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Kinetics-controlled growth of bimetallic RhAg on Au nanorods and their catalytic properties† Cite this: DOI: 10.1039/c3nr05775e

Wei Ye,a Xia Guo,b Fang Xie,a Rui Zhu,c Qing Zhaoc and Jian Yang*a Controlled growth of hybrid metallic nanocomposites for a desirable structure in a combination of selected components is highly important for their applications. Herein, the controllable growth of RhAg on the gold nanorods is achieved from the dumbbell-like RhAg-tipped nanorods to the brushy RhAg-coated nanorods, or the rod-like Au@Ag–Rh nanorattles. These different growth modes of RhAg on the gold nanorods are correlated with the reducing kinetics of RhCl3 and AgNO3. In view of the promising catalytic properties of Rh, the gold nanorods modified by RhAg in different structures are examined as catalysts for the Received 30th October 2013 Accepted 26th January 2014

oxidation of o-phenylenediamine. It is found that brushy RhAg-coated nanorods present a higher catalytic efficiency than dumbbell-like RhAg-tipped nanorods and rod-like Au@Ag–Rh nanorattles. These

DOI: 10.1039/c3nr05775e www.rsc.org/nanoscale

results would benefit the overgrowth control on the one-dimensional metallic nanorods and the rational design of new generation heterogeneous catalysts and optical devices.

Introduction Controllable growth of highly dispersed and structurally dened metallic nanocomposites is of timely interest for a wide range of applications,1–3 because their performances highly depend on the physical properties of different metals such as size, shape and structure.4,5 These types of growth control on one-dimensional metallic nanostructures are particularly interesting, due to a highly anisotropic shape, unique crystal facets, and distinctive physical properties. So far, these types of growth control have been focused on Pt, Pd, Ag and some of their alloys on the gold nanorods.6–13 A typical example is the growth of the core–shell Au@Ag nanorods with intense surface plasmon resonances, which were applied for light-responsive drug delivery and surface enhanced Raman scattering.7,8,12 Further control of the growth behaviour of these metals on the gold nanorods would lead to many interesting hierarchical structures, such as dumbbell-like nanorods, brushy nanorods, and rod-like nanorattles.14–16 The structures greatly alter the original properties of the gold nanorods and affect their performances in many applications. For instance, the catalytic

a

Key Laboratory of Colloid and Interface Chemistry, Ministry of Education and School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, P.R. China. E-mail: [email protected]; Fax: +86-0531-88364489

b

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, P.R. China

c State Key Laboratory for Mesoscopic Physics and Electron Microscopy Laboratory, Department of Physics, Peking University, Beijing 100871, P.R. China

† Electronic supplementary information (ESI) available: EDS spectra and line-scanning of the dumbbell-like RhAg-tipped gold nanorods; TEM images of rod-like Au@Ag–Rh nanorattles prepared by the core–shell Au@Ag nanorods. See DOI: 10.1039/c3nr05775e

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efficiency of the Ag-tipped gold nanorods in the reduction of p-nitrophenol is much higher than that of the core–shell Au@Ag nanorods.9 Thus, how to control the growth of other metals on the gold nanorods is important, although the controlled growth is a great challenge in synthesis. The synthesis becomes even difficult for the case of bimetals, because it needs simultaneous control both on the formation of bimetals and on the nucleation/growth sites. Very recently, the controllable growth of bimetals on the gold nanorods has been achieved by the formation of the dumbbell-like PdAg-tipped gold nanorods and the brushy PdAg-coated gold nanorods.13 Although a couple of bimetals (PtAg, PdAg, PdPt, etc.) have been grown on the gold nanorods, the Rh-based ones are rarely reported. This could be attributed to its large lattice mismatch with gold (7%, Rh 0.380 nm vs. Au 0.408 nm),17 high bulk cohesive energy (Rh 5.8 eV per atom vs. Au 3.8 eV per atom)17 and large surface energy (Rh 2.8 J m2 vs. Au 1.6 J m2),18 all of which make the growth of Rh on Au different from the cases of Ag, Pd and Pt.7–13 Very recently, monometallic Rh was successfully grown on the surface of gold nanorods with Na3RhCl6$12H2O as a reagent.19 Scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy revealed that Rh grown on the gold nanorods followed an island growth mode aer the formation of a few layers.19,20 But the growth of Rh-based bimetals on the gold nanorods is not reported yet. In addition, the good catalytic properties of Rh-based bimetal nanoparticles in many reactions also make them attractive to us.21–23 Herein, the controllable growth of RhAg on the gold nanorods is realized from the dumbbell-like RhAg-tipped nanorods to the brushy RhAg-coated nanorods, or to the rod-like Au@Ag– Rh nanorattles. The different growth behaviors of RhAg on the

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gold nanorods are controlled by the reducing kinetics of RhCl3 and AgNO3 that is dependent on the pH of the solution, the reaction temperature, and the concentration of the surfactant. All the nanocomposites are characterized by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), EDS spectra, and UV-Vis absorption spectra. To the best of our knowledge, this is the rst time that these morphologies have been obtained within one case, which offers an excellent opportunity to understand the relationship between different structures and their physicochemical properties. In view of the promising catalytic properties of Rh, the RhAg-modied gold nanorods in different structures are examined as catalysts for the oxidation of o-phenylenediamine (OPD).

Results and discussion The controlled growth of bimetallic RhAg on gold nanorods could be achieved by different concentrations of NaOH, because the reducing ability of ascorbic acid (AA) is strongly dependent on the pH of the solution. The addition of NaOH into the reaction would increase the pH of the solution, and then enhance the reducing ability of AA. As a result, the reducing

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kinetics of RhCl3 and AgNO3 and the growth behaviours of RhAg are greatly affected, which has been demonstrated by the products in Fig. 1. Fig. 1a shows the product obtained by reducing RhCl3 and AgNO3 at 60  C without NaOH. A dumbbelllike shape could be readily visualized throughout the image, suggesting the high yield of the dumbbell-like nanorods. Meanwhile, the dumbbell-like nanorods exhibit a narrow size distribution of about 60.4  5.0 nm in length and 12.3  1.3 nm in diameter, which might be inherited from the gold nanorods used as a seed. The high magnication TEM image (Fig. 1b) further reveals that the bulky tips (33.1  2.8 nm) of the dumbbell-like nanorods consist of aggregated nanoparticles attached to the ends of the nanorods. In order to identify the composition of these nanoparticles, HRTEM image, EDS spectra and element distributions along the axis direction are obtained. As presented in Fig. 1c, the nanoparticles display clear lattice fringes, indicating their high crystallinity. The random orientation of the lattice fringes conrms their polycrystalline nature. The distances between neighboring lattice fringes could be attributed to either {220} planes of Au or {111} planes of RhAg. EDS spectra (ESI, Fig. S1†) support the presence of Rh and Ag for the dumbbell-like

Fig. 1 TEM images and the HRTEM image of the hybrid nanorods obtained at different concentrations of NaOH. (a–c) 0 mM NaOH, (d–f) 0.41 mM NaOH and (g–i) 1.66 mM NaOH.

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nanorods, giving a molar ratio of Rh/Ag of about 86 : 14. The elemental analysis along the axis direction (ESI, Fig. S2†) further conrms that the majority of Rh and Ag are distributed at the tips of the gold nanorods, indicative of their preferential growth. The preferential growth of RhAg on the gold nanorods could be associated with the absence of NaOH in the reaction. The absence of NaOH in the solution implies a low rate of reduction of RhCl3 and AgNO3, producing a limited number of RhAg clusters. The clusters would preferentially nucleate and grow at the high active sites of the gold nanorods that usually locate at the tips of the nanorods. Thus, the growth of RhAg on the gold nanorods results in the dumbbell-like nanorods. Although the preferential growth of another metal at the tips of the gold nanorods has been reported,24,25 the case of bimetals is more difficult, particularly for the case of Rh on Au. The addition of NaOH would increase the pH of the solution, and then enhance the reducing ability of AA. As a result, more RhAg would be reduced and then grown on the surface of the gold nanorods. As shown in Fig. 1d and e, the product obtained at 0.41 mM of NaOH is composed of brushy nanorods with an average length of 52.6  5.2 nm and diameter of 29.0  2.5 nm, where RhAg nanoparticles are randomly dispersed on the surface of the gold nanorods. The result is also conrmed by the HRTEM image (Fig. 1f). The molar ratio of Rh/Ag in the brushy nanorods is 83 : 17, close to that in the dumbbell-like nanorods. Further increase of the concentration of NaOH to 1.66 mM produces many rod-like nanorattles, as displayed in Fig. 1g and h. In the rod-like nanorattles, the core–shell Au@Ag nanorods are encapsulated by a porous shell of Rh, which is supported by a HRTEM image and line-scanning EDS spectra (Fig. 1i and S3†). The formation of the rod-like nanorattles could be attributed to the signicant differences in the rates of reduction of Rh and Ag by AA at a high pH. It is believed that Ag ions are rst reduced at the surface of the gold nanorods, due to the high affinity of Ag to Au. This is supported by the fact that the characteristic colour of the core–shell Au@Ag nanorods appears in the solution immediately aer the addition of all the related reactants. Then, RhCl3 reacts with the Ag shell on the Au nanorods, producing the rod-like nanorattles. In order to verify this mechanism, the pre-prepared core–shell Au@Ag nanorods are used as the reactant to react with RhCl3. As shown in Fig. S4,† similar rod-like nanorattles are also observed. This mechanism also explains the signicant differences between the molar ratio of Rh/Ag (62 : 38) in the nanorattles and those in RhAg-tipped nanorods or in RhAg-coated nanorods. It should be pointed that the titration of pH offers an effective pathway to tailor the surface structure of RhAg on the gold nanorods, from dumbbell-like nanorods to brushy nanorods, to rod-like nanorattles. The optical properties of the gold nanorods modied by RhAg in different structures are shown in Fig. 2. It is found that the two surface plasmon resonances of the gold nanorods are greatly dampened and broadened aer the growth of RhAg, no matter what structure it is. A similar result has been documented in the literature,26,27 which is usually attributed to the changes from the dielectric constant, aspect ratio and surface structure around the gold nanorods caused by the growth of

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UV-Vis absorption spectra of the hybrid nanorods obtained at different concentrations of NaOH. (a) Dumbbell-like nanorods; (b) brushy nanorods and (c) rod-like nanorattles.

Fig. 2

another metal. This effect from RhAg is relatively less for the rod-like nanorattles compared with the dumbbell-like nanorods and brushy nanorods. This might be due to the porous structures and the less contact between the shells and the nanorods in the nanorattles. Ag plays an important role in the growth of bimetallic RhAg on the gold nanorods, which has been conrmed by the following control experiments. As shown in Fig. 3a and d, the product obtained without AgNO3 is almost unchanged in comparison to the gold nanorods, in terms of its shape and optical properties. This indicates the absence of Rh on the gold nanorods, which could be assigned to the large nucleation barrier caused by the high lattice mismatch between Rh and Au. As AgNO3 is used for the reaction at 23.3 mM, the product is dominant by the dumbbell-like nanorods, as presented in Fig. 3b. Compared with the case without AgNO3, the successful reduction of RhCl3 together with AgNO3 suggests the importance of Ag in the formation of RhAg, which might originate from the low specic surface energy of Ag, the high cohesive energy of Ag to Au, and the small lattice mismatch between Ag and Au.28 With Ag as a bridge, the nucleation barrier of Rh on Au could be overcome, producing the RhAg bimetal on the

(a–c) TEM images and (d) UV-Vis absorption spectra of the products prepared at different concentrations of AgNO3: (a) 0 mM, (b) 23.3 mM, and (c) 63.3 mM. Fig. 3

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nanorods. Unfortunately, the longitudinal surface plasmon resonance (LSPR) band of the hybrid nanorods greatly broadens and shis to the red (Fig. 3d), which can be explained by the changes in the dielectric constant around the gold nanorods and the aspect ratio as well. When the amount of AgNO3 arises to 63.3 mM, RhAg nanoparticles could be easily observed on the body of the gold nanorods, as displayed in Fig. 3c. This result could be assigned to the increased rate of reduction of RhAg due to the presence of more AgNO3 in the reaction. Its absorption spectrum exhibits similar optical features to that of the dumbbell-like RhAg-tipped nanorods. The surfactant, CTAC, has a similar effect on the products, in terms of their morphology and optical properties. As shown in Fig. 4a, the dumbbell-like nanorods are quite prevailing in the product obtained with 10 mM of CTAC. As the concentration of CTAC gradually increases to 50 mM, more and more RhAg nanoparticles appear on the tips of the nanorods in Fig. 4b. In the case of 70 mM CTAC, the gold nanorods are completely covered by RhAg nanoparticles, as shown in Fig. 4c. This phenomenon could be attributed to the formation of CTAC–Ag complexes in the solution, which destabilizes the AgUPD layer on the gold nanorods and weakens its protection on the surface.29 So, increasing the amount of CTAC would facilitate the deposition of RhAg, which is conrmed by element analysis of Rh/Au and Ag/Au in the hybrid nanorods (Fig. 4d). The effect of the reaction temperature on the product should be taken into account. As shown in Fig. 5a, the product obtained at 40  C consists of smooth nanorods without any signs of the growth of RhAg. The result could be explained by the weak reducing ability of ascorbic acid at a low temperature. So, as the reaction temperature gradually rises to 60  C, the growth of RhAg on the gold nanorods could be easily observed, as illustrated in Fig. 1a–c. The high reaction temperature at 80  C would promote the growth of RhAg on the gold nanorods, producing a large number of RhAg-coated nanorods (Fig. 5b).

Fig. 4 (a–c) TEM images of the products prepared at different

concentrations of CTAC: (a) 10 mM, (b) 50 mM, and (c) 70 mM. (d) Temporal evolution of the molar ratios of Rh/Au and Ag/Au in the hybrid nanorods.

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(a–b) TEM images of the product obtained at different temperatures, (a) 40  C and (b) 80  C.

Fig. 5

Because of the well-reported catalytic properties of Rh, Ag and Au, the hybrid nanorods with different structures are examined as a catalyst for the oxidation of o-phenylenediamine (OPD) to 2,3-diaminophenazine (DAP) (Fig. 6a). The oxidation reaction could be easily monitored by UV-Vis absorption spectroscopy, because DAP shows a characteristic absorption at 420 nm.30 A typical evolution of the absorption spectra is presented in Fig. 6b, using the brushy RhAg-coated gold nanorods as a catalyst. The absorption at 420 nm gradually increases with the reaction time, indicating the continuous formation of DAP in the solution. Aer 30 min, 80% of OPD is converted to DAP. In order to identify the active component for the catalytic reaction, the gold nanorods and the core–shell Au@Ag nanorods are also screened under the same conditions. Their conversion efficiencies are 2.2% for the gold nanorods and 0.2% for the core– shell Au@Ag nanorods, suggesting the catalytic role of Rh. Thus, the conversion efficiencies of the different hybrid nanorods are normalized on Rh. As shown in Fig. 6c, the conversion efficiency of the brushy nanorods is higher than 68% for the dumbbell-like gold nanorods and 58% for the rod-like nanorattles. The differences might be due to different surface structures in catalysts, which greatly affect the active surface area of Rh and then the catalytic performances. Aer the catalysis, the unique structures of the catalysts are basically retained, as shown in Fig. S5.† But the loss of the catalysts during their collection by centrifugation for the next cycle and the potential risk from particle aggregation make the cycling

Fig. 6 (a) Oxidation of OPD to DAP. (b) Absorption spectra of the reaction solution at different times, using the brushy RhAg-coated Au nanorods as a catalyst. The time interval between the neighboring spectra is 1.5 min. (c) Time dependence of the conversion efficiencies of OPD with the different hybrid nanorods as the catalysts. The plots are normalized by the mass of the active component, Rh.

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performances of the free-standing nanohybrids unsatisfactory (Fig. S6†), which deserve further improvements.

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Conclusions In summary, bimetallic RhAg is successfully grown on the gold nanorods in different modes, resulting in dumbbell-like RhAgtipped gold nanorods, brushy RhAg-coated gold nanorods and rod-like Au@Ag–Rh nanorattles in high yields. The different growth modes of RhAg could be controlled by varying the rates of reduction of RhCl3 and AgNO3 under different experiment conditions. Low rates of reduction of RhCl3 and AgNO3 realized by a neutral solution (pH  7), a low temperature (60  C), or low concentrations of CTAC and reactants produce a limited number of RhAg clusters that usually prefer to nucleate and grow at the high active sites of the gold nanorods. Therefore, RhAg-tipped nanorods are obtained. High rates of reduction of RhCl3 and AgNO3 would make the growth of RhAg expand to the entire surface. Because the high active sites on the gold nanorods are limited, a large number of as-prepared RhAg clusters have to occupy other surface sites for nucleation and growth. Further increase of the rates even results in the reduction of AgNO3 on the gold nanorods rst and then RhCl3 reacts with metallic Ag to form rod-like Au@Ag–Rh nanorattles. The catalytic properties of the hybrid nanostructures are tested by the oxidation of OPD to DAP. The brushy RhAg-coated gold nanorods exhibit a superior performance for the catalytic reaction in comparison to the dumbbell-like RhAg-tipped gold nanorods and the rod-like Au@Ag–Rh nanorattles.

Experimental section Chemicals HAuCl4$xH2O (x ¼ 3–5, Au  47.8%), cetyltrimethyl-ammonium chloride (CTAC, 98%), silver nitrate ($99.8%), sodium borohydride ($96%), ascorbic acid ($99.7%), H2O2 ($30.0%), and o-phenylenediamine (OPD, $98.5%) were purchased from Sinopharm Chemical Reagent Co. Ltd. RhCl3 (Rh  39%) was obtained from July Chemical Co. Ltd. All these reagents were used without any further purication. Instruments UV-Vis absorption spectra were recorded in the range of 300– 900 nm at room temperature using a Shimadzu UV-2450 spectrophotometer. Transmission electron microscopy (TEM) images were acquired using a transmission electron microscope of JEM1011 at an accelerating voltage of 100 kV. High-resolution TEM (HRTEM) images and line-scanning energy-dispersive X-ray spectra (EDS) were obtained using an analytical transmission electron microscope of JEOL 2010 at 200 kV. The samples for the TEM images were puried by centrifugation twice to remove the surfactants and/or excess of the reactants. Then, the deposit was dispersed in distilled water. The resulting solution was dropped on a copper grid coated with an amorphous carbon lm. EDS spectra were recorded with a eld-emission scanning electron microscope of JEOL-7600F at 200 kV.

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Controlled growth of bimetallic RhAg on Au nanorods Gold nanorods were synthesized by a well-established protocol based on a seed-mediated process.31,32 Then, they were collected by centrifugation and washed with distilled water once. Then, they were dispersed in distilled water as a stock solution. In a typical procedure for RhAg-tipped Au nanorods, 2 mL of the stock solution containing the gold nanorods, 1.5 mL of 0.2 M CTAC, 65 mL of 4 mM AgNO3, 60 mL of 10 mM RhCl3, and 0.2 mL of 0.1 M ascorbic acid were mixed together. This solution was diluted to 6 mL with distilled water. Aer that, the solution was vigorously stirred and kept in a water bath at 60  C. Aer 4 hours, the solution was cooled to room temperature. The product was collected by centrifugation and dispersed in distilled water for the characterization. The brushy RhAg-coated Au nanorods and the rod-like Au@Ag–Rh nanorattles were synthesized by the same protocol except the addition of NaOH. Catalytic oxidation of OPD 20 mL of 50 mM OPD and 0.1 mL of 30 wt% H2O2 were mixed with 2 mL of H2O. Then, 0.5 mL of the solution with different catalysts, such as the RhAg-tipped Au nanorods, the RhAgcoated Au nanorods, the Au@Ag–Rh nanorattles, Au nanorods and core–shell Au@Ag nanorods, was added. The solution was immediately transferred into a clean quartz cuvette for UV-Vis absorption spectra. The catalytic oxidation could be monitored by measuring the absorption spectra of the solution at regular intervals.

Acknowledgements We would like to thank the nancial support from the Natural Science Foundation of China (21071055 and 21172076), Shandong Provincial Natural Science Foundation for Distinguished Young Scholar (JQ201205), New Century Excellent Talents in University (NCET-10-0369), Independent Innovation Foundations of Shandong University (2012 ZD007), and new-faculty start-up funding in Shandong University. We also want to thank Prof. Chunjiang Jia for valuable help in experiments.

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Kinetics-controlled growth of bimetallic RhAg on Au nanorods and their catalytic properties.

Controlled growth of hybrid metallic nanocomposites for a desirable structure in a combination of selected components is highly important for their ap...
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