Hybrid Photocatalysts with Earth-Abundant Plasmonic Materials

Plasmonic catalysis utilises light energy to drive chemical reactions. Compared to conventional catalytic processes, which are run by high temperatures and pressures, light-driven processes can lower energy consumption and increase selectivity. Conventional plasmonic nanoparticles (Ag, Au) are relatively scarce and expensive, and therefore the use of materials with earth-abundant elements in plasmonic catalysis is widely pursued. Despite their good optical properties, plasmonic nanoparticles are often unsuitable catalysts. Hybrid catalysts, structures consisting of a light-harvesting plasmonic part and a catalytical centre of different material, have emerged as an opportunity to address these challenges and obtain desired properties. This thesis consists of two parts: In the first part, properties of plasmonic materials are described, and previous studies of hybrid catalysts with earth-abundant plasmonic materials are reviewed. Experimental work on plasmonic-catalytic nanohybrids, with TiN as the plasmonic part and Pd as the catalytic entity, is described in the second part. In this context, a Pd/TiN (Pd nanoparticles supported into TiN) catalyst was synthesised, characterised and applied to test catalytical reactions. Contrary to the hypothesis, light-induced rate enhancement was not observed in our current catalytical studies. These results call for further optimisation of synthesis and reaction conditions to prepare an earth-abundant, light-active catalyst

Photophysics of photothermal activation of plasmonic nanostructures

This thesis primarily focuses on photophysics of charge and energy transfer in plasmonic structures. After an initial study of photothermal catalysis using silver bromide for methane activation which found the process to be unsuitable at 1 bar, the focus pivoted towards plasmonic nanostructures for photothermal applications. Charge transfer across a 4-mercaptobenzoic acid linker between silver nanocubes and cerium dioxide was examined using Raman spectroscopy, this found an increase in charge transfer with thicker shells and higher energy light. The photophysics of a similar system were then probed, using time-resolved spectroscopy and gold nanoparticles embedded in a cerium praseodymium mixed metal oxide. That study indicated that charge and energy transfer could occur and were influenced by the composition: with ballistic charge transfer dominating in cerium oxide, chemical interfacial damping being prevalent with low levels of praseodymium, and plasmon induced resonant energy transfer occurring with higher loadings of praseodymium. A final study is, at first glance, somewhat of a non-sequitur and examines the photophysics of hafnium nitride. In existing literature there was a discrepancy between theoretical and experimental work regarding the lifetime of photo-excited electrons in hafnium nitride, in this study this discrepancy is resolved and shows that the photo-excited electrons rapidly (<50 fs) couple to the lattice and generate heat. Final remarks suggest linking the work on hafnium nitride to the gold/mixed-metal-oxide work to examine the potential energy transfer

Plasmonics and its Applications

Plasmonics is a rapidly developing field that combines fundamental research and applications ranging from areas such as physics to engineering, chemistry, biology, medicine, food sciences, and the environmental sciences. Plasmonics appeared in the 1950s with the discovery of surface plasmon polaritons. Plasmonics then went through a novel propulsion in the mid-1970s, when surface-enhanced Raman scattering was discovered. Nevertheless, it is in this last decade that a very significant explosion of plasmonics and its applications has occurred. Thus, this book provides a snapshot of the current advances in these various areas of plasmonics and its applications, such as engineering, sensing, surface-enhanced fluorescence, catalysis, and photovoltaic devices

High-Yield Synthesis and Applications of Anisotropic Gold Nanoparticles

This work will describe research directed towards the synthesis of anisotropic gold nanoparticles as well as their functionalization and biological applications. The thesis will begin by describing a new technique for the high-yield synthesis of gold nanorods using hydroquinone as a reducing agent. This addresses important limitations of the traditional nanorod synthesis including low yield of gold ions conversion to metallic form and inability to produce rods with longitudinal surface plasmon peak above 850 nm. The use of hydroquinone was also found to improve the synthesis of gold nanowires via the nanorod-seed mediated procedure developed in our lab. The thesis will next present the synthesis of novel starfruit-shaped nanorods, mesorods, and nanowires using a modified nanorod-seed mediated procedure. The starfruit particles displayed increased activity as surface-enhanced Raman spectroscopy (SERS) substrates as compared to smooth structures. Next, a method for the functionalization of gold nanorods using a cationic thiol, 16-mercaptohexadecyltrimethylammonium bromide (MTAB), will be described. By using this thiol, we were able to demonstrate the complete removal of toxic surfactant from the nanorods and were also able to precisely quantify the grafting density of thiol molecules on the nanorod surface through a combination of several analytical techniques. Finally, this thesis will show that MTAB-functionalized nanorods are nontoxic and can be taken up in extremely high numbers into cancer cells. The thesis will conclude by describing the surprising uptake of larger mesorods and nanowires functionalized with MTAB into cells in high quantities

Spectroscopy-Based Biosensors

Biosensors are analytical devices capable of providing quantitative or semi-quantitative information by using a biological recognition element and a transducer. Depending upon the nature of the recognition element, different surface sensitive techniques can be applied to monitor these molecular interactions. In order to increase sensitivities and to lower detection limits down to even individual molecules, nanomaterials are promising candidates. This is possible due to the potential to immobilize more bioreceptor units at reduced volumes and their ability to act as transduction elements by themselves. Among such nanomaterials, gold nanoparticles, quantum dots, polymer nanoparticles, carbon nanotubes, nanodiamonds, and graphene are intensively studied. Biosensors provide rapid, real-time, accurate, and reliable information about the analyte under investigation and have been envisioned in a wide range of analytical applications, including medicine, food safety, bioprocessing, environmental/industrial monitoring, and electronics. A variety of biosensors, such as optical, spectroscopic, molecular, thermal, and piezoelectric, have been studied and applied in countless fields. In this book, examples of spectroscopic and optical biosensors and immunoassays are presented. Furthermore, two comprehensive reviews on optical biosensors are include

Size and composition dependent electrochemical oxidation and deposition of metal nanostructures.

This dissertation describes (1) size-dependent electrochemical oxidation/stripping of gold and silver nanoparticles (NPs), (2) alloying of copper with gold nanoparticles at underpotential deposition potentials, (3) electrochemical characterization of Au/Ag core-shell structures, (4) characterization of metal nanoparticle alloys by stripping voltammetry, and (5) layer-by-layer assembly of metal nanoparticle/polymer structures. The motivation of this work is to better understand fundamental properties of metal nanostructures as a function of size, shape, and composition. We synthesized Au and Ag NPs with different size by electrochemical reduction of the metal salt directly on the electrode surface and by seed-mediated growth in solution followed by chemisorption on a silane functionalized electrode surface, respectively. Linear sweep voltammetry results demonstrated a negative shift in peak potential for oxidation with decrease in size. For Ag NPs, the oxidation potential is 275 mV and 382 mV for 8 and 50 nm particles, respectively. In the case of Au NPs, the peak potentials are 734 and 913 mV for 4 and 250 nm particles, respectively. This shift in oxidation potential with change in size of metal nanoparticles is consistent with Plieth theory. Underpotential deposition of copper on Au NPs of different size led to alloying of Au and Cu. Several peaks were observed on linear sweep voltammograms. We assigned these peaks to different copper locations in the alloy structure: (1) Cu UPD on the surface of Au NP, (2) outer-shell Cu-Au alloy, and (3) core of Cu-Au NP alloy. Au/Ag core-shell nanostructures were synthesized by seed-mediated growth directly on the electrode surface and characterized with electrochemical techniques. During electrochemical characterization, dealloying of Au from Au/Ag alloy structures occurred by cycling in bromide containing electrolyte solution. Composition analysis based on LSV showed that less than 3% of Au remained on the electrode surface. SEM images showed that the morphology of Au/Ag nanostructures changes after electrochemical oxidation. Particles become bigger and form hollow bulbs , porous structures, and networks. We also synthesized Au/Ag alloy nanoparticles through a high temperature seed-mediated growth procedure and characterized them by UV-Vis and LSV at different stages of synthesis. LSV results provided information about the composition and atomic arrangements of alloy nanoparticles synthesized using 1:1 Au:Ag ratio, but a different synthesis method. After a 24-hour heating time, the (Au 4nm )Ag NPs did not show the oxidation peak for Ag, indicating that it stabilized during the alloy formation. In the case of (Ag 8nm )Au NPs, Ag oxidation peak appeared on LSVs regardless the heating time. Electrochemical characterization and UV-vis spectroscopy results for metal nanoparticle-polymer multilayer films showed that, with increase in the number of metal-polymer layers, absorbance and coverage increases due to an increase of the amount of metal assembled on the surface. A red shift in peak wavelength indicates an increase in size and aggregation of NPs on the electrode surface. SEM analysis shows that the morphology of the film depends on the nature of the metal deposited and the size of NPs. Films of Ag NPs consisted of large aggregated structures on the electrode surface, while films of Au NPs were uniform and porous. Experiments on the electron transfer through the polymer film to the metal NPs, demonstrated that electron transport depends on the number of polymer bilayers and the nature of the NPs. After deposition of 5 polymer bilayers, Au oxidation peak disappeared, while Ag oxidation peak was lower compared to 1 layer, but still observable. This dissertation describes a few sets of experiments on fundamental electrochemical properties of metal nanostructures. The results of these experiments are crucial for the application areas such as catalysis and sensing. It is important to study the stability of these nanoparticles, and also their recycling potential, since it can be affected by changes in the shape and size of the nanoparticles during the course of a reaction. This will not only provide information about electrochemical stability but may also prove useful as a method for analyzing nanoparticles and using them as labels for analytical applications by electrochemical stripping voltammetry

The Optical Properties of Spiky Gold Nanoshells

Plasmonic nanoparticles are a powerful and versatile tool for molecular sensing, drug delivery, and cancer treatment. When exposed to incident light, these nanoparticles have greatly increased far-field scattering and near-field enhancement. Spiky gold nanoshells are a recently developed class of nanoparticles composed of sharp gold spikes decorating a polystyrene core. Spiky nanoshells are synthesized using the templated surfactant-assisted seed growth method, which enables extensive control of the nanoparticle morphology. Here, it is shown that these particles have a tailorable far-field resonance, extremely uniform single-particle surface enhanced Raman scattering, and modal interference in dark-field microscopy measurements. Finite-difference time-domain simulations are performed to determine the morphological features which control these unusual behaviors. Additionally, a T-matrix method was developed to use finite-difference time-domain simulations to analyze mode mixing in these particles. These studies show that the lengths of spikes are critical in determining the far-field scattering peak. Additionally, simulation of the electric field near the particle surface show that the near-field Raman surface enhancement is dominated by the quadrupole modes, resulting in Quadrupole Enhanced Raman Scattering. Due to the large number of spikes, the near-field enhancement is relatively insensitive to variations in individual spikes, resulting in emergent homogeneity in optical properties due to heterogeneity in the structure. The disorder induced asymmetry of the spiky nanoshell enables mode-mixing between the dipole and quadrupole modes, which is observed experimentally in dark-field measurements and predicted theoretically in a T-matrix analysis of finite-difference time-domain simulations. This mode mixing was found to be of the order of 5% between the quadrupole and dipole modes. Such mode mixing is responsible for the broadening of the quadrupole modes towards the infrared and for the activation of all six quadrupole moments, partially explaining how heterogeneity can result in reliable and robust near-field enhancement