"Small-particle limit" the second harmonic generation from noble metal nanoparticles

In this paper, we have probed the origin of SHG in copper nanoparticles by polarization-resolved hyper-Rayleigh scattering (HRS). Results obtained with various sizes of copper nanoparticles at four different wavelengths covering the wavelength range 738-1907 nm reveal that the origin of second harmonic generation (SHG) in these particles is purely dipolar in nature as long as the size (d) of the particles remains smaller compared to the wavelength (;.) of light ("small-particle limit"). However, contribution of the higher order multipoles coupled with retardation effect becomes apparent with an increase in the d/lambda ratio. We have identified the "small-particle limit" in the second harmonic generation from noble metal nanoparticles by evaluating the critical d/lambda ratio at which the retardation effect sets in the noble metal nanoparticles. We have found that the second-order nonlinear optical property of copper nanoparticles closely resembles that of gold, but not that of silver. (C) 2009 Elsevier B.V. All rights reserved.

The Surface Plasmon Resonance of Supported Noble Metal Nanoparticles: Characterization, Laser Tailoring, and SERS Application

This work deals with the optical properties of supported noble metal nanoparticles, which are dominated by the so-called Mie resonance and are strongly dependent on the particles’ morphology. For this reason, characterization and control of the dimension of these systems are desired in order to optimize their applications. Gold and silver nanoparticles have been produced on dielectric supports like quartz glass, sapphire and rutile, by the technique of vapor deposition under ultra-high vacuum conditions. During the preparation, coalescence is observed as an important mechanism of cluster growth. The particles have been studied in situ by optical transmission spectroscopy and ex situ by atomic force microscopy. It is shown that the morphology of the aggregates can be regarded as oblate spheroids. A theoretical treatment of their optical properties, based on the quasistatic approximation, and its combination with results obtained by atomic force microscopy give a detailed characterization of the nanoparticles. This method has been compared with transmission electron microscopy and the results are in excellent agreement. Tailoring of the clusters’ dimensions by irradiation with nanosecond-pulsed laser light has been investigated. Selected particles are heated within the ensemble by excitation of the Mie resonance under irradiation with a tunable laser source. Laser-induced coalescence prevents strongly tailoring of the particle size. Nevertheless, control of the particle shape is possible. Laser-tailored ensembles have been tested as substrates for surface-enhanced Raman spectroscopy (SERS), leading to an improvement of the results. Moreover, they constitute reproducible, robust and tunable SERS-substrates with a high potential for specific applications, in the present case focused on environmental protection. Thereby, these SERS-substrates are ideally suited for routine measurements.

Noble-metal nanoparticles produced with colloidal lithography: fabrication, optical properties and applications

In this work, metal nanoparticles produced by nanosphere lithography were studied in terms of their optical properties (in connection to their plasmon resonances), their potential application in sensing platforms - for thin layer sensing and bio-recognition events -, and for a particular case (the nanocrescents), for enhanced spectroscopy studies. The general preparation procedures introduced early in 2005 by Shumaker-Parry et al. to produce metallic nanocrescents were extended to give rise to more complex (isolated) structures, and also, by combining colloidal monolayer fabrication and plasma etching techniques, to arrays of them. The fabrication methods presented in this work were extended not only to new shapes or arrangements of particles, but included also a targeted surface tailoring of the substrates and the structures, using different thiol and silane compounds as linkers for further attachment of, i.e. polyelectrolyte layers, which allow for a controlled tailoring of their nanoenvironment. The optical properties of the nanocrescents were studied with conventional transmission spectroscopy; a simple multipole model was adapted to explain their behaviour qualitatively. In terms of applications, the results on thin film sensing using these particles show that the crescents present an interesting mode-dependent sensitivity and spatial extension. Parallel to this, the penetrations depths were modeled with two simplified schemes, obtaining good agreement with theory. The multiple modes of the particles with their characteristic decay lengths and sensitivities represent a major improvement for particle-sensing platforms compared to previous single resonance systems. The nanocrescents were also used to alter the emission properties of fluorophores placed close to them. In this work, green emitting dyes were placed at controlled distances from the structures and excited using a pulsed laser emitting in the near infrared. The fluorescence signal obtained in this manner should be connected to a two-photon processes triggered by these structures; obtaining first insight into plasmon-mediated enhancement phenomena. An even simpler and faster approach to produce plasmonic structures than that for the crescents was tested. Metallic nanodiscs and nanoellipses were produced by means of nanosphere lithography, extending a procedure reported in the literature to new shapes and optical properties. The optical properties of these particles were characterized by extinction spectroscopy and compared to results from the literature. Their major advantage is that they present a polarization-dependent response, like the nanocrescents, but are much simpler to fabricate, and the resonances can be tailored in the visible with relative ease. The sensing capabilities of the metallic nanodiscs were explored in the same manner as for the nanocrescents, meaning their response to thin layers and to bio-recognition events on their surface. The sensitivity of these nanostructures to thin films proved to be lower than that of the crescents, though in the same order of magnitude. Experimental information about the near field extension for the Au nanodiscs of different sizes was also extracted from these measurements. Further resonance-tailoring approaches based on electrochemical deposition of metals on the nanodiscs were explored, as a means of modifying plasmon resonances by changing surface properties of the nanoparticles. First results on these experiments would indicate that the deposition of Ag on Au on a submonolayer coverage level can lead to important blue-shifts in the resonances, which would open a simple way to tailor resonances by changing material properties in a local manner.

Capillary microreactors based on hierarchical SiO2 monoliths incorporating noble metal nanoparticles for the Preferential Oxidation of CO

Novel hierarchical SiO2 monolithic microreactors loaded with either Pd or Pt nanoparticles have been prepared in fused silica capillaries and tested in the Preferential Oxidation of CO (PrOx) reaction. Pd and Pt nanoparticles were prepared by the reduction by solvent method and the support used was a mesoporous SiO2 monolith prepared by a well-established sol–gel methodology. Comparison of the activity with an equivalent powder catalyst indicated that the microreactors show an enhanced catalytic behavior (both in terms of CO conversion and selectivity) due to the superior mass and heat transfer processes that take place inside the microchannel. TOF values at low CO conversions have been found to be ∼2.5 times higher in the microreactors than in the powder catalyst and the residence time seems to have a noticeable influence over the selectivity of the catalysts designed for this reaction. The Pd and Pt flexible microreactors developed in this work have proven to be effective for the CO oxidation reaction both in the presence and absence of H2, standing out as a very interesting and suitable option for the development of CO purification systems of small dimensions for portable and on-board applications.

Selective photo-oxidation of cellobiose with tio2-supported metal nanoparticles

Upgrade of biomass to valuable chemicals is a central topic in modern research due to the high availability and low price of this feedstock. For the difficulties in biomass treatment, different pathways are still under investigation. A promising way is in the photodegradation, because it can lead to greener transformation processes with the use of solar light as a renewable resource. The aim of my work was the research of a photocatalyst for the hydrolysis of cellobiose under visible irradiation. Cellobiose was selected because it is a model molecule for biomass depolymerisation studies. Different titania crystalline structures were studied to find the most active phase. Furthermore, to enhance the absorption of this semiconductor in the visible range, noble metal nanoparticles were immobilized on titania. Gold and silver were chosen because they present a Surface Plasmon Resonance band and they are active metals in several photocatalytic reactions. The immobilized catalysts were synthesized following different methods to optimize the synthetic steps and to achieve better performances. For the same purpose the alloying effect between gold and silver nanoparticles was examined.

Silica coated noble metal nanoparticle hydrosols as supported catalyst precursors

Synthesis of well-defined nanoparticles has been intensively pursued not only for their fundamental scientific interest, but also for many technological applications. One important development of the nanomaterial is in the area of chemical catalysis. We have now developed a new aqueous-based method for the synthesis of silica encapsulated noble metal nanoparticles in controlled dimensions. Thus, colloid stable silica encapsulated similar to 5 nm platinum nanoparticle is synthesized by a multi-step method. The thickness of the silica coating could be controlled using a different amount of silica precursor. These particles supported on a high surface area alumina are also demonstrated to display a superior hydrogenation activity and stability against metal sintering after thermal activation.

In situ synthesis and catalytic activity in CO oxidation of metal nanoparticles supported on porous nanocrystalline silicon

Reactive surface of mesoporous nanocrystalline silicon was used to synthesise noble metal nanoparticles via in situ reduction of the precursor salt solutions. The synthetic methodology for metal nanoparticle formation was systematically developed, and reaction conditions of metal salts reduction were optimised to prepare nanoparticles of controlled size distribution in the order 5–10 nm inside the mesoporous silicon template. CO oxidation was used as a test reaction for the synthesised Pt/porous silicon catalysts. Sharp reaction light-off was observed at about 120 °C on the optimised catalysts. The catalysts were shown to be stable in the extended steady-state runs and in the catalysts re-use experiments. Metal nanoparticles were shown to be stable to sintering at elevated temperatures up to 1000 °C. However, after thermal treatment on air, Pt nanoparticles were covered by a SiOx layer and were less active in CO oxidation.

Catalytic decomposition of formic acid using supported metal nanoparticles

Upgrade of hydrogen to valuable fuel is a central topic in modern research due to its high availability and low price. For the difficulties in hydrogen storage, different pathways are still under investigation. A promising way is in the liquid-phase chemical hydrogen storage materials, because they can lead to greener transformation processes with the on line development of hydrogen for fuel cells. The aim of my work was the optimization of catalysts for the decomposition of formic acid made by sol immobilisation method (a typical colloidal method). Formic acid was selected because of the following features: it is a versatile renewable reagent for green synthesis studies. The first aim of my research was the synthesis and optimisation of Pd nanoparticles by sol-immobilisation to achieve better catalytic performances and investigate the effect of particle size, oxidation state, role of stabiliser and nature of the support. Palladium was chosen because it is a well-known active metal for the catalytic decomposition of formic acid. Noble metal nanoparticles of palladium were immobilized on carbon charcoal and on titania. In the second part the catalytic performance of the “homemade” catalyst Pd/C to a commercial Pd/C and the effect of different monometallic and bimetallic systems (AuxPdy) in the catalytic formic acid decomposition was investigated. The training period for the production of this work was carried out at the University of Cardiff (Group of Dr. N. Dimitratos).

Noble metal photocatalysts under visible light and UV light irradiation

One of the greatest challenges for the study of photocatalysts is to devise new catalysts that possess high activity under visible light illumination. This would allow the use of an abundant and green energy source, sunlight, to drive chemical reactions. Gold nanoparticles strongly absorb both visible light and UV light. It is therefore possible to drive chemical reactions utilising a significant fraction of full sunlight spectrum. Here we prepared gold nanoparticles supported on various oxide powders, and reported a new finding that gold nanoparticles on oxide supports exhibit significant activity for the oxidation of formaldehyde and methanol in the air at ambient temperature, when illuminated with visible light. We suggested that visible light can greatly enhance local electromagnetic fields and heat gold nanoparticles due to surface plasmon resonance effect which provides activation energy for the oxidation of organic molecules. Moreover, the nature of the oxide support has an important influence on the activity of the gold nanoparticles. The finding reveals the possibility to drive chemical reactions with sunlight on gold nanoparticles at ambient temperature, highlighting a new direction for research on visible light photocatalysts. Gold nanoparticles supported on oxides also exhibit significant dye oxidation activity under visible light irradiation in aqueous solution at ambient temperature. Turnover frequencies of the supported gold nanoparticles for the dye degradation are much higher than titania based photocatalysts under both visible and UV light. These gold photocatalysts can also catalyse phenol degradation as well as selective oxidation of benzyl alcohol under UV light. The reaction mechanism for these photocatalytic oxidations was studied. Gold nanoparticles exhibit photocatalytic activity due to visible light heating gold electrons in 6sp band, while the UV absorption results in electron holes in gold 5d band to oxidise organic molecules. Silver nanoparticles also exhibit considerable visible light and UV light absorption due to surface plasmon resonance effect and the interband transition of 4d electrons to the 5sp band, respectively. Therefore, silver nanoparticles are potentially photocatalysts that utilise the solar spectrum effectively. Here we reported that silver nanoparticles at room temperature can be used to drive chemical reactions when illuminated with light throughout the solar spectrum. The significant activities for dye degradation by silver nanoparticles on oxide supports are even better than those by semiconductor photocatalysts. Moreover, silver photocatalysts also can degrade phenol and drive the oxidation of benzyl alcohol to benzaldehyde under UV light. We suggested that surface plasmon resonance effect and interband transition of silver nanoparticles can activate organic molecule oxidations under light illumination.

Research progress on noble metal composite nanoparticles

Noble metal composite nanoparticles, as attractive building blocks of advanced functional materials, have received enormous attentions due to their specific optical, electronic and catalytic properties that are distant from those of the corresponding monometal nanoparticles. Such materials have important applications in such areas as sensors, optical materials, catalysis and biology, and developed into an increasingly important research area in nanomaterials science. This article reviews the recent progress in the synthesis, properties, and applications of noble metal composite nanoparticles with core-shell, heterostructure, and alloy structure.

On the Mechanism of Phase Transfer Catalysis in Brust-Schiffrin Synthesis of Metal Nanoparticles

The two-phase Brust-Schiffrin method (BSM) is used to synthesize highly stable nanoparticles of noble metals. A phase transfer catalyst (PTC) is used to bring in aqueous phase soluble precursors into the organic phase to enable particle synthesis there. Two different mechanisms for phase transfer are advanced in the literature. The first mechanism considers PTC to bring in an aqueous phase soluble precursor by complexing with it. The second mechanism considers the ionic species to be contained in inverse micelles of PTC, with a water core inside. A comprehensive experimental study involving measurement of interfacial tension, viscosity, water content by Karl-Fischer titration, static light scattering, H-1 NMR, and small-angle X-ray scattering is reported in this work to establish that the phase transfer catalyst tetraoctylammonium bromide transfers ions by complexing with them, instead of encapsulating them in inverse micelles. The findings have implications for particle synthesis in two-phase methods such as BSM and their modification to produce more monodispersed particles.

An effective hydrothermal route for the synthesis of multiple PDDA-protected noble-metal nanostructures

In this Article, we demonstrate an effective hydrothermal route for the synthesis of multiple PDDA-protected (PDDA = poly(diallyl dimethylammonium) chloride) noble-metal (including silver, platinum, palladium, and gold) nanostructures in the absence of any seeds and surfactants, in which PDDA, an ordinary and water-soluble polyelectrolyte, acts as both a reducing and a stabilizing agent. Under optimal experimental conditions, Ag nanocubes, Pt and Pd nanopolyhedrons, and Au nanoplates can be obtained, which were characterized by transmission electron microscopy, scanning electron microscopy, energy-dispersive spectroscopy, and X-ray diffraction. More importantly, the nanostrucfures synthesized show potential applications in surface-enhanced Raman scattering and electrocatalysis, in which Ag nanocubes and Pt nanopolyhedrons were chosen as the examples, respectively.

New catalysts for organic synthesis driven by light and efficient sorbents for removal of radioactive ions from water

The body of the thesis contained two separate elements which made an original contribution to fundamental understanding in the areas of photocatalysis, chemical synthesis and water treatment. Research on chemical reactions catalyzed by noble metal nanoparticles (such as gold) or surface complex grafted metal oxides which can be driven by sunlight at ambient temperature and the second element on radioactive cesium (137Cs+) cations and iodine (125I-) anions recovery by the unique structural features of titanate nanostructures for firmly capture and safe storage; the works has been all published in journals that are rated at the top of their respective fields.