescence (PL) microscopy was used to clarify the mechanism of photocatalytic hydrogen generation in Au-TNP/rGO/Pt-NF. Using a 405 nm CW laser as the excitation light, PL images and spectra were detected by the electron-multiplying charge-coupled device (EMCCD) camera. Figure 4a shows single-particle PL images of individual

3-fold greater than that of Pt-edged Au TNPs (0.23 mg) in photocatalytic hydrogen generation. Consequently, two-dimensional Au-TNP/rGO/Pt-NF exhibits higher efficiency of photocatalytic hydrogen generation than directly contacted Pt−Au metallic plasmonic photocatalyst. Compared to traditional plasmonic photocatalysts Au/TiO2 (11.6 μmol g−1 h−1),28 Au-TNP/rGO/Pt-NF (1.0 mmol g−1 h−1) exhibited much higher activity in photocatalytic hydrogen generation. SPR of Au nanoparticles is greatly influenced by their nanostructures. To investigate the effect of Au shapes on hydrogen generation of plasmonic photocatalyst Au/rGO/PtNF, Au nanosphere (NS) with 52 nm diameter and nanorod (NR) with 83 nm length and 16 nm width were synthesized for preparation of composites Au-NS/rGO/Pt-NF and Au-NR/ rGO/Pt-NF as references, containing the same amount of Au0 as Au-TNP/rGO/Pt-NF. Structures of Au-NS/rGO/Pt-NF and Au-NR/rGO/Pt-NF are shown in TEM images of panels a and b of Figure 3, respectively. It is clearly observed that Au-NS,

Figure 4. Single-particle PL images of Au-TNP (a) and Au-TNP/rGO (c). Single-particle PL spectra of Au-TNP (b) and Au-TNP/rGO (d) corresponding to numbered points in panels a and c, respectively. Figure 3. TEM images of Au-NS/rGO/Pt-NF (a), Au-NR/rGO/PtNF (b), and Au-TNP/rGO/Pt-NF (c). Visible−NIR extinction spectra (d) and photocatalytic hydrogen generation rates (e) of AuNS/rGO/Pt-NF, Au-NR/rGO/Pt-NF, and Au-TNP/rGO/Pt-NF under visible−NIR light irradiation (>420 nm).

Au-TNP, while single-particle PL spectra of individual Au-TNP particles with the numbers in Figure 4a are shown in Figure 4b. PL spectra of five individual Au-TNP particles exhibited the similar shape with two distinct PL bands. The strong band at shorter wavelength is assigned to PL from the MSPR mode, while the weak band at longer wavelength is assigned to PL from the DSPR mode.10,11 Because of the large energy difference between the initially generated excited state and DSPR mode, the population of hot electrons transferring from the original excited state to DSPR mode decreased to result in the weak PL. When Au-TNP was loaded on the surface of rGO, the brightness of single-particle PL images of Au-TNP became dark because of the PL quenching by rGO (Figure 4c). The reduced intensity in single-particle PL spectra of Au-TNP/rGO (Figure 4d) demonstrated the occurrence of PL quenching. PL from the DSPR mode was completely quenched, while PL from the MSPR mode became very weak. Hot electrons transfer from Au to rGO, competitive with SPR modes for recombination, resulting in the quenching of SPR PL. Therefore, the quenching of DSPR and MSPR modes of AuTNP/rGO demonstrated that hot electrons transfer from AuTNP to rGO, leading to hydrogen generation as observed for Au-TNP/rGO (Figure 2c). When Pt-NF was loaded on rGO to give the composite Au-TNP/rGO/Pt-NF, single-particle PL spectra of Au-TNP were found to have slight difference from

Au-NR, and Pt-NF are dispersed on the surface of rGO. Visible−NIR extinction spectra of Au-NS/rGO/Pt-NF and AuNR/rGO/Pt-NF are shown in Figure 3d. For Au-NR, the weak band at shorter wavelengths is assigned to transverse SPR (TSPR) mode, while the strong band at longer wavelength is assigned to LSPR mode.12 Given the same amount of Au0, due to the weak polarization of the nanosphere, only one weak SPR band was observed around 550 nm for Au-NS. TEM images and visible−NIR extinction spectra of Au-NS and Au-NR are shown in Figure S6. The photocatalytic hydrogen generation rates of Au-NS/rGO/Pt-NF, Au-NR/rGO/Pt-NF, and AuTNP/rGO/Pt-NF were measured to be 0.08, 0.23, and 0.52 μmol h−1, respectively (Figure 3e). Photocatalytic activity of Au-TNP/rGO/Pt-NF was 2- and 6-fold greater than those of Au-NR/rGO/Pt-NF and Au-NS/rGO/Pt-NF, respectively. The action spectra of AQE of Au-NR/rGO/Pt-NF and Au-NS/ rGO/Pt-NF for hydrogen generation (Figure S7b,c) were in agreement with their SPR absorption spectra, demonstrating the SPR hot electrons for hydrogen generation. Low photocatalytic activity of Au-NS/rGO/Pt-NF is due to the weak SPR 846

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were calculated (Figure S9). The quenching efficiencies of PL from MSPR and DSPR modes for Au-TNP were 83% and 99%, respectively, while those from TSPR and LSPR modes for AuNR are 3% and 86%, respectively. It is clear that Au-NR has only one LSPR mode for hot electron transfer to rGO, while Au-TNP has two DSPR and MSPR modes for hot electron transfer to rGO, leading to highly efficient electron transfer. Radiative decay of plasmonic hot electrons on Au-TNP (left) and photocatalytic mechanism for hydrogen generation (right) of Au-TNP/rGO/Pt-NF are illustrated in Scheme 1. PL of AuTNP is assigned to the radiative decay of photoinduced electron−hole pairs from MSPR and DSPR modes.10,11 In the single-particle PL experiment, a 405 nm CW laser was used to excite d−sp interband transition of Au-TNP to generate hole− electron pairs. Because of the rapid interconversion between electron−hole pairs and the MSPR mode, radiative decay occurs to generate a band at shorter wavelength in the PL spectrum. Simultaneously, for electron−hole pairs, the energy transfer occurs from the MSPR to DSPR mode. Moreover, excitons of d−sp interbands transfer directly to the DSPR mode by the nonradiative energy loss, and the DSPR mode leads to emission of a photon through a radiative decay, resulting in a PL band at longer wavelength. The energy difference between the initially generated excited state and DSPR mode is greater than that to the MSPR mode. Therefore, a smaller population of electron−hole pairs transfers to the DSPR mode than to the MSPR mode, leading to a weak DSPR PL band in the singleparticle PL spectrum. After Au-TNP was combined with rGO, the radiative decay on Au-TNP was limited to generate the DSPR and MSPR PL quenching, as demonstrated by the singleparticle PL experiments. Hot electrons generated on Au-TNP transfer from Au to rGO, leading to charge separation. Quenching efficiencies of DSPR and MSPR PL were 83% and 99%, respectively, indicating that the DSPR and MSPR modes are the main PL-quenching channels. Therefore, hot electrons transfer from DSPR and MSPR modes of Au-TNP to rGO, leading to hydrogen generation. The holes left on AuTNP react with methanol. However, the main decay process of hot electrons is still charge recombination. In the photocatalytic reaction, under visible−NIR light irradiation, the hole−electron pairs are generated simultaneously on d−sp interband transition, from MSPR and DSPR modes. Hot electrons transfer from MSPR and DSPR modes of Au-TNP to rGO (Scheme 1, left), leading to hydrogen generation. When Pt-NF was loaded on rGO, hot electrons on rGO were collected on Pt-NF to decrease charge recombination, leading to highly efficient hydrogen generation by Pt-NF as cocatalyst. For AuNR, it has been demonstrated that the LSPR mode is the main

those of Au-TNP/rGO, implying that the quenching of SPR PL is mainly caused by rGO. Interestingly, Au-TNP/rGO/Pt-NF showed activity of photocatalytic hydrogen generation that was 2.5-fold higher than that of Au-TNP/rGO, demonstrating that Pt-NF collects hot electrons on rGO and acts as cocatalyst for hydrogen generation. Single-particle plasmonic PL spectra of individual Au-NR particles were also measured (Figure 5) to show two distinct

Figure 5. Single-particle PL images of Au-NR (a) and Au-NR/rGO (c). Single-particle PL spectra of Au-NR (b) and Au-NR/rGO (d) corresponding to numbered points in panels a and c, respectively.

PL bands at shorter and longer wavelengths, which are assigned to PL from TSPR and LSPR, respectively. Because of the different aspect ratio of individual Au-NR particles, the position and intensity of LSPR PL bands have some differences. When Au-NR was loaded on rGO, the brightness of the single-particle PL image has a slight change compared with pure Au-NR. The brighter points 5 and 7 are due to the aggregation of Au-NR particles. The PL quenching was clearly observed in the singleparticle PL spectra of Au-NR/rGO. Compared with pure AuNR, the intensity of the PL band from LSPR mode of Au-NR/ rGO became weak but still was observed, demonstrating the low efficiency of the hot electron transfer from Au-NR to rGO. To compare the efficiency of hot electron transfer in Au-TNP/ rGO and Au-NR/rGO, the averaged intensity and quenching efficiency of SPR PL bands of 10 Au-TNP and Au-NR particles

Scheme 1. SPR Radiative Decay of Hot Electron on Au-TNP (Left Panel) and Photocatalytic Mechanism for Hydrogen Generation (Right Panel) of Au-TNP/rGO/Pt-NF

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The Journal of Physical Chemistry Letters PL-quenching channel.12,13 Because the LSPR mode of Au-NR has the lowest energy level, hot electrons generated by TSPR and sp−d interband excitations can transfer to LSPR for radiative decay as LSPR PL. When Au-NR was loaded on rGO, hot electrons on LSPR can transfer to rGO, resulting in the quenching of LSPR PL. The quenching efficiency of LSPR PL of Au-NR was 86%, which is similar to the 83% of MSPR PL and smaller than the 99% of DSPR PL of Au-TNP. Consequently, the single-particle PL experiment demonstrates that the more efficient photocatalytic performance of Au-TNP/ rGO/Pt-NF is attributed to multiplasmon modes of Au-TNP for hot electron transfer from Au to rGO, leading to efficient hydrogen generation. In conclusion, a two-dimensional Au-TNP/rGO/Pt-NF nanostructure was synthesized by compositing Au-TNP, rGO, and Pt-NF as plasmonic photocatalysts. The photocatalytic hydrogen generation rate over Au-TNP/rGO/Pt-NF was 1.5fold greater than that of reported bimetallic plasmonic photocatalyst Pt-edged Au-TNP.10 Au-TNP/rGO/Pt-NF exhibited 6- and 2.5-fold higher activity in hydrogen generation under visible−NIR light irradiation, compared to that of AuNS/rGO/Pt-NF and Au-NR/rGO/Pt-NF, respectively. The single-particle PL study demonstrated that Au-TNP has MSPR and DSPR modes as PL-quenching channels, while Au-NR has only LSPR mode as PL-quenching channel. Therefore, Au-TNP has multiplasmon modes for hot electron transfer from Au to rGO. Moreover, Pt-NF collects hot electrons and acts as cocatalyst for hydrogen generation, leading to the high photocatalytic efficiency of Au-TNP/rGO/Pt-NF. Our work not only provides a new two-dimensional plasmonic photocatalyst but also gives a deep understanding of hot electron transfer in a plasmonic photocatalyst at the single-particle level.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b03045. Experimental section and Figures S1−S9 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mamoru Fujitsuka: 0000-0002-2336-4355 Tetsuro Majima: 0000-0003-1805-1677 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been partly supported by a Grant-in-Aid for Scientific Research (project 25220806 and others) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government. Z.L. thanks the JSPS for a Postdoctoral Fellowship for Foreign Researchers (No. P15073).

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