Controlling Shape and Plasmon Resonance of Pt-etched Au@Ag

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Controlling Shape and Plasmon Resonance of Pt-etched Au@Ag Nanorods Rongkai Ye, Yanping Zhang, Yuyu Chen, Liangfeng Tang, Qiong Wang, Qianyu Wang, Bishan Li, Xuan Zhou, Jianyu Liu, and Jianqiang Hu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00328 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on April 30, 2018

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Controlling Shape and Plasmon Resonance of Ptetched Au@Ag Nanorods Rongkai Ye,†,‡ Yanping Zhang, †,‡ Yuyu Chen,† Liangfeng Tang,† Qiong Wang,† Qianyu Wang,† Bishan Li,† Xuan Zhou,† Jianyu Liu†,* and Jianqiang Hu†,*

†

Key Laboratory of Fuel Cell Technology of Guangdong Province, Department of Chemistry,

College of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China.

‡

Rongkai Ye and Yanping Zhang equally contributed to this work.

*

E-mail: [email protected] (J. Y. Liu) or [email protected] (J. Q. Hu)

Tel: +86-020-22236670; Fax: +86-020-22236670.

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ABSTRACT

Pt-based catalysts with novel structure have been attracted great attention due to their outstanding performance. In this work, H2PtCl6 was used as both precursor and etching agent, to realize the shape-controlled synthesis of Pt-modified Au@Ag nanorods (NRs). During the synthesis, the as-prepared Ag shell played a crucial role in both protecting the Au NRs from being etched away by PtCl62- and leading to an unusual growth mode of Pt component. The sitespecified etching and/or growth depended on the concentration of H2PtCl6, where high-yield core-shell structure or dumbbell-like structure could be obtained. The shape-controlled synthesis also led to a tunable longitudinal surface plasmon resonance from ca. 649 to 900 nm. Meanwhile, the core-shell Pt-modified Au@Ag NRs showed approximately 4-fold enhancement in catalytic reduction reaction of p-nitrophenol than the Au NRs, suggesting the great potential for photocatalytic reaction.

Introduction The shape controlling of well-defined novel nanostructures has attracted extensive attention due to their shape-dependence properties in catalysis,1-3 biomedicine4-6 and optics.7-9 Comparing to monometallic nanoparticles (NPs), the fabrication of multimetallic NPs provides an efficient approach to enable innovative applications due to the synergistic effects among different


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components.10-14 Up to date, the construction of specific multimetallic NPs could be achieved by controlling either selective growth15-17 or oxidative etching.18-21 Au nanorods (NRs) is an ideal substrate for manufacturing multimetallic NPs, whose highly anisotropic shape and resulting unique surface plasmon resonance (SPR) effect make it different from the sphere one.9,22 A typical example is Pd-coated Au NRs with light response over the visible to near-infrared region, which realized plasmonic harvesting for Suzuki coupling reaction.23 Pt is also a favorite component with superb performance in catalysis,24,25 despite of its weak optical properties and high cost.8,26 From what has been discussed above, combining Pt with Au NRs could effectively improve its SPR property as well as catalytic performance. However, it is still not easy to realize a controllable growth of Pt on Au, due to the large surface energy difference (Pt 2.49 J m-2 vs. Au 1.6 J m-2) and high cohesive energy difference (Pt 5.48 eV per atom vs. Au 3.8 eV per atom).27 So far, the synthesis of Pt-modified Au NPs were mainly focused on controlling the selective growth of Pt including silver ions-assisted method16,28 and CO-block method2 by reducing Pt(II) species like K2PtCl4. For example, Liz-Marzan et al. have realized the growth of Pt on Au NRs in both tip preferential way or complete coating in the presence of Ag+ from the synthesis of Au NRs.16 However, Pt could only form pyramid-like patches on Au NRs irregularly by this way, which might not make full use of Pt component. Therefore, a new approach for controllable growth of Pt on Au is still necessary. Recently, selectively oxidative etching was widely reported as a powerful tool to manufacture specific nanostructures,18-21,29,30 while the shape-controlled synthesis of Pt-modified Au NPs are rarely


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reported. The etching process could be completed by Fe3+, H2O2, or oxygen in the presence of AA and Cu2+.31,32 For example, Guo et al. realized the selective etching of Au NRs by K2PtCl4, which formed dumbbell-like NRs with the diameter of less than 12 nm.20 H2PtCl6, on the other hand, is usually thought to be unsuitable for the growth of Pt on Au, since Au(0) in Au NRs are likely to be oxidized by Pt(IV).16 However, this etching ability might exhibit a novel way to fabricate NPs. Hence, constructing specific Pt-modified Au@Ag NRs by Pt-eching process could not only enhance the catalytic property of Pt, but also provide a new approach for shapecontrolled synthesis. Herein, we developed a facile and shape-controlled aqueous synthesis of Pt-modified Au@Ag NRs with H2PtCl6. Different from the silver ions-assisted method developed by Liz-Marzán,16 thin Ag shell was first deposited on the Au NRs before the growth of Pt. In addition, H2PtCl6 was used as both etching agent and precursor to realize the controllable growth of Pt, and two typical structures (named as Au@PtAg NRs and Pt-tip Au@Ag NRs, respectively) could be successfully obtained. The products were characterized by transmission electron microscopy (TEM), UV-vis spectrum, X-ray photoelectron spectroscopy (XPS), high-angle annular dark-filed scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray spectrum (EDS) elemental mapping as well as line scanning EDS. The growth mechanism was explained by the surface energy differences of crystalline facets in Au@Ag NRs, which is different from the case of Ag underpotential deposition (UPD). These Pt-modified Au@Ag NRs shows distinctive shifts in the absorption peaks of UV-vis spectrum due to the changes in structure. Finally, the


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Au@PtAg NRs was further examined as a potential catalyst by the reduction of 4-nitrophenol (4NP) with approximately 4-fold enhancement than Au NRs. Experimental Section Chemicals and Instruments. Chloroauric acid (HAuCl4·3H2O), 5-bromine salicylic acid (5BrSA), cetyltrimethylammonium bromide (CTAB), ascorbic acid (AA), sodium borohydride (NaBH4) and 4-nitrophenol (4-NP) were purchased from National Medicine Group Chemical Reagent. Cetyltrimethylammonium chloride (CTAC), silver nitrate (AgNO3), chloroplatinic acid (H2PtCl6·6H2O) and potassium chloroplatinate (K2PtCl4) were purchased from Aladdin. All the reagents were analytical grade and used without any further purification. All aqueous solutions were prepared with Milli-Q purified water (> 18.0 MΩ cm). All glassware was treated with aqua regia and rinsed with ultrapure water prior to use. TEM was performed with a TECNAI 10 microscope operated at 100 kV. EDS spectra, mapping and STEM were performed on a JEOL 2100F microscope operated at 200 kV. The optical absorption spectra were recorded by a Hewlett–Packard 8452 diode array spectrometer (U–3010). XPS was performed on a K-Alpha X-ray photoelectron spectroscopy. Synthesis of Au NRs. Au NRs were prepared according to the previous literature with some modifications.22 Briefly, 0.9 g of CTAB and 0.11 g of 5-BrSA were firstly dissolved in 25 mL of ultrapure water of 60 °C. Then, 2.4 mL of 4 mM AgNO3 solution was added to the mixture and kept undisturbed for 15 min when cooled down to 30 °C, after which 25 mL of 1 mM HAuCl4 solution was added and kept stirring for 15 min. Next, 200 µL of 0.064 M AA was added with


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vigorous stir until it became colorless. Finally, 80 µL of seed solution was introduced to the above solution with stir for 30 s and kept undisturbed for 12 h. The product was separated by centrifugation at 8000 rpm for 15 min and dispersed in 5 mL of ultrapure water. Synthesis of Pt-modified Au@Ag NRs. In a typical synthesis, 1 mL of the as-prepared Au NRs and 10 µL of freshly prepared 0.1 M AA were injected into 10 mL of 0.08 M CTAC in a 50-mL round-bottom flask. The mixture was stirred at 60 °C for 10 min. Then, 10 µL of 0.1 M AgNO3 was added and the solution was stirred for 15 min, after which 10 µL of 0.02 M H2PtCl6 was introduced to the solution and kept stirring for 2 min. The mixture was kept undisturbed at 60 °C for 5 h to gain a navy color. The product was separated by centrifugation at 8000 rpm for 15 min, followed by the removal of the supernatant. The final product was washed several times with ultrapure water and dispersed in 1 mL ultrapure water for further characterizations. Catalytic reduction of 4-NP. The catalytic reduction of 4-NP was conducted by a procedure similar to those reported methods.18,33,34 Both of the 4-NP and NaBH4 aqueous solutions were freshly prepared. First, 3 mL of 0.6 M ice-cold NaBH4 was mixed with 27 mL of 0.18 mM 4-NP and stirred for 30 min at room temperature. Then, 3 mL of the mixture was added into a 4-mL quartz cuvette, and the appropriate amount of the Au NRs or Pt-modified Au@Ag NRs with different concentration of H2PtCl6 was added into the cuvette to start the reducing reaction of 4NP. The reducing process of 4-NP was monitored by measuring the UV-vis absorption spectra of the solution at regular intervals. Results and Discussion


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Structural characterizations of the products. Figure 1a shows the TEM image of Au NRs synthesized with CTAB and 5-BrSA as previously reported.22 The Au NRs obtained have a narrow size distribution of 54.7 ± 2.5 nm in length and 15.1 ± 0.9 nm in width, giving a strong longitudinal surface plasmon resonance (LSPR) peak centered at about 783 nm (Figure 1c). The Au@Ag NRs were further synthesized by depositing Ag on the as-prepared Au NRs at 60 °C for 5 h. It can be seen from the high-magnified image in the Figure 1b that Ag is deposited evenly on the surface of Au NRs, and an obvious core-shell structure could be easily discerned due to the large contrast difference of Au and Ag. The length of the products is 56.3 ± 3.0 nm with the diameter increasing to 30.1 ± 1.6 nm, which indicates a faster growth rate on the {110} facet of Au NRs than {100} facet.35 Compared with Au NRs, the UV-vis spectrum (Figure 1c) of Au@Ag NRs reveals blue shifts especially in LSPR peak from 783 nm to 645 nm, which could be attributed to the changes in both aspect ratio and local dielectric constant.36 EDS analysis in Table S1 in the supporting information gives a Au/Ag molar ratio of nearly 1:3. The growth process of Ag on Au NRs at the initial stage was not easy to examine by TEM, however, the change in its optical properties provided a possible way by UV-vis spectra. As shown in Figure 1c, the LSPR peak of NRs shows constant blue shift against time, while the transversal surface plasmon resonance (TSPR) peak remains almost unchanged particularly after 15 min. Therefore, we chose 15 min as a suitable point for the following Pt-etching process. The Au@Ag NRs obtained at 15 min (marked as Au@Ag NRs-15 min) was then examined by TEM, EDS spectra and XPS spectra. Although core-shell structure is not easily discerned in TEM image (Figure


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S1a), the EDS data in Table S1 shows that the Ag/Au molar ratio increased to 0.111 compared with 0.016 in Au NRs. Additionally, the survey spectrum of Au@Ag NRs-15 min in Figure S1b shows the presence of Au and Ag in the product, which is further confirmed by the highresolution spectra in Figure S1c, d. It is worth noted that the binding energies at 368.1 and 374.2 eV in Figure S1d could be attributed to 3d5/2 and 3d3/2 of Ag,18 proving the existence of Ag on the surface of Au NRs. The shape-controlled synthesis of Pt-modified Au@Ag NRs could be achieved by simply altering the concentration of H2PtCl6, which would affect the etching rate and growth behaviors, as shown in Figure 2. Figure 2a shows the TEM image of the product obtained with 0.02 mM H2PtCl6 (referred to as Au@PtAg NRs), which exhibits a worm-like structure. The NRs obtained are 56.3 ± 2.8 nm in length and 17.2 ± 1.9 nm in width. EDS analysis in Table S1 shows that the molar ratio of Ag/Pt is nearly 5:8. The rough surface of NRs could be owing to the high cohesive energy of Pt (5.84 eV per atom) and its large lattice mismatch to Au atoms,27 resulting in the island growth pattern of Pt atoms.15 Line scanning EDS of a single particle along transversal (Figure 2c) and axial direction (Figure 2d) show that both the Ag and Pt were distributed evenly around the middle Au nanorod. The inset STEM images in Figure 2c and Figure 2d shows the scanning direction of a single Au@PtAg NRs. The EDS mapping of a single Au@PtAg nanorod further confirms the core-shell structure (Figure S2). The further increase of concentration of H2PtCl6 from 0.02 mM to 0.08 mM could greatly affect the morphology of the final product. As shown in the Figure 2b, the product (referred to as


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Pt-tip Au@Ag NRs) exhibits a dumbbell-like structure with a thinner body compared with Au@PtAg NRs. EDS analysis in Table S1 shows that the molar ratio of Ag/Pt was closed to Au@PtAg NRs. To clearly exhibit the Pt-tip structure, we conducted an EDS mapping of a single Pt-tip Au@Ag nanorod (Figure S3). As shown in Figure S3, Pt is distributed at the both tips of nanorod, while a thin Ag shell is formed on the Au nanorod. Additionally, the lighter dots which could be observed in the TEM image should be identified as AgCl, giving a further evidence of the dissolving of Ag. The growth mechanism of Pt-modified Au@Ag NRs. The etching ability of H2PtCl6 was firstly confirmed by a control experiment in Figure 3. The same amount of H2PtCl6 for Au@PtAg NRs was added to as-prepared Au@Ag NRs solution in 60 °C, and then an instant color shift from bright green to pink was observed, as shown in the digital pictures in Figure 3a. This result should be due to the galvanic reaction between Pt(IV)/ Pt(II) and Ag(I)/Ag. The UVvis spectrum of the product in Figure 3a shows almost the same absorption peaks as the asprepared Au NRs, which was further confirmed by the TEM image in Figure 3b. It is worth pointing out that both UV-vis spectrum and TEM image show no significant change in the morphology of the product compared with the initial Au NRs. EDS spectra was also used to confirm the composition of the product. As shown in Table S1, the molar ratio of Au/Ag/Pt is nearly 90:8:2, indicating that most Ag has been etched away and only few Pt is deposited on the particles. The Ag residue in the product would result in increasing intensity in TSPR, which could be owing to local dielectric constant change.36 To validate the stronger etching ability of


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Pt(IV) than Pt(II), H2PtCl6 was replaced with K2PtCl4 (equal amount) in the experiment with Au@Ag NRs, as shown in Figure S4. The inset of digital picture in Figure S4 shows that the solution was brown other than pink, indicating that the Ag shell might be not etched away due to the weak oxidative ability of Pt(II). The UV-vis spectrum of the product in Figure S4 is consistent with Au@Ag NRs, which further confirms the existence of Ag shell. Hence, Pt(II) species is unsuitable for the shape-controlled growth in this work. This growth behavior could be explained by Scheme 1. In the synthesis process, AgNO3 was firstly added to the solution, and Au@Ag NRs was formed. After 15 min, a certain amount of H2PtCl6 was injected, resulting in two competitive reactions: the galvanic reaction between Pt(IV)/ Pt(II) and Ag(I)/Ag (identified as Pt-etching process in this work), and the reduction of PtAg by AA. At a relatively high concentration of H2PtCl6, the etching process is dominant, resulting in the etching of Au@Ag NRs, which explains the decrease in diameter of final product. Since the high active sites are located at the tip of Au NRs,16,37 a dumbbell-like structure would be formed. On the other hand, the etching process is not significant at such a low H2PtCl6 concentration, facilitating the deposition of PtAg on Au NRs. Interestingly, these growth modes are quite different from the work of Liz-Marzán, where a tip-preferential growth was observed at a low Pt concentration.16 This difference could be explained by the surface condition of different NRs. In Liz-Marzán’ work, the growth of Pt on Au NRs was realized only in the presence of Ag+, where the Ag UPD is likely to form a Ag monolayer on the {110} facet compared to {100} facet of Au NRs. In that case, the reduction rate on {110} facet (which is often referred to the


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lateral sides of Au NRs) is much lower than the {100} facet, resulting in a tip-preferential growth at a low Pt concentration. Obviously, increasing the Pt concentration is bound to a full-coating growth. However, in our work, Au@Ag NRs was formed before the addition of H2PtCl6. Different from the UPD Ag monolayer, fewer capping agent was found on the surface of Ag shell.18 In this case, the selective stabilization on specific facets by capping agent is weakened, and the active sites on the Au@Ag NRs are determined by the surface energies of crystalline facets. For fcc structure, the energetic sequence is γ{111}

Controlling Shape and Plasmon Resonance of Pt-etched Au@Ag

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