CO ₂ electroreduction is a promising technology to produce chemicals and fuels from renewable resources. Polycrystalline and nanostructured metals have been tested extensively while less effort has been spent on understanding the performance of bimetallic alloys. In this work, we study compositionally variant, smooth Au-Pd thin film alloys to discard any morphological or mesoscopic effect on the electrocatalytic performance. We find that the onset potential of CO formation exhibits a strong dependence on the Pd content of the alloys. Strikingly, palladium, a hydrogen evolution catalyst with reasonable exchange current density, suppresses hydrogen evolution when alloyed with gold in the presence of CO ₂ . Cyclic voltammetry, in situ surface enhanced infrared absorption spectroscopy, and potential-dependent online product analysis strongly suggest that by alloying Au with Pd a significant increase in the surface coverage of adsorbed CO occurs with increasing Pd content at low overpotentials (e.g., approximately -0.35 V vs RHE). Such an increase in CO coverage suppresses H ₂ evolution due to the lack of vacant active sites. Moreover, the overall increase in the binding energy with the CO ₂ intermediates gained with the addition of Pd increases the CO production at low overpotentials, where polycrystalline Au suffers from poor CO ₂ adsorption and poor selectivity for CO production. These results show that promising CO ₂ reduction electrode materials (e.g., Au) can be alloyed not only to tune the catalyst's activity but also to deliberately decrease the availability of surface sites for competitive H ₂ evolution.

@en

Au-Cu bimetallic thin films with controlled composition were fabricated by magnetron sputtering co-deposition, and their performance for the electrocatalytic reduction of CO ₂ was investigated. The uniform planar morphology served as a platform to evaluate the electronic effect isolated from morphological effects while minimizing geometric contributions. The catalytic selectivity and activity of Au-Cu alloys was found to be correlated with the variation of electronic structure that was varied with tunable composition. Notably, the d-band center gradually shifted away from the Fermi level with increasing Au atomic ratio, leading to a weakened binding energy ofCO, which is consistent with low CO coverage observed in CO stripping experiments. The decrease in theCO binding strength results in the enhanced catalytic activity for CO formation with the increase in Au content. In addition, it was observed that copper oxide/hydroxide species are less stable on Au-Cu surfaces compared to those on the pure Cu surface, where the surface oxophilicity could be critical to tuning the binding strength ofOCHO. These results imply that the altered electronic structure could explain the decreased formation of HCOO ^- on the Au-Cu alloys. In general, the formation of CO and HCOO ^- as main CO ₂ reduction products on planar Au-Cu alloys followed the shift of the d-band center, which indicates that the electronic effect is the major governing factor for the electrocatalytic activity of CO ₂ reduction on Au-Cu bimetallic thin films.

@en

The increasing penetration of nano-products to the market is raising big concerns about the potential toxic and environmental effects of their constituent engineered nanoparticles (ENPs). Contradictory toxicity test results reported in the literature thus far can be explained by differences in the ENP production methods, which can strongly affect nanoparticle purity and therefore the outcome of the tests. In this paper we investigate the toxicity of Ag nanoparticles (AgNPs) produced by spark ablation - a gas-phase technique that can deliver well-defined nanoparticles of high purity - on Lemna minor. Our results show that AgNPs exhibit a toxic behavior at concentrations as low as 5 μg L⁻¹, which is considerably lower compared to the threshold concentrations reported in other studies. This difference can be attributed to the purity of the ENPs used in our measurements, which can release higher concentrations of toxic Ag⁺ ions upon dilution in the test solutions.

@en

The use of disc diffusion susceptibility tests to determine the antibacterial activity of engineered nanoparticles (ENPs) is questionable because their low diffusivity practically prevents them from penetrating through the culture media. In this study, we investigate the ability of such a test, namely the Kirby-Bauer disc diffusion test, to determine the antimicrobial activity of Au and Ag ENPs having diameters from 10 to 40 nm on Escherichia coli cultures. As anticipated, the tests did not show any antibacterial effects of Au nanoparticles (NPs) as a result of their negligible diffusivity through the culture media. Ag NPs on the other hand exhibited a strong antimicrobial activity that was independent of their size. Considering that Ag, in contrast to Au, dissolves upon oxidation and dilution in aqueous solutions, the apparent antibacterial behavior of Ag NPs is attributed to the ions they release. The Kirby-Bauer method, and other similar tests, can therefore be employed to probe the antimicrobial activity of ENPs related to their ability to release ions rather than to their unique size-dependent properties. [Figure not available: see fulltext.].

@en

This work exploits the ability of the spark discharge particle generator (SDG) to produce metallic nanoparticles (NPs) with control over the size, shape and composition, to unravel the plasmonic mechanisms by which NPs can enhance the photoelectrochemical performance of semiconductor photoanodes. Chapter 1 gives an overview of the SDG and the aerosol technology used in this thesis to synthesize the NPs. Chapter 2 summarizes the different aerosol NP immobilization techniques (both on solids and in liquids) and introduces for the first time an electrospray technique to efficiently capture neutral NPs in liquids. In chapter 3, an extensive literature review on plasmonic photoelectrocatalysis is given to introduce the plasmonic mechanisms that are experimentally studied in Chapter 4, 5, 6 and 7. Chapter 4 and 5 are dedicated to study the hot electron injection (HEI) mechanism by which plasmonic NPs create light-induced “hot” charge carriers upon illumination that can drive photoelectrochemical reactions. Chapter 4 reveals that alloying Ag NPs with Au can be used to shift in a control way the absorption and utilization of light to longer wavelengths. However, due to the low interband energy of Au (i.e., 2.3 eV) compared to that of Ag (i.e., 3.6 eV), the alloy NPs exhibited more interband excitations when illuminated with visible light than pure Ag NPs. Such increase in interband excitations resulted in lower hot electron energies and HEI efficiencies in the alloy NPs than in pure Ag NPs. Chapter 5, reveals the HEI size dependency of Ag NPs. It is found that smaller NPs (< 10 nm) where the surface-induced excitations are prominent result in higher HEI efficiencies, while for larger light absorbing NPs (in the range 10-25 nm) a maximum in the performance is found that corresponds well with the size of the Ag NP with the largest nearfield enhancement. Chapter 6, studies the ability of Ag NPs to concentrate and scatter light into thin film semiconductors to enhance their absorption. It is found that most of the solar energy absorbed by pure 15 nm Ag NPs is lost through heat dissipation. However, larger NPs preferentially scatter the incoming light to the neighbour 6 semiconductor, improving its absorption above their band gap energy. Finally, two configurations of plasmonic NP/semiconductor composites were studied to enhance the semiconductor absorption. In the first configuration the NPs were placed at the semiconductor-electrolyte interface and in the second configuration, the NPs were embedded in the semiconductor at the back-contact/semiconductor interface. It was found that an absorption enhancement at the semiconductor/electrolyte interface was better utilized due to the ability of the surface charge layer to efficiently separate the extra electron holes induced by the plasmonic NPs. @en

The conversion of light to electrical and chemical energy has the potential to provide meaningful advances to many aspects of daily life, including the production of energy, water purification, and optical sensing. Recently, plasmonic nanoparticles (PNPs) have been increasingly used in artificial photosynthesis (e.g., water splitting) devices in order to extend the visible light utilization of semiconductors to light energies below their band gap. These nanoparticles absorb light and produce hot electrons and holes that can drive artificial photosynthesis reactions. For n-type semiconductor photoanodes decorated with PNPs, hot charge carriers are separated by a process called hot electron injection (HEI), where hot electrons with sufficient energy are transferred to the conduction band of the semiconductor. An important parameter that affects the HEI efficiency is the nanoparticle composition, since the hot electron energy is sensitive to the electronic band structure of the metal. Alloy PNPs are of particular importance for semiconductor/PNPs composites, because by changing the alloy composition their absorption spectra can be tuned to accurately extend the light absorption of the semiconductor. This work experimentally compares the HEI efficiency from Ag, Au, and Ag/Au alloy nanoparticles to TiO₂ photoanodes for the photoproduction of hydrogen. Alloy PNPs not only exhibit tunable absorption but can also improve the stability and electronic and catalytic properties of the pure metal PNPs. In this work, we find that the Ag/Au alloy PNPs extend the stability of Ag in water to larger applied potentials while, at the same time, increasing the interband threshold energy of Au. This increasing of the interband energy of Au suppresses the visible-light-induced interband excitations, favoring intraband excitations that result in higher hot electron energies and HEI efficiencies.

@en

The electrocatalytic reduction of CO₂ on Au-Pt bimetallic catalysts with different compositions was evaluated, offering a platform for uncovering the correlation between the catalytic activity and the surface composition of bimetallic electrocatalysts. The Au-Pt alloy films were synthesized by a magnetron sputtering co-deposition technique with tunable composition. It was found that the syngas ratio (CO:H₂) on the Au-Pt films is able to be tuned by systematically controlling the binary composition. This tunable catalytic selectivity is attributed to the variation of binding strength of COOH and CO intermediates, influenced by the surface electronic structure (d-band center energy) which is linked to the surface composition of the bimetallic films. Notably, a gradual shift of the d-band center away from the Fermi level was observed with increasing Au content, which correspondingly reduces the binding strength of the COOH and CO intermediates, leading to the distinct catalytic activity for the reduction of CO₂ on the compositionally variant Au-Pt bimetallic films. In addition, the formation of formic acid in the bimetallic systems at reduced overpotentials and higher yield indicates that synergistic effects can facilitate reaction pathways for products that are not accessible with the individual components.

@en

Ag nanoparticles (NPs) are deposited on BiVO4 photoanodes to study their effect on the photoelectrochemical (PEC) water splitting performance of the semiconductor. 15 nm light-absorbing NPs and 65 nm light scattering NPs were studied separately to compare their light trapping ability for enhancing the semiconductor's absorption through light concentration and light scattering, respectively. The 15 nm NPs enhanced the BiVO4 external quantum efficiency throughout the semiconductor's absorption range (e.g., ≈2.5 fold at λ=400 nm). However, when a hole scavenger was added to the electrolyte, no enhancement was observed upon NP deposition, indicating that the NPs only facilitate the injection of holes from the semiconductor surface to the electrolyte but do not enhance its absorption. On the other hand, the 65 nm scattering NPs not only facilitated hole injection to the electrolyte, but also enhanced the absorption of the semiconductor (by ≈6 %) through light scattering. Such a dual effect, i.e., of enhancing both the surface properties and the absorption of the semiconductor, makes light scattering Ag NPs an ideal decoration for PEC water splitting photoelectrodes.
@en

Tungsten oxides (WO_x) thin films are currently used in electro-chromic devices, solar-cells and gas sensors as a result of their versatile and unique characteristics. In this study, we produce nanoparticulate WO_x films by spark ablation and focused inertial deposition, and demonstrate their application for NO₂ sensing. The primary particles in the as-deposited film samples are amorphous with sizes ranging from 10 to 15 nm. To crystallize the samples, the as-deposited films are annealed at 500°C in air. This also caused the primary particles to grow to 30-50 nm by sintering. The morphologies and crystal structures of the resulting materials are studied using scanning and transmission electron microscopy and X-ray diffraction, whereas information on composition and oxidation states are determined by X-ray photoemission spectroscopy. The observed sensitivity of the resistance of the annealed films is ∼100 when exposed to 1 ppm of NO₂ in air at 200°C, which provides a considerable margin for employing them in gas sensors for measuring even lower concentrations. The films show a stable and repeatable response pattern. Considering the numerous advantages of spark ablation for fabricating nanoparticulate thin films, the results reported here provide a promising first step toward the production of high sensitivity and high accuracy sensors.

@en

Photoelectrochemical (PEC) water splitting is a promising technology that uses light absorbing semiconductors to convert solar energy directly into a chemical fuel (i.e., hydrogen). PEC water splitting has the potential to become a key technology in achieving a sustainable society, if high solar to fuel energy conversion efficiencies are obtained with earth abundant materials. This review article discusses recent developments and discoveries in the mechanisms by which the localized surface plasmon resonance (LSPR) in metallic nanoparticles can increase or complement a neighbouring semiconductor in light absorption for catalytic water splitting applications. These mechanisms can mitigate the intrinsic optical limitations of semiconductors (e.g., metal oxides) for efficient solar water splitting. We identify four types of enhancement mechanisms in the recent literature: (i) light scattering, (ii) light concentration, (iii) hot electron injection (HEI), and (iv) plasmon-induced resonance energy transfer (PIRET). (i) Light scattering and (ii) light concentration are light trapping mechanisms that can increase the absorption of light with energies above the semiconductor optical band-edge. These two mechanisms are ideal to enhance the absorption of promising semiconductors with narrow bandgap energies that suffer from limited absorption coefficients and bulk charge recombination. On the other hand, (iii) HEI and the recently discovered (iv) PIRET are mechanisms that can enhance the absorption also below the semiconductor optical band-edge. Therefore, HEI and PIRET have the potential to extend the light utilization to visible and near-infrared wavelengths of semiconductors with excellent electrochemical properties, but with large bandgap energies. New techniques and theories that have been developed to elucidate the above mentioned plasmonic mechanisms are presented and discussed for their application in metal oxide photoelectrodes. Finally, other plasmonic and non-plasmonic effects that do not increase the device absorption, but affect the electrochemical properties of the semiconductor (e.g., charge carrier transport) are also discussed, since a complete understanding of these phenomena is fundamental for the design of an efficient plasmonic NP-semiconductor water splitting device.

@en