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Seed-assisted synthesis of Pd@Au core–shell nanotetrapods and their optical and catalytic properties† Ruopeng Zhao,b Mingxing Gong,b Huimin Zhu,b Yu Chen,*ab Yawen Tang*b and Tianhong Lub The synthesis of noble metal nanostructures with special morphology, structure, composition, and size has been an attractive research area because of their valuable applications in various fields, including optics, electronics, sensing and catalysis. In this work, the first Pd@Au core–shell nanotetrapods (Pd@Au CSNTPs) were synthesized through a facile seeded growth method. Specifically, Pd nanotetrapods were utilized as the substrate for Au coating through chemically reducing HAuCl4 with ascorbic acid (AA) in the presence of polyvinylpyrrolidone (PVP). The morphology, composition, and structure of Pd@Au CSNTPs were fully characterized by scanning and transmission electron microscopy, energy dispersive spectroscopy element mapping, X-ray powder diffraction, X-ray photoelectron spectroscopy techniques,
Received 24th April 2014 Accepted 29th May 2014
etc. Different from conventional spherical Au nanoparticles, the Pd@Au CSNTPs had a very wide surface plasmon resonance (SPR) absorption band in the visible and near-infrared regions (500–1400 nm),
DOI: 10.1039/c4nr02214a
showing special SPR absorption features. Meanwhile, the Pd@Au CSNTPs exhibited remarkably enhanced catalytic activity for the hydrogenation reduction of nitro functional groups and the C]N bond because
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of their specific structural characteristics.
Introduction Noble metal nanostructures have wide applications in the eld of catalysis because of their unique chemical and physical properties.1–5 The catalytic properties of noble metal nanostructures critically depend on their size, morphology, chemical composition, and surface structure.6–11 In particular, bimetallic core–shell noble metal nanostructures with designed compositions and morphologies generally show enhanced catalytic activities compared with their monometallic counterparts, owing to the geometric effect, electronic effect and synergistic effect between different components.12–15 For example, the Pd@Pt core–shell nanowires exhibited remarkably enhanced electrocatalytic activity and durability for the oxygen reduction reaction compared to commercial Pt black.16 The Au@Pd concave nanocubes exhibited signicantly improved catalytic activity for the ethanol oxidation reaction relative to the conventional Pd nanoparticles.17 The Au@Pt nanoparticles
a
School of Materials Science and Engineering, Shaanxi Normal University, Xi'an 710062, PR China. E-mail: [email protected]
b
Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Centre of Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, PR China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental details: MB reduction. See DOI: 10.1039/c4nr02214a
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exhibited improved electrocataytic activity for the methanol oxidation reaction compared to Pt nanoparticles.18 These facts demonstrate that the catalytic activity of the shell metal in core– shell nanostructures can be effectively tuned by manipulating the compositions and/or structural features. Among various core–shell nanostructures, bimetallic core– shell nanostructures composed of Au and Pd are one of the most interesting core–shell nanostructures in catalysis research. Although focus has been mainly on the Au@Pd core– shell nanostructures,19–23 the Pd@Au core–shell nanostructures have attracted growing interest, and various invaluable applications in catalysis and sensing have been reported.24–32 For example, the concave cubic Pd@Au core–shell nanocrystals exhibited an improved cathodic electrochemiluminescence signal in the luminal-H2O2 system due to their excellent electrocatalytic activity for H2O2 reduction.30,31 The concave Pd@Au core–shell nanocubes exhibited higher electrocatalytic activity than the conventional nanocubes for ascorbic acid oxidation.32 The Pd@Au core–shell nanoparticles exhibited higher electrocatalytic activity than PdAu alloy nanoparticles for the ethanol oxidation reaction.29 Noble metal multipods, one of the nanostructures with highly branched morphologies, are of particular interest for catalysis because of their attractive structural features such as porosity, large specic surface area, interconnected nanostructure, excellent electrical connectivity, and high density of low-coordinated atoms including steps, kinks, terraces, islands
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Scheme 1
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Procedure to design the Pd@Au CSNTPs.
and vacancies.33–39 For example, the Pd nanotetrapods exhibited remarkably enhanced mass activity, specic activity and stability for the formic acid oxidation reaction compared with commercial Pd black.2 Till now, to the best of our knowledge, the Pd@Au core–shell nanostructures with a tetrapod morphology have not yet been reported. In this work, we describe a simple and effective route to synthesize Pd@Au core– shell nanotetrapods (Pd@Au CSNTPs) by a facile seeded growth method using Pd nanotetrapods as the core (Scheme 1). The asprepared Pd@Au CSNTPs show particular optical properties and the remarkably enhanced catalytic activity for the hydrogenation reduction of nitro functional groups.
Experimental section Reagents and chemicals Ethylenediamine-tetramethylene phosphonic acid was purchased from Shandong Taihe Water Treatment Co., Ltd. (Shandong, China). Palladium chloride (PdCl2), hydrogen tetrachloroaurate(III) tetrahydrate (HAuCl4$4H2O), formaldehyde solution (HCHO, 40%), ascorbic acid (AA), polyvinylpyrrolidone (PVP), 4-nitrophenol (4-NP), methylene blue (MB) and sodium borohydride (NaBH4) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Other reagents were of analytical reagent grade and used without further purication. Preparation of the Pd@Au CSNTPs The Pd nanotetrapods were synthesized by a complex-reduction method according to our previous work.2 For the synthesis of the Pd@Au CSNTPs, 10 mg as-prepared Pd nanotetrapods, 0.6 mL of 0.05 M HAuCl4, and 0.01 g PVP were added to 10 mL deionized water with continuous stirring. Aer adding 0.1 mL of 0.1 M AA, the mixture was heated at 30 C for 1 h. Finally, the products were separated by centrifugation at 16 000 rpm for 5 min, washed several times with water, and then dried at 60 C for 5 h in a vacuum dryer. For comparison, the single-component Au nanoparticles were also prepared under similar experimental conditions.
microscope at an accelerating voltage of 20 kV. Energy dispersive X-ray (EDX) analysis was carried out on a JEOL JSM-7600F SEM. X-ray diffraction (XRD) patterns of nanocrystals were obtained with a Model D/max-rC X-ray diffractometer using a ˚ and operating at 40 kV Cu Ka radiation source (l ¼ 1.5406 A) and 100 mA. Ultraviolet-visible spectroscopy (UV-vis) measurements were performed at room temperature on a Shimadzu UV3600 spectrophotometer. X-ray photoelectron spectroscopy (XPS) measurements were performed with a Thermo VG Scientic ESCALAB 250 spectrometer with a monochromatic Al Ka Xray source (1486.6 eV photons). The binding energy was calibrated by means of the C1s peak energy of 284.6 eV. Catalytic reduction of 4-NP Typically, 1.0 mL of 0.1 M NaBH4 and 2 mL of 1.0 105 M 4-NP solutions were rst put in a quartzy cuvette having 1 cm path length. Then, 5 mL of Pd@Au CSNTP aqueous solution (1.0 g L1) was added to the mixture solution. The reduction progress of 4-NP was then monitored by recording the time-dependent absorption UV-vis spectra of the reaction system at a regular time interval of 3 min. For comparison, the as-prepared Au nanoparticles were also used as heterogeneous catalysts for the reduction of 4-NP. Catalytic reduction of MB Typically, 0.5 mL of 0.1 M NaBH4 and 2 mL of 5 105 M MB solutions were rst put in a quartzy cuvette having 1 cm path length. Then, 5 mL of catalyst aqueous solution (1.0 g L1) was added to the mixture solution. The reduction progress of MB was then monitored by recording the time-dependent absorption spectra of the reaction system at a regular time interval of 1 min.
Results and discussion Characterization of the Pd@Au CSNTPs The products were initially characterized by SEM. The representative SEM image shows that the products are highly branched nanostructures with the fourth branches at tetrahedral dimensions (Fig. 1A). The same information can be got from the TEM image (Fig. 1B). Because nanotetrapods preferred to lie at on the substrate using three of their four apexes, most
Physical characterization Transmission electron microscopy (TEM) measurements were made on a JEOL JEM-2100F transmission electron microscope operating at an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) images were captured on a JSM-2010
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Fig. 1 (A) Representative large-area SEM image of the Pd@Au CSNTPs. Inset: SEM image of an individual Pd@Au CSNTPs. (B) Representative large-area TEM image of the Pd@Au CSNTPs. Inset: HAADF-STEM image of an individual Pd@Au CSNTPs.
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of the particles were observed as three-branched planar tripodlike projections.35,40 In the high-angle annular dark eld scanning TEM (HAADF-STEM) image, the bright spot is the tip radiating either inward or outward from the planes of the gures (inset in Fig. 1B). The chemical composition of the as-prepared nanotetrapods was investigated by EDX analysis. EDX analysis shows that the chemical composition of the nanotetrapods is around Pd76Au24 (Fig. 2A). This value coincides with the Pt/Au atomic ratio in the initial precursors, which means that all AuIII precursors are completely reduced by an ascorbic acid solution. The structure of the Pd–Au nanotetrapods was investigated preliminarily by XRD. As observed, the peak positions of each diffraction peak match well with those of Au (JCPDS no. 04-0784) and Pd (JCPDS no. 46-1043) with face centered cubic (fcc) crystal structure, respectively (Fig. 2B). Interestingly, the intensity ratio of {110} to {111} for the Pd–Au nanotetrapods is 0.58, which is much higher than that of the standard powder sample for fcc Au (0.32). The relatively stronger {110} diffraction peak hints that the Pd–Au nanotetrapods have abundant {110} facets.41 In order to clearly distinguish structural features of the Pd– Au nanotetrapods, EDX elemental mapping patterns and EDX line scanning proles were observed. The corresponding EDX elemental mapping investigation reveals that the Pd–Au nanotetrapods were bimetallic nanostructures consisting of a Pd core and an Au outer shell (Fig. 3A). The compositional line proles on a branch further conrm its core–shell structure and the thickness of Au shell is about 3–5 nm (Fig. 3B). The detailed structural features of the Pd@Au CSNTPs were characterized by high-resolution TEM (HRTEM). The HRTEM
Fig. 2
image shows that the length of each branch of the Pd@Au CSNTPs is about 150 nm (Fig. 4A), close to that of Pd nanotetrapod seeds (Fig. 4B). The corresponding selected-area electron diffraction (SAED) indicates that the Au shell is a single crystal (Fig. 4C). The magnied HRTEM image of a branch shows that the Pd@Au CSNTPs have a concave surface topology marked by yellow arrows (Fig. 4D). The further magnied HRTEM image displays a large number of surface atomic steps, edges and corner atoms marked by yellow arrows (Fig. 4E). These rich edges and corner atoms derived from the Au surface are highly valuable for the enhancement of the catalytic activity.42–44 Meanwhile, a continuous lattice fringe pattern with a fringe interval of ca. 0.145 nm is observed in the magnied HRTEM image (Fig. 4E), which is indexed to the {110} facet of the fcc Au crystal (0.1442 nm). As we know, the fcc noble metal nanocrystals are generally bound by low-index {111} and/or {100} facets owing to minimization of surface energy during nanocrystal growth. However, the high-quality fcc noble metal nanocrystals with high energy {110} facets can be achieved by kinetically controlled synthesis (i.e., the slow reduction rate) in the presence of a surfactant because of the specic adsorption of the surfactant on {110} facets (this will stabilize the {110} facets).45,46 UV-vis measurements show that the reduction of the AuIII precursor is complete at 40 min by monitoring the absorption behavior of the HAuCl4 at 243 nm (Fig. 5A and B). The slow reduction rate may be ascribed to the weak reduction capability of AA, which allows Au deposit/growth on the Pd nanotetrapod seed surface through a kinetic method. As shown in Fig. 4D, the surface of Au shell is smooth, which is indicative of the layer-by-layer kinetic epitaxial growth for Au shell on Pd seeds through a kinetic method.47 So we assumed that the appearance of a crystalline Au shell with
(A) EDX spectrum and (B) XRD pattern of the Pd@Au CSNTPs.
Fig. 3 (A) EDX elemental mapping patterns of an individual Pd@Au CSNTPs. (B) EDX line scanning profiles recorded form regions 1, 2, 3, and 4 marked by arrows in the HAADF-STEM image in (A).
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Fig. 4 (A) HRTEM image of an individual Pd@Au CSNTPs. (B) HRTEM image of an individual Pd nanotetrapod. (C) The SAED pattern of the Pd@Au CSNTPs. (D) The magnified HRTEM image taken from the blue region marked by a square in (A). (E) The further magnified HRTEM image taken from the red region marked by a square in (D).
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(A) UV-vis spectra of the reaction system under different reaction times. (B) The dependence of the absorbance value of HAuCl4 at 243 nm on reaction times.
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Fig. 5
the {110} facet is most likely due to the specic adsorption of PVP on Au{110} facets and the slow reduction rate of the AuIII precursor. The surface composition of the Pd@Au CSNTPs and electronic interaction between the Pd core and the Au shell were investigated by XPS. As observed, the Pd signal is weaker than the Au signal (Fig. 6A), although the content of Pd is much higher than that of Au (the Pd–Au atomic rate of the Pd@Au CSNTPs is 76 : 24). This is apparently due to the Au coating, and indirectly conrms the formation the Pd@Au core–shell nanostructures. The Pd 3d and Au 4f spectra show that the binding energies of Pd0 3d (3d5/2 ¼ 335.18 eV; 3d3/2 ¼ 340.42 eV, Fig. 6B) and Au0 4f (4f7/2 ¼ 83.4 eV; 4f5/2 ¼ 87.12 eV, Fig. 6C) slightly deviate from the standard values of bulk Pd (3d5/2 ¼ 334.90 eV; 3d3/2 ¼ 340.15 eV) and bulk Au (4f7/2 ¼ 83.80 eV; 4f5/2 ¼ 87.45 eV), originating from the electronic interaction between the Au shell and the Pd core because of the difference in electronegativity.48,49 Optical properties of the Pd@Au CSNTPs Compared to the black colour of the aqueous Pd nanotetrapod suspension, the Pd@Au CSNTP suspension shows obvious bluepurple colour (Fig. 7A). The optical properties of the Pd@Au
Fig. 6 XPS spectra of the Pd@Au CSNTPs: (A) survey scan and narrow scan for (B) Pd and (C) Au elements.
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(A) The photographs of (a) Pd nanotetrapod suspension and (b) Pd@Au CSNTP suspension. (B) UV-vis-NIR spectra of the Pd@Au CSNTPs and Pd nanotetrapods.
Fig. 7
CSNTPs was investigated by UV-vis-NIR in detail. Different from conventional spherical Au nanoparticles that have surface plasmon resonance (SPR) absorption peaks in the range of 500– 900 nm, the Pd@Au CSNTPs with three-dimensional and branched Au nanotetrapod shell should display uniquely different SPR absorption features. As observed, the Pd@Au CSNTPs give a very wide SPR absorption band in the visible and near-infrared regions (500–1400 nm, Fig. 7B). Meanwhile, no obvious SPR signal is observed for the Pd nanotetrapods, indicating that the SPR absorption of the Pd@Au CSNTPs originates from the Au shell. Similar to self-assembled Au nanostructures at the solid phase surface (i.e., superstructures),26,50 the particular SPR absorption of the Pd@Au CSNTPs is most likely caused by the strong plasmon coupling of isolated Au nanoparticles on the Pd@Au CSNTP surface because of the compact particle-toparticle contact. Catalytic properties of the Pd@Au CSNTPs One of the most important applications of noble metal nanostructures is to catalyze or activate some reactions that will otherwise be unlikely to occur. 4-Aminophenol (4-AP), as an important ne chemical, is widely applied in the manufacturing industry, such as in pharmaceuticals, dyestuffs, rubber antioxidants and other industrially important products. However, the 4-AP used to be produced by the selective catalytic hydrogenation of the corresponding nitro-precursors (4-nitrophenol, 4NP) by using tri-n-butylamine or pyridine as solvents at high pressure and temperature, which is costly and less benign. With regard to this, the catalytic property of the Pd@Au CSNTPs for the reduction of 4-NP by NaBH4 was investigated in this study, where the overall reaction is presented in Scheme 2.51 The absorption peak of light-yellow 4-NP is centered at 317 nm whereas the absorption peak of colorless 4-AP is centered at
Scheme 2
The overall reaction process for converting 4-NP to 4-AP
by NaBH4.
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properties compared to conventional spherical Au nanoparticles. The SPR of the Pd@Au CSNTPs in the near-infrared range may endow them with potential applications in surface enhancement spectroscopic methods and chemical or biological sensing. For example, the specic chemical sensitivity of Pd toward hydrogen makes the Pd@Au CSNTPs potentially useful for SPR-based hydrogen sensors. Meanwhile, the Pd@Au CSNTPs possess excellent catalytic activity for the hydrogenation reduction of nitro functional groups because of high density of low-coordinated defect atoms and the complete Au {110} facet. Obviously, this new kind of highly branched Pd@Au core–shell nanostructures may also nd use as catalysts beyond 4-NP reduction and MB reduction.
(A) UV-vis spectra of 4-NP and 4-AP (inset: photographs of 4NP and 4-AP solution). (B and C) UV-vis spectra for successive reduction of 4-NP with NaBH4 using (B) the Pd@Au CSNTPs and (C) as-prepared Au nanoparticles as catalysts at 3 min intervals. (D) The relationship between ln(Ct/C0) and reaction time (t), where the ratio of the 4-NP concentration (Ct at time t) to its initial value C0 was directly given by the relative intensity of the respective absorbance At/A0. Fig. 8
400 nm (Fig. 8A and inset). Thus, the reaction processes can be monitored by UV-vis. For comparison, the single-component Au nanoparticles were prepared under similar experimental conditions. As observed, the reduction of 4-NP to 4-AP is nished within 34 min using the Pd@Au CSNTPs as the catalyst (Fig. 8B), which is much shorter than that of the as-prepared Au nanoparticles (52 min, Fig. 8C). The reaction rate constant k is calculated from the slope of the linear section of the plots of ln(Ct/C0) versus t (Fig. 8D). Compared with the as-prepared Au nanoparticles (k: 0.081 min1), the Pd@Au CSNTPs exhibit a better catalytic activity for the 4-AP reduction (k: 0.139 min1). Remarkably, the k value is also higher than those of the Au/Au– polythiophene core–shell nanospheres (k: 0.039 min1)40 and Resin@Au core–shell nanocomposites (k: 0.097 min1).52 The excellent catalytic activity of the Pd@Au CSNTPs may originate from the following reasons: (i) the rich edges and corner atoms derived from the Au surface are active for the catalytic reaction because of their low-coordination number. (ii) Among the lowindex facets, the Au{110} facet has the highest catalytic activity for the hydrogenation reduction of nitro functional groups (Au {110} > Au{100} >Au{111}) because of its high surface energy.53 Aer selecting the MB reduction as a model reaction, UV-vis investigations demonstrate that the catalytic activity of Pd@Au CSNTPs is also better than that of the as-prepared Au nanoparticles (Fig. S1 in ESI†), indicating that Pd@Au CSNTPs have also excellent catalytic activity for the hydrogenation reduction of the C]N double bond.
Conclusions In summary, the Pd@Au CSNTPs with smooth surface are synthesized by a seed-based chemical reduction route. The asprepared Pd@Au CSNTPs show the interesting optical
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Acknowledgements The authors are grateful for the nancial support of NSFC (21376122 and 21273116), Natural Science Foundation of Jiangsu Province (BK20131395), United Fund of NSFC and Yunnan Province (U1137602), Industry-Academia Cooperation Innovation Fund Project of Jiangsu Province (BY2012001), Fundamental Research Funds for the Central Universities (GK201402016), University Postgraduate Research and Innovation Project in Jiangsu Province (CXLX13-369), and Starting Funds of Shaanxi Normal University.
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Seed-assisted synthesis of Pd@Au core–shell nanotetrapods and their optical and catalytic properties† Ruopeng Zhao,b Mingxing Gong,b Huimin Zhu,b Yu Chen,*ab Yawen Tang*b and Tianhong Lub The synthesis of noble metal nanostructures with special morphology, structure, composition, and size has been an attractive research area because of their valuable applications in various fields, including optics, electronics, sensing and catalysis. In this work, the first Pd@Au core–shell nanotetrapods (Pd@Au CSNTPs) were synthesized through a facile seeded growth method. Specifically, Pd nanotetrapods were utilized as the substrate for Au coating through chemically reducing HAuCl4 with ascorbic acid (AA) in the presence of polyvinylpyrrolidone (PVP). The morphology, composition, and structure of Pd@Au CSNTPs were fully characterized by scanning and transmission electron microscopy, energy dispersive spectroscopy element mapping, X-ray powder diffraction, X-ray photoelectron spectroscopy techniques,
Received 24th April 2014 Accepted 29th May 2014
etc. Different from conventional spherical Au nanoparticles, the Pd@Au CSNTPs had a very wide surface plasmon resonance (SPR) absorption band in the visible and near-infrared regions (500–1400 nm),
DOI: 10.1039/c4nr02214a
showing special SPR absorption features. Meanwhile, the Pd@Au CSNTPs exhibited remarkably enhanced catalytic activity for the hydrogenation reduction of nitro functional groups and the C]N bond because
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of their specific structural characteristics.
Introduction Noble metal nanostructures have wide applications in the eld of catalysis because of their unique chemical and physical properties.1–5 The catalytic properties of noble metal nanostructures critically depend on their size, morphology, chemical composition, and surface structure.6–11 In particular, bimetallic core–shell noble metal nanostructures with designed compositions and morphologies generally show enhanced catalytic activities compared with their monometallic counterparts, owing to the geometric effect, electronic effect and synergistic effect between different components.12–15 For example, the Pd@Pt core–shell nanowires exhibited remarkably enhanced electrocatalytic activity and durability for the oxygen reduction reaction compared to commercial Pt black.16 The Au@Pd concave nanocubes exhibited signicantly improved catalytic activity for the ethanol oxidation reaction relative to the conventional Pd nanoparticles.17 The Au@Pt nanoparticles
a
School of Materials Science and Engineering, Shaanxi Normal University, Xi'an 710062, PR China. E-mail: [email protected]
b
Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Centre of Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, PR China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental details: MB reduction. See DOI: 10.1039/c4nr02214a
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exhibited improved electrocataytic activity for the methanol oxidation reaction compared to Pt nanoparticles.18 These facts demonstrate that the catalytic activity of the shell metal in core– shell nanostructures can be effectively tuned by manipulating the compositions and/or structural features. Among various core–shell nanostructures, bimetallic core– shell nanostructures composed of Au and Pd are one of the most interesting core–shell nanostructures in catalysis research. Although focus has been mainly on the Au@Pd core– shell nanostructures,19–23 the Pd@Au core–shell nanostructures have attracted growing interest, and various invaluable applications in catalysis and sensing have been reported.24–32 For example, the concave cubic Pd@Au core–shell nanocrystals exhibited an improved cathodic electrochemiluminescence signal in the luminal-H2O2 system due to their excellent electrocatalytic activity for H2O2 reduction.30,31 The concave Pd@Au core–shell nanocubes exhibited higher electrocatalytic activity than the conventional nanocubes for ascorbic acid oxidation.32 The Pd@Au core–shell nanoparticles exhibited higher electrocatalytic activity than PdAu alloy nanoparticles for the ethanol oxidation reaction.29 Noble metal multipods, one of the nanostructures with highly branched morphologies, are of particular interest for catalysis because of their attractive structural features such as porosity, large specic surface area, interconnected nanostructure, excellent electrical connectivity, and high density of low-coordinated atoms including steps, kinks, terraces, islands
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Scheme 1
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Procedure to design the Pd@Au CSNTPs.
and vacancies.33–39 For example, the Pd nanotetrapods exhibited remarkably enhanced mass activity, specic activity and stability for the formic acid oxidation reaction compared with commercial Pd black.2 Till now, to the best of our knowledge, the Pd@Au core–shell nanostructures with a tetrapod morphology have not yet been reported. In this work, we describe a simple and effective route to synthesize Pd@Au core– shell nanotetrapods (Pd@Au CSNTPs) by a facile seeded growth method using Pd nanotetrapods as the core (Scheme 1). The asprepared Pd@Au CSNTPs show particular optical properties and the remarkably enhanced catalytic activity for the hydrogenation reduction of nitro functional groups.
Experimental section Reagents and chemicals Ethylenediamine-tetramethylene phosphonic acid was purchased from Shandong Taihe Water Treatment Co., Ltd. (Shandong, China). Palladium chloride (PdCl2), hydrogen tetrachloroaurate(III) tetrahydrate (HAuCl4$4H2O), formaldehyde solution (HCHO, 40%), ascorbic acid (AA), polyvinylpyrrolidone (PVP), 4-nitrophenol (4-NP), methylene blue (MB) and sodium borohydride (NaBH4) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Other reagents were of analytical reagent grade and used without further purication. Preparation of the Pd@Au CSNTPs The Pd nanotetrapods were synthesized by a complex-reduction method according to our previous work.2 For the synthesis of the Pd@Au CSNTPs, 10 mg as-prepared Pd nanotetrapods, 0.6 mL of 0.05 M HAuCl4, and 0.01 g PVP were added to 10 mL deionized water with continuous stirring. Aer adding 0.1 mL of 0.1 M AA, the mixture was heated at 30 C for 1 h. Finally, the products were separated by centrifugation at 16 000 rpm for 5 min, washed several times with water, and then dried at 60 C for 5 h in a vacuum dryer. For comparison, the single-component Au nanoparticles were also prepared under similar experimental conditions.
microscope at an accelerating voltage of 20 kV. Energy dispersive X-ray (EDX) analysis was carried out on a JEOL JSM-7600F SEM. X-ray diffraction (XRD) patterns of nanocrystals were obtained with a Model D/max-rC X-ray diffractometer using a ˚ and operating at 40 kV Cu Ka radiation source (l ¼ 1.5406 A) and 100 mA. Ultraviolet-visible spectroscopy (UV-vis) measurements were performed at room temperature on a Shimadzu UV3600 spectrophotometer. X-ray photoelectron spectroscopy (XPS) measurements were performed with a Thermo VG Scientic ESCALAB 250 spectrometer with a monochromatic Al Ka Xray source (1486.6 eV photons). The binding energy was calibrated by means of the C1s peak energy of 284.6 eV. Catalytic reduction of 4-NP Typically, 1.0 mL of 0.1 M NaBH4 and 2 mL of 1.0 105 M 4-NP solutions were rst put in a quartzy cuvette having 1 cm path length. Then, 5 mL of Pd@Au CSNTP aqueous solution (1.0 g L1) was added to the mixture solution. The reduction progress of 4-NP was then monitored by recording the time-dependent absorption UV-vis spectra of the reaction system at a regular time interval of 3 min. For comparison, the as-prepared Au nanoparticles were also used as heterogeneous catalysts for the reduction of 4-NP. Catalytic reduction of MB Typically, 0.5 mL of 0.1 M NaBH4 and 2 mL of 5 105 M MB solutions were rst put in a quartzy cuvette having 1 cm path length. Then, 5 mL of catalyst aqueous solution (1.0 g L1) was added to the mixture solution. The reduction progress of MB was then monitored by recording the time-dependent absorption spectra of the reaction system at a regular time interval of 1 min.
Results and discussion Characterization of the Pd@Au CSNTPs The products were initially characterized by SEM. The representative SEM image shows that the products are highly branched nanostructures with the fourth branches at tetrahedral dimensions (Fig. 1A). The same information can be got from the TEM image (Fig. 1B). Because nanotetrapods preferred to lie at on the substrate using three of their four apexes, most
Physical characterization Transmission electron microscopy (TEM) measurements were made on a JEOL JEM-2100F transmission electron microscope operating at an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) images were captured on a JSM-2010
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Fig. 1 (A) Representative large-area SEM image of the Pd@Au CSNTPs. Inset: SEM image of an individual Pd@Au CSNTPs. (B) Representative large-area TEM image of the Pd@Au CSNTPs. Inset: HAADF-STEM image of an individual Pd@Au CSNTPs.
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of the particles were observed as three-branched planar tripodlike projections.35,40 In the high-angle annular dark eld scanning TEM (HAADF-STEM) image, the bright spot is the tip radiating either inward or outward from the planes of the gures (inset in Fig. 1B). The chemical composition of the as-prepared nanotetrapods was investigated by EDX analysis. EDX analysis shows that the chemical composition of the nanotetrapods is around Pd76Au24 (Fig. 2A). This value coincides with the Pt/Au atomic ratio in the initial precursors, which means that all AuIII precursors are completely reduced by an ascorbic acid solution. The structure of the Pd–Au nanotetrapods was investigated preliminarily by XRD. As observed, the peak positions of each diffraction peak match well with those of Au (JCPDS no. 04-0784) and Pd (JCPDS no. 46-1043) with face centered cubic (fcc) crystal structure, respectively (Fig. 2B). Interestingly, the intensity ratio of {110} to {111} for the Pd–Au nanotetrapods is 0.58, which is much higher than that of the standard powder sample for fcc Au (0.32). The relatively stronger {110} diffraction peak hints that the Pd–Au nanotetrapods have abundant {110} facets.41 In order to clearly distinguish structural features of the Pd– Au nanotetrapods, EDX elemental mapping patterns and EDX line scanning proles were observed. The corresponding EDX elemental mapping investigation reveals that the Pd–Au nanotetrapods were bimetallic nanostructures consisting of a Pd core and an Au outer shell (Fig. 3A). The compositional line proles on a branch further conrm its core–shell structure and the thickness of Au shell is about 3–5 nm (Fig. 3B). The detailed structural features of the Pd@Au CSNTPs were characterized by high-resolution TEM (HRTEM). The HRTEM
Fig. 2
image shows that the length of each branch of the Pd@Au CSNTPs is about 150 nm (Fig. 4A), close to that of Pd nanotetrapod seeds (Fig. 4B). The corresponding selected-area electron diffraction (SAED) indicates that the Au shell is a single crystal (Fig. 4C). The magnied HRTEM image of a branch shows that the Pd@Au CSNTPs have a concave surface topology marked by yellow arrows (Fig. 4D). The further magnied HRTEM image displays a large number of surface atomic steps, edges and corner atoms marked by yellow arrows (Fig. 4E). These rich edges and corner atoms derived from the Au surface are highly valuable for the enhancement of the catalytic activity.42–44 Meanwhile, a continuous lattice fringe pattern with a fringe interval of ca. 0.145 nm is observed in the magnied HRTEM image (Fig. 4E), which is indexed to the {110} facet of the fcc Au crystal (0.1442 nm). As we know, the fcc noble metal nanocrystals are generally bound by low-index {111} and/or {100} facets owing to minimization of surface energy during nanocrystal growth. However, the high-quality fcc noble metal nanocrystals with high energy {110} facets can be achieved by kinetically controlled synthesis (i.e., the slow reduction rate) in the presence of a surfactant because of the specic adsorption of the surfactant on {110} facets (this will stabilize the {110} facets).45,46 UV-vis measurements show that the reduction of the AuIII precursor is complete at 40 min by monitoring the absorption behavior of the HAuCl4 at 243 nm (Fig. 5A and B). The slow reduction rate may be ascribed to the weak reduction capability of AA, which allows Au deposit/growth on the Pd nanotetrapod seed surface through a kinetic method. As shown in Fig. 4D, the surface of Au shell is smooth, which is indicative of the layer-by-layer kinetic epitaxial growth for Au shell on Pd seeds through a kinetic method.47 So we assumed that the appearance of a crystalline Au shell with
(A) EDX spectrum and (B) XRD pattern of the Pd@Au CSNTPs.
Fig. 3 (A) EDX elemental mapping patterns of an individual Pd@Au CSNTPs. (B) EDX line scanning profiles recorded form regions 1, 2, 3, and 4 marked by arrows in the HAADF-STEM image in (A).
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Fig. 4 (A) HRTEM image of an individual Pd@Au CSNTPs. (B) HRTEM image of an individual Pd nanotetrapod. (C) The SAED pattern of the Pd@Au CSNTPs. (D) The magnified HRTEM image taken from the blue region marked by a square in (A). (E) The further magnified HRTEM image taken from the red region marked by a square in (D).
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(A) UV-vis spectra of the reaction system under different reaction times. (B) The dependence of the absorbance value of HAuCl4 at 243 nm on reaction times.
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Fig. 5
the {110} facet is most likely due to the specic adsorption of PVP on Au{110} facets and the slow reduction rate of the AuIII precursor. The surface composition of the Pd@Au CSNTPs and electronic interaction between the Pd core and the Au shell were investigated by XPS. As observed, the Pd signal is weaker than the Au signal (Fig. 6A), although the content of Pd is much higher than that of Au (the Pd–Au atomic rate of the Pd@Au CSNTPs is 76 : 24). This is apparently due to the Au coating, and indirectly conrms the formation the Pd@Au core–shell nanostructures. The Pd 3d and Au 4f spectra show that the binding energies of Pd0 3d (3d5/2 ¼ 335.18 eV; 3d3/2 ¼ 340.42 eV, Fig. 6B) and Au0 4f (4f7/2 ¼ 83.4 eV; 4f5/2 ¼ 87.12 eV, Fig. 6C) slightly deviate from the standard values of bulk Pd (3d5/2 ¼ 334.90 eV; 3d3/2 ¼ 340.15 eV) and bulk Au (4f7/2 ¼ 83.80 eV; 4f5/2 ¼ 87.45 eV), originating from the electronic interaction between the Au shell and the Pd core because of the difference in electronegativity.48,49 Optical properties of the Pd@Au CSNTPs Compared to the black colour of the aqueous Pd nanotetrapod suspension, the Pd@Au CSNTP suspension shows obvious bluepurple colour (Fig. 7A). The optical properties of the Pd@Au
Fig. 6 XPS spectra of the Pd@Au CSNTPs: (A) survey scan and narrow scan for (B) Pd and (C) Au elements.
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(A) The photographs of (a) Pd nanotetrapod suspension and (b) Pd@Au CSNTP suspension. (B) UV-vis-NIR spectra of the Pd@Au CSNTPs and Pd nanotetrapods.
Fig. 7
CSNTPs was investigated by UV-vis-NIR in detail. Different from conventional spherical Au nanoparticles that have surface plasmon resonance (SPR) absorption peaks in the range of 500– 900 nm, the Pd@Au CSNTPs with three-dimensional and branched Au nanotetrapod shell should display uniquely different SPR absorption features. As observed, the Pd@Au CSNTPs give a very wide SPR absorption band in the visible and near-infrared regions (500–1400 nm, Fig. 7B). Meanwhile, no obvious SPR signal is observed for the Pd nanotetrapods, indicating that the SPR absorption of the Pd@Au CSNTPs originates from the Au shell. Similar to self-assembled Au nanostructures at the solid phase surface (i.e., superstructures),26,50 the particular SPR absorption of the Pd@Au CSNTPs is most likely caused by the strong plasmon coupling of isolated Au nanoparticles on the Pd@Au CSNTP surface because of the compact particle-toparticle contact. Catalytic properties of the Pd@Au CSNTPs One of the most important applications of noble metal nanostructures is to catalyze or activate some reactions that will otherwise be unlikely to occur. 4-Aminophenol (4-AP), as an important ne chemical, is widely applied in the manufacturing industry, such as in pharmaceuticals, dyestuffs, rubber antioxidants and other industrially important products. However, the 4-AP used to be produced by the selective catalytic hydrogenation of the corresponding nitro-precursors (4-nitrophenol, 4NP) by using tri-n-butylamine or pyridine as solvents at high pressure and temperature, which is costly and less benign. With regard to this, the catalytic property of the Pd@Au CSNTPs for the reduction of 4-NP by NaBH4 was investigated in this study, where the overall reaction is presented in Scheme 2.51 The absorption peak of light-yellow 4-NP is centered at 317 nm whereas the absorption peak of colorless 4-AP is centered at
Scheme 2
The overall reaction process for converting 4-NP to 4-AP
by NaBH4.
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properties compared to conventional spherical Au nanoparticles. The SPR of the Pd@Au CSNTPs in the near-infrared range may endow them with potential applications in surface enhancement spectroscopic methods and chemical or biological sensing. For example, the specic chemical sensitivity of Pd toward hydrogen makes the Pd@Au CSNTPs potentially useful for SPR-based hydrogen sensors. Meanwhile, the Pd@Au CSNTPs possess excellent catalytic activity for the hydrogenation reduction of nitro functional groups because of high density of low-coordinated defect atoms and the complete Au {110} facet. Obviously, this new kind of highly branched Pd@Au core–shell nanostructures may also nd use as catalysts beyond 4-NP reduction and MB reduction.
(A) UV-vis spectra of 4-NP and 4-AP (inset: photographs of 4NP and 4-AP solution). (B and C) UV-vis spectra for successive reduction of 4-NP with NaBH4 using (B) the Pd@Au CSNTPs and (C) as-prepared Au nanoparticles as catalysts at 3 min intervals. (D) The relationship between ln(Ct/C0) and reaction time (t), where the ratio of the 4-NP concentration (Ct at time t) to its initial value C0 was directly given by the relative intensity of the respective absorbance At/A0. Fig. 8
400 nm (Fig. 8A and inset). Thus, the reaction processes can be monitored by UV-vis. For comparison, the single-component Au nanoparticles were prepared under similar experimental conditions. As observed, the reduction of 4-NP to 4-AP is nished within 34 min using the Pd@Au CSNTPs as the catalyst (Fig. 8B), which is much shorter than that of the as-prepared Au nanoparticles (52 min, Fig. 8C). The reaction rate constant k is calculated from the slope of the linear section of the plots of ln(Ct/C0) versus t (Fig. 8D). Compared with the as-prepared Au nanoparticles (k: 0.081 min1), the Pd@Au CSNTPs exhibit a better catalytic activity for the 4-AP reduction (k: 0.139 min1). Remarkably, the k value is also higher than those of the Au/Au– polythiophene core–shell nanospheres (k: 0.039 min1)40 and Resin@Au core–shell nanocomposites (k: 0.097 min1).52 The excellent catalytic activity of the Pd@Au CSNTPs may originate from the following reasons: (i) the rich edges and corner atoms derived from the Au surface are active for the catalytic reaction because of their low-coordination number. (ii) Among the lowindex facets, the Au{110} facet has the highest catalytic activity for the hydrogenation reduction of nitro functional groups (Au {110} > Au{100} >Au{111}) because of its high surface energy.53 Aer selecting the MB reduction as a model reaction, UV-vis investigations demonstrate that the catalytic activity of Pd@Au CSNTPs is also better than that of the as-prepared Au nanoparticles (Fig. S1 in ESI†), indicating that Pd@Au CSNTPs have also excellent catalytic activity for the hydrogenation reduction of the C]N double bond.
Conclusions In summary, the Pd@Au CSNTPs with smooth surface are synthesized by a seed-based chemical reduction route. The asprepared Pd@Au CSNTPs show the interesting optical
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Acknowledgements The authors are grateful for the nancial support of NSFC (21376122 and 21273116), Natural Science Foundation of Jiangsu Province (BK20131395), United Fund of NSFC and Yunnan Province (U1137602), Industry-Academia Cooperation Innovation Fund Project of Jiangsu Province (BY2012001), Fundamental Research Funds for the Central Universities (GK201402016), University Postgraduate Research and Innovation Project in Jiangsu Province (CXLX13-369), and Starting Funds of Shaanxi Normal University.
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