Pedro H C Camargo

University of São Paulo, San Paulo, São Paulo, Brazil

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Publications (54)311.09 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We have investigated the use of x-ray energy dispersive spectroscopy during tomographic hyperspectral imaging experiments in the scanning transmission electron microscope. In this work, we have found that for an analytical system employing a commercial high-tilt tomography holder the measured x-ray signal is limited by shadowing caused by the penumbra of the holder relative to the x-ray detector system. This limits the ability to perform quantitative, elemental tomographic analysis.
    Journal of Physics Conference Series 06/2014; 522(1):012025.
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    ABSTRACT: Sub-nanosecond charge dynamics in monodisperse Au nanoparticles (NPs) supported on TiO2 colloidal spheres are studied as a function of NP diameter using ultrafast transient absorption spectroscopy. The decay of the transmittance changes observed in the region of the plasmon resonance of the Au NPs following photoexcitation of the TiO2 spheres are well-described by a bi-exponential function consisting of a fast component of 2 ps duration associated with electron-phonon scattering, followed by a slow and relatively weak component associated with phonon-phonon scattering. The decay constant characterising the latter component was found to be dependent on the size of the Au NPs, rising from 49 ± 3 to 128 ± 6 ps as the diameter of the Au NPs increased from 12.2 ± 2.2 nm to 24.5 ± 2.8 nm, respectively.
    Physical Chemistry Chemical Physics 06/2014; · 3.83 Impact Factor
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    ABSTRACT: This paper describes the development of a facile and environmentally friendly strategy for supporting palladium nanoparticles (Pd NPs) on multiwalled carbon nanotubes (MWCNTs) with ethanol as the solvent/reducing agent, potassium tetrachloropalladate(II) (K2PdCl4) as the Pd precursor, and polyvinylpyrrolidone (PVP) as the surface modifier. More specifically, our approach was based on the nonbonding interaction between MWCNTs and PVP, which is a cheap, nontoxic, and commercial polymer. As PVP can serve not only as a surface modifier but also as a stabilizing agent for Pd NPs, the utilization of additional functionalization steps, reducing agents, and stabilizers was not required to achieve uniform Pd deposition over the MWCNTs. Our results demonstrate that Pd NPs below 5 nm in diameter can be directly supported on MWCNTs by this route. Also, sequential Pd-reduction steps can be employed to improve the coverage of Pd NPs at the MWCNT surfaces, although this can also lead to the formation of larger Pd particles or aggregates. The electrocatalytic activity for ethanol oxidation was investigated as a function of the composition and structure of the materials produced, in which MWCNTs decorated with Pd NPs of smaller sizes and lower coverages displayed the highest activities. The results described herein suggest that our approach may serve as a simple platform for the synthesis of MWCNTs decorated with metal NPs with well-defined morphologies and uniform dispersion for electrochemical and catalytic applications.
    Berichte der deutschen chemischen Gesellschaft 03/2014; · 2.94 Impact Factor
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    ABSTRACT: Significant elemental segregation is shown to exist within individual hollow silver-gold (Ag-Au) bimetallic nanoparticles produced from the galvanic reaction between Ag particles and AuCl4-. Three dimensional compositional mapping using energy dispersive X-ray (EDX) tomography within the scanning transmission electron microscope (STEM) reveals that nanoparticle surface segregation inverts from Au-rich to Ag-rich as Au content increases. Maximum Au surface coverage was observed for nanoparticles with approximately 25at% Au which correlates to the optimal catalytic performance in a three-component coupling reaction between cyclohexanecarboxyaldehyde, piperidine and phenylacetylene. These results provide important fundamental insights towards optimizing the properties and reducing the cost of precious metal catalysts.
    Nano Letters 02/2014; · 13.03 Impact Factor
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    ABSTRACT: We describe herein a systematic investigation on the optical properties of Ag, Au, and Cu triangular nanoprisms as a function of size and excitation wavelength using the discrete dipole approximation. Specifically, the edge length was varied from 40 to 100 nm while the thickness was kept at 10 nm. In the far field, our results suggest that the in-plane localized surface plasmon resonance (LSPR) peaks red-shifted as the edge length increased. In the near field, the magnitude of the electric fields generated close to the surface of the nanoprisms were calculated considering 514, 633, and 785 nm as the excitation wavelengths. The variation on the magnitude of the electric fields can be understood based on the matching between the excitation wavelength and the position of the in-plane dipole and quadrupole LSPR modes. We believe that these results can have important implications in the design of metal nanoprisms for plasmonic applications.
    Chemical Physics 09/2013; · 1.96 Impact Factor
  • Caio C S de Oliveira, Rômulo A Ando, Pedro H C Camargo
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    ABSTRACT: The polyol method has been widely employed for the synthesis of uniform silver nanowires (Ag NWs) in high yields. In this article, we describe the utilization of HCl oxidative etching as an effective strategy to control the width of Ag NWs produced by the polyol approach. More specifically, the width of the produced Ag NWs could be tuned from 65 to 765 nm by varying the HCl concentration in the polyol recipe. Our results indicate that the obtained widths displayed a linear and steady increase according to the HCl concentration employed in the reaction. Although the width was also dependent on other experimental parameters such as the AgNO(3) and polyvinylpirrolidone (PVP) concentrations and temperature, the HCl oxidative etching enabled the controlled synthesis of Ag NWs over the widest range of widths. The size-dependent optical property investigations revealed that the transverse mode surface plasmon resonance peak for the produced Ag NWs red-shifted from 378 to 467 nm as their width increased from 77 to 584 nm. The application of Ag NWs (77 nm) as SERS substrates for the detection of 4-mercaptopyridine was also demonstrated. As the properties of metal nanostructures are strongly dependent upon size, the results reported herein can have important implications for designing the synthesis of uniform Ag NWs in high yields displaying controlled and/or desired dimensions for applications in areas including plasmonics, electronics, and sensing.
    Physical Chemistry Chemical Physics 12/2012; · 3.83 Impact Factor
  • Jean C. S. Costa, Paola Corio, Pedro H. C. Camargo
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    ABSTRACT: We report the synthesis of silver–gold nanotubes containing hot spots along their surface. The Ag–Au nanotubes exhibited exceptional SERS properties compared to silver nanowires, enabling the detection of crystal violet in the 10−10 M regime, as well as 9-nitroanthracene and benzo[a]pyrene at 3.3 × 10−7 M.
    RSC Advances 10/2012; 2(26):9801-9804. · 2.56 Impact Factor
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    ABSTRACT: This Letter reports an investigation on the optical properties of copper nanocubes as a function of size as modeled by the discrete dipole approximation. In the far-field, our results showed that the extinction resonances shifted from 595 to 670 nm as the size increased from 20 to 100 nm. Also, the highest optical efficiencies for absorption and scattering were obtained for nanocubes that were 60 and 100 nm in size, respectively. In the near-field, the electric-field amplitudes were investigated considering 514, 633 and 785 nm as the excitation wavelengths. The E-fields increased with size, being the highest at 633 nm.
    Chemical Physics Letters 08/2012; 544:64–69. · 2.15 Impact Factor
  • Marcos V. Petri, Rômulo A. Ando, Pedro H.C. Camargo
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    ABSTRACT: This Letter reports on the synthesis of Ag–Au nanoparticles (NPs) with controlled structures and compositions via a galvanic replacement reaction between Ag NPs and AuCl4-(aq) followed by the investigation of their optical and catalytic properties. Our results showed the formation of porous walls, hollow interiors and increased Au content in the Ag–Au NPs as the volume of AuCl4-(aq) employed in the reaction was increased. These variations led to a red shift and broadening of the SPR peaks and an increase of up to 10.9-folds in the catalytic activity towards the reduction of 4-nitrophenol relative to Ag NPs.
    Chemical Physics Letters 04/2012; 531:188–192. · 2.15 Impact Factor
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    ABSTRACT: This paper describes a surface‐enhanced Raman scattering (SERS) systematic investigation regarding the functionalization of gold (Au) and silver (Ag) nanoparticles with diphenyl dichalcogenides, i.e. diphenyl disulfide, diphenyl diselenide, and diphenyl ditelluride. Our results showed that, in all cases, functionalization took place with the cleavage of the chalcogen–chalcogen bond on the surface of the metal. According to our density functional theory calculations, the molecules assumed a tilted orientation with respect to the metal surface for both Au and Ag, in which the angle of the phenyl ring relative to the metallic surface decreased as the mass of the chalcogen atom increased. The detected differences in the ordinary Raman and SERS spectra were assigned to the distinct stretching frequencies of the carbon–chalcogen bond and its relative contribution to the ring vibrational modes. In addition, the SERS spectra showed that there was no significant interaction between the phenyl ring and the surface, in agreement with the tilted orientation observed from our density functional theory calculations. The results described herein indicate that diphenyl dichalcogenides can be successfully employed as starting materials for the functionalization of Au nanoparticles with organosulfur, organoselenium, and organotellurium compounds. On the other hand, diphenyl disulfide and diphenyl diselenide could be employed for the functionalization of Ag nanoparticles, while the partial oxidation of the organotellurium unit could be detected on the Ag surface. Copyright © 2011 John Wiley & Sons, Ltd.
    Journal of Raman Spectroscopy 01/2012; 43(6). · 2.68 Impact Factor
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    ABSTRACT: This article describes a systematic study of the galvanic replacement reaction between PtCl(6)(2-) ions and Pd nanocrystals with different shapes, including cubes, cuboctahedrons, and octahedrons. It was found that Br(-) ions played an important role in initiating, facilitating, and directing the replacement reaction. The presence of Br(-) ions led to the selective initiation of galvanic replacement from the {100} facets of Pd nanocrystals, likely due to the preferential adsorption of Br(-) ions on this crystallographic plane. The site-selective galvanic replacement resulted in the formation of Pd-Pt bimetallic nanocrystals with a concave structure owing to simultaneous dissolution of Pd atoms from the {100} facets and deposition of the resultant Pt atoms on the {111} facets. The Pd-Pt concave nanocubes with different weight percentages of Pt at 3.4, 10.4, 19.9, and 34.4 were also evaluated as electrocatalysts for the oxygen reduction reaction (ORR). Significantly, the sample with a 3.4 wt.% of Pt exhibited the largest specific electrochemical surface area and was found to be four times as active as the commercial Pt/C catalyst for the ORR in terms of equivalent Pt mass.
    Journal of the American Chemical Society 03/2011; 133(15):6078-89. · 10.68 Impact Factor
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    ABSTRACT: This paper studies the selectivity of well-defined Au and Ag nanostructures as substrates for the SERS (surface-enhanced Raman scattering) detection of simazine (6-chloro-N,N′-diethyl-1,3,5-triazine-2,4-diamine) and atrazine (6-chloro-N-ethyl-N′-isopropyl-1,3,5-triazine-2,4-diamine). Our data showed that simazine and atrazine displayed similar SERS spectra when the Au was employed as substrate. Conversely, distinct SERS signatures were obtained upon the utilization of Ag substrates. Density functional theory (DFT) calculations and vibrational assignments suggested that, while simazine and atrazine adsorbed on Au via the N3 position of the triazine ring, simazine adsorbed on Ag via N3 and atrazine via N5. The results presented herein demonstrated that the adsorption geometry of analyte molecules can play a central role over substrate selectivity in SERS, which is particularly important in applications involving ultrasensitive analysis of mixtures containing structurally similar molecules.
    The Journal of Physical Chemistry C. 02/2011; 115(10).
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    ABSTRACT: This paper describes a systematic study on the epitaxial overgrowth of Pt on well-defined Pd nanocrystals with different shapes (and exposed facets), including regular octahedrons, truncated octahedrons, and cubes. Two different reducing agents, i.e., citric acid and L-ascorbic acid, were evaluated and compared for the reduction of K₂PtCl₄ in an aqueous solution in the presence of Pd nanocrystal seeds. When citric acid was used as a reducing agent, conformal overgrowth of octahedral Pt shells on regular and truncated octahedrons of Pd led to the formation of Pd-Pt core-shell octahedrons, while non-conformal overgrowth of Pt on cubic Pd seeds resulted in the formation of an incomplete octahedral Pt shell. On the contrary, localized overgrowth of Pt branches was observed when L-ascorbic acid was used as a reducing agent regardless of the facets expressed on the surface of Pd nanocrystal seeds. This work shows that both the binding affinity of a reducing agent to the Pt surface and the reduction kinetics for a Pt precursor play important roles in determining the mode of Pt overgrowth on Pd nanocrystal surface.
    Nanoscale 11/2010; 2(11):2406-11. · 6.23 Impact Factor
  • Eun Chul Cho, Sung-Wook Choi, Pedro H C Camargo, Younan Xia
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    ABSTRACT: This article describes a simple method for controlling the assembly of gold nanoparticles (Au NPs) into chainlike structures with tunable lengths and interparticle separations. The chainlike assemblies were induced to form by adding HS(CH(2))(n)COOH (n = 2, 10, and 15) into a suspension of Au NPs in a mixture of ethanol and water (98:2 by volume). The number of Au NPs in the chainlike assemblies could be altered by varying the concentration of the thiol while the interparticle distance between Au NPs in the chain could be adjusted by using thiols of different chain lengths. The chainlike assemblies of Au NPs were structurally unstable during storage and purification. We solved this problem by fixing the chainlike assemblies with silica coating via the Stober method or by encapsulating them inside gelatin microspheres with the use of a fluidic device. After fixing, the chainlike assemblies of Au NPs could be preserved for a long period of time, during which their characteristic optical properties remained unchanged.
    Langmuir 03/2010; 26(12):10005-12. · 4.38 Impact Factor
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    ABSTRACT: This perspective article highlights recent developments in a class of surface-enhanced Raman scattering (SERS) experiments that aim to correlate SERS enhancement factors with the physical parameters of metal nanostructures. In a typical study, the SERS substrate is fabricated by depositing colloidal nanoparticles on a silicon wafer to obtain individual particles isolated from each other, or small aggregates such as dimeric units. With the help of registration marks, the same nanoparticle, or dimer of nanoparticles, can be quickly located under a Raman microscope (for SERS spectra) and a scanning electron microscope (for structural characterization). The nanoscale characterization achieved by these studies has resulted in unparalleled investigations into the nature of polarization dependency for SERS, the hot spot nature of single nanoparticles and dimers, and the manipulation of hot spots through shape-controlled synthesis and self-assembly. We discuss the new insights these studies have offered, and the future progress they can deliver to the advancement of SERS.
    Journal of Physical Chemistry Letters 02/2010; 1(4):696-703. · 6.59 Impact Factor
  • Eun Chul Cho, Pedro H C Camargo, Younan Xia
    Advanced Materials 02/2010; 22(6):744-8. · 14.83 Impact Factor
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    ABSTRACT: We describe a systematic investigation on the SERS enhancement factors of individual dimers (EF(dimer)) constructed from two Ag nanocubes that display a face-to-face, edge-to-face, or edge-to-edge structure. The highest field-enhancements were obtained for the dimers displaying a face-to-face and edge-to-face configuration. In these two systems, EF(dimer) was insensitive the dimer geometry and corresponded to 2.0×10(7) and 1.5×10(7), respectively. However, EF(dimer) was decreased to 5.6×10(6) for the edge-to-edge structure. These variations in the detected field-enhancements could be explained based on the relative orientation of the nanocubes and the number of probe molecules enclosed in the hot-spot region for each dimer configuration.
    Chemical Physics Letters 01/2010; 484(4-6):304-308. · 2.15 Impact Factor
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    ABSTRACT: This Letter describes a simple, aqueous-phase route to the synthesis of Pd–Rh bimetallic nanodendrites consisting of Rh branches anchored to a Pd nanocrystal core. Palladium nanocrystals with various shapes, including truncated octahedron, cube, octahedron, and thin plate, have all been successfully employed as seeds to grow Rh branches via the reduction of Na3RhCl6 with l-ascorbic acid in an aqueous solution. The degree of Rh branching could be controlled by varying the concentration of Na3RhCl6 involved in a synthesis. Electron microscopy analysis revealed that growth of Rh branches proceeded via attachment of small Rh particles that had been formed via homogeneous nucleation in solution.
    Chemical Physics Letters 01/2010; 494(4):249-254. · 2.15 Impact Factor
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    Angewandte Chemie International Edition 11/2009; 49(1):164-8. · 11.34 Impact Factor
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    ABSTRACT: This paper describes a systematic study of the surface-enhanced Raman scattering (SERS) activity of hot spots formed between a Ag nanowire and a Ag nanocube with sharp corners. We investigated two distinct dimer structures: (i) a nanocube having one side face nearly touching the side face of a nanowire, and (ii) a nanocube having one edge nearly touching the side face of a nanowire. The field enhancements for the dimers displayed a strong dependence on laser polarization, and the strongest SERS intensities were observed for polarization along the hot-spot axis. Moreover, the detected SERS intensities were dependent on the hot-spot structure, i.e., the relative orientation of the Ag nanocube with respect to the nanowire's side face. When the dimer had a face-to-face configuration, the enhancement factor EF(dimer) was 1.4 x 10(7). This corresponds to 22-fold and 24-fold increases compared to those for individual Ag nanowires and nanocubes, respectively. Conversely, when the dimer had an edge-to-face configuration, EF(dimer) was 4.3 x 10(6). These results demonstrated that the number of probe molecules adsorbed at the hot spot played an important role in determining the detected SERS intensities. EF(dimer) was maximized when the dimer configuration allowed for a larger number of probe molecules to be trapped within the hot-spot region.
    Nanotechnology 10/2009; 20(43):434020. · 3.84 Impact Factor

Publication Stats

1k Citations
311.09 Total Impact Points

Institutions

  • 2012–2014
    • University of São Paulo
      • Departamento de Química Fundamental (IQ) (São Paulo)
      San Paulo, São Paulo, Brazil
  • 2008–2011
    • Washington University in St. Louis
      • Department of Biomedical Engineering
      Saint Louis, MO, United States
  • 2007–2008
    • University of Washington Seattle
      • Department of Chemistry
      Seattle, WA, United States
    • Yonsei University
      • Department of Materials Science and Engineering
      Seoul, Seoul, South Korea
  • 2003–2008
    • Universidade Federal do Paraná
      • Department of Chemistry
      Pontal do Paraná, Paraná, Brazil