Thenner S. Rodrigues

Plasmonic Nanorattles as Next-Generation Catalysts for Surface Plasmon Resonance-Mediated Oxidations Promoted by Activated Oxygen

Nanorattles, comprised of a nanosphere inside a nanoshell, were employed as the next generation of plasmonic catalysts for oxidations promoted by activated O2 . After investigating how the presence of a nanosphere inside a nanoshell affected the electric-field enhancements in the nanorattle relative to a nanoshell and a nanosphere, the SPR-mediated oxidation of p-aminothiophenol (PATP) functionalized at their surface was investigated to benchmark how these different electric-field intensities affected the performances of Au@AgAu nanorattles, AgAu nanoshells and Au nanoparticles having similar sizes. The high performance of the nanorattles enabled the visible-light driven synthesis of azobenzene from aniline under ambient conditions. As the nanorattles allow the formation of electromagnetic hot spots without relying on the uncontrolled aggregation of nanostructures, it enables their application as catalysts in liquid phase under mild conditions using visible light as the main energy input.

Rapid Synthesis of Hollow Ag–Au Nanodendrites in 15 Seconds by Combining Galvanic Replacement and Precursor Reduction Reactions

Metallic nanomaterials displaying hollow interiors as well as sharp tips/branches at their surface (such as hollow nanodendrites) are attractive, because these features enable higher surface-to-volume ratios than their solid and/or rounded counterparts. This paper describes a simple strategy for the synthesis of Ag-Au nanodendrites in 15 s using Ag nanospheres prepared in a previous synthetic step as seeds. Our approach was based on the utilization of Ag nanospheres as seeds for Au deposition by a combination of galvanic replacement reaction between Ag and AuCl4(-)(aq) and AuCl4(-)(aq) reduction using hydroquinone in the presence of polyvinylpyrrolidone (PVP) as a stabilizer and water as the solvent. The produced Ag-Au nanodendrites presented monodisperse sizes, and their surface morphologies could be tuned as a function of growth time. Owing to their hollow interiors and sharp tips, the Ag-Au nanodendrites performed as effective substrates for surface-enhanced Raman scattering (SERS) detection of 4-MPy (4-mercaptopyridine) and R6G (rhodamine 6G) as probe molecules. We believe that the approach described herein can serve as a protocol for the fast and one-step synthesis of Ag-Au hollow nanondendrites with a wide range of sizes, compositions, and surface morphologies for applications in SERS and catalysis.

Ce1−xSmxO1.9−δ nanoparticles obtained by microwave-assisted hydrothermal processing: an efficient application for catalytic oxidation of α-bisabolol

Heterogeneous catalysts based on Sm-doped ceria were employed for the first time in the liquid-phase oxidation of α-bisabolol. Nanometer-sized catalysts were obtained by microwave-hydrothermal synthesis and were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), Raman spectroscopy and N2-physisorption. The influence of Sm doping, temperature and the solvent used on the catalytic behavior was investigated. Conversions up to 84% and a combined selectivity for the products up to 77% were obtained for Ce0.9Sm0.15O1.85−δ catalysts. The reactions were highly selective for the epoxidation products (only bisabolol oxides A and B were obtained) and shown to be strongly dependent on the temperature and solvent employed. Best results were achieved for higher Sm concentrations, which indicate that changes in the textural properties due to doping produced a significantly more active catalyst.

An undergraduate level experiment on the synthesis of Au nanoparticles and their size-dependent optical and catalytic properties

The synthesis of gold nanoparticles (Au NPs) 15, 26, and 34 nm in diameter, followed by the investigation of their size-dependent optical and catalytic properties, is described herein as an undergraduate level experiment. The proposed experiment covers concepts on the synthesis, stabilization, and characterization of Au NPs, their size-dependent optical and catalytic properties at the nanoscale, chemical kinetics, and the role of a catalyst. The experiment should be performed by groups of two or three students in three lab sessions of 3 h each and organized as follows: i) synthesis of Au NPs of different sizes and investigation of their optical properties; ii) evaluation of their catalytic activity; and iii) data analysis and discussion. We believe that this activity enables students to integrate these multidisciplinary concepts in a single experiment as well as to become introduced/familiarized with an active research field and current literature in the areas of nanoparticle synthesis and catalysis.

Rational design of plasmonic catalysts: matching the surface plasmon resonance with lamp emission spectra for improved performance in AgAu nanorings

In order to enable practical applications of SPR-excitation in heterogeneous catalysis, facile procedures for the synthesis of plasmonic catalysts as well as the use of commercially available and inexpensive lamps as the excitation source are highly desirable. In this context, the development of catalysts displaying SPR extinction that matches, as much as possible, the emission spectra of commercially available lamps represent an intuitive strategy to maximize performance. We report the design and facile synthesis of AgAu nanorings displaying SPR extinction that closely matches the emission spectra of a commercial halogen–tungsten lamp. The AgAu nanorings were employed as catalysts for the SPR-mediated oxidation of methylene blue in the liquid phase (water as the solvent), under ambient conditions, and using a halogen–tungsten lamp as the only energy input. The activity of the nanorings was benchmarked against Ag and Au nanospheres. We found that the activity of the nanorings was higher relative to the nanospheres, and that both hot electrons and holes generated as a result of the SPR excitation participated in the methylene blue oxidation reaction with similar relative contributions. Our results show that the rational design of metallic nanostructures plays an important role for enabling practical applications in the field of plasmonic catalysis, in which facile procedures can be employed for the synthesis of the catalysts (attractive for large-scale production) and commercial lamps may be used as the only energy input.

Surface Segregated AgAu Tadpole-Shaped Nanoparticles Synthesized Via a Single Step Combined Galvanic and Citrate Reduction Reaction.

New AgAu tadpole nanocrystals were synthesized in a one-step reaction involving simultaneous galvanic replacement between Ag nanospheres and AuCl4(-)(aq.) and AuCl4(-)(aq.) reduction to Au in the presence of citrate. The AgAu tadpoles display nodular polycrystalline hollow heads, while their undulating tails are single crystals. The unusual morphology suggests an oriented attachment growth mechanism. Remarkably, a 1 nm thick Ag layer was found to segregate so as to cover the entire surface of the tadpoles. By varying the nature of the seeds (Au NPs), double-headed Au tadpoles could also be obtained. The effect of a number of reaction parameters on product morphology were explored, leading to new insights into the growth mechanisms and surface segregation behavior involved in the synthesis of bimetallic and anisotropic nanomaterials.

Hollow AgPt/SiO2 nanomaterials with controlled surface morphologies: is the number of Pt surface atoms imperative to optimize catalytic performances?

We describe herein an investigation on how the number of Pt surface atoms and nature of exposed surface facets affect the catalytic performances of AgPt nanomaterials displaying controlled surface morphologies (smooth or rough surfaces), shapes (spherical or one-dimensional), and hollow interiors towards CO oxidation. More specifically, we focused on AgPt nanoshells (smooth surfaces), assembled nanoparticles (rough surfaces), nanotubes with smooth surfaces, and nanotubes with rough surfaces. We found that their catalytic performances followed the order: nanotubes with smooth surfaces > nanoshells, nanotubes with rough surfaces > assembled nanoparticles. The better catalytic activity observed for the nanoshells relative to the assembled nanoparticles can be associated with their higher number of Pt surface atoms. Even though the nanotubes with rough surfaces had a higher number of Pt surface atoms relative to the nanotubes with smooth surfaces, the latter displayed higher catalytic activities as a result of the preferential exposure of {100} facets, which are the most active towards CO oxidation relative to {111} and {110}. Interestingly, the nanotubes with smooth surfaces also displayed higher catalytic activities when compared to the nanoshells, showing that the preferential exposure of {100} side facets compensated the decrease in their number of Pt surface atoms relative to the nanoshells. Our data showed that the catalytic performances were strongly dependent on the surface morphologies, in which the preferential exposure of more active surface facets may play a significant role in the optimization of performances relative to the number of Pt surface atoms.

Bimetallic Nanoshells as Platforms for Metallo‐ and Biometallo‐Catalytic Applications

The use of gold, silver, platinum and palladium for preparation of bimetallic nanoshells (AgAu, AgPt, and AgPdNSs, respectively) and their use for metallo‐ and bio‐metallo catalytic applications have been described. Bimetallic nanoshells (metallo‐catalysts) were employed for silane oxidation to silanols and hydrogen (H2) production. Fast and efficient oxidation of several silanes was observed after only 1 h at room temperature, by employing AgPd NSs as catalyst, acetone as solvent, and water as oxidant. Interestingly, bio‐metallo‐catalysts (NSs‐CALB) prepared from lipase attachment to the bimetallic nanoshells, displayed promising bi‐catalytic activities (enzymatic: transesterification; metallic: silane oxidation).

Controlled Synthesis: Nucleation and Growth in Solution

The controlled synthesis of metallic nanomaterials in solution is central to realize many applications that arise from their fascinating properties. As properties in metal nanomaterials are strongly dependent upon size, shape, composition, structure (solid versus hollow interiors), and surface functionality, controlled synthesis is a powerful approach to tailor and optimize properties as well as to establish how they are dependent on the several physical and chemical parameters that define a nanostructure. In this context, this chapter focuses on the fundamentals of the controlled synthesis of metal nanomaterials in solution phase in terms of the available theoretical framework. Specifically, it starts by introducing the mechanisms employed for the stabilization of nanomaterials during solution-phase synthesis (Sect. 2.2). The basics of nucleation and growth in solution will be discussed in Sect. 2.3. After that, the shape-controlled synthesis of Ag nanomaterials will be employed as proof-of-concept example of how thermodynamic versus kinetic considerations, oxidative etching, and surface capping can be employed to effectively maneuver the shape of a metal nanocrystal in solution (Sect. 2.4). Finally, some of the current challenges and outlook regarding the controlled synthesis of metal-based nanomaterials will be presented (Sect. 2.5).

Catalytic Properties of AgPt Nanoshells as a Function of Size: Larger Outer Diameters Lead to Improved Performances

We report herein a systematic investigation on the effect of the size of silver (Ag) nanoparticles employed as starting materials over the morphological features and catalytic performances of AgPt nanoshells produced by a combination of galvanic replacement between Ag and PtCl6(2-) and PtCl6(2-) reduction by hydroquinone. More specifically, we focused on Ag nanoparticles of four different sizes as starting materials, and found that the outer diameter, shell thickness, and the number of Pt surface atoms of the produced nanoshells increased with the size of the starting Ag nanoparticles. The produced AgPt nanoshells were supported into SiO2, and the catalytic performances of the AgPt/SiO2 nanocatalysts toward the gas-phase oxidation of benzene, toluene, and o-xylene (BTX oxidation) followed the order: AgPt 163 nm/SiO2 > AgPt 133 nm/SiO2 > AgPt 105 nm/SiO2 > AgPt 95 nm/SiO2. Interestingly, bigger AgPt nanoshell sizes lead to better catalytic performances in contrast to the intuitive prediction that particles having larger outer diameters tend to present poorer catalytic activities due to their lower surface to volume ratios as compared to smaller particles. This is in agreement with the H2 chemisorption results, and can be assigned to the increase in the Pt surface area with size due to the presence of smaller NPs islands at the surface of the nanoshells having larger outer diameters. This result indicates that, in addition to the overall diameters, the optimization of the surface morphology may play an important role over the optimization of catalytic activities in metal-based nanocatalysts, which can be even more pronounced that the size effect. Our data demonstrate that the control over surface morphology play a very important role relative to the effect of size to the optimization of catalytic performances in catalysts based on noble-metal nanostructures.