420 research outputs found
Review: Doing educational research : a guide for first time researchers
No full textTitle: Doing Educational Research : a guide for first time researchers
Author/Editor: Clive Opie
Publisher: Sage Publications
Publication Date: 2004
ISBN: Paperback 0761970029
Price: £18.99
Reviewed by: Torben Steeg, Independent D&T Consultan
Light-driven transformation processes of anisotropic silver nanoparticles
No full textThe photoinduced formation of silver nanoprisms from smaller silver seed particles in the presence of citrate anions is a classic example of a photomorphic reaction. In this case, light is used as a convenient tool to dynamically manipulate the shape of metal nanoparticles. To date, very little is known about the prevailing reaction mechanism of this type of photoreaction. Here we provide a detailed study of the shape transformation dynamics as a function of a range of different process parameters, such as photon energy and photon flux. For the first time, we provide direct evidence that the photochemical synthesis of silver nanoprisms from spherical seed nanoparticles proceeds via a light-activated two-dimensional coalescence mechanism. On the other hand, we could show that Ostwald ripening becomes the dominant reaction mechanism when larger silver nanoprisms are grown from photochemically synthesized smaller nanoprisms. This two-step reaction proceeds significantly faster and yields more uniform, sharper nanoprisms than the classical one-step photodevelopment process from seeds. The ability to dynamically control nanoparticle shapes and properties with light opens up novel synthesis avenues but also, more importantly, allows one to conceive new applications that exploit the nonstatic character of these nanoparticles and the ability to control and adjust their properties at will in a highly dynamic fashion.George P. Lee, Yichao Shi, Ellen Lavoie, Torben Daeneke, Philipp Reineck, Ute B. Cappel, David M. Huang and Udo Bac
Visual Author-ship: Creativity and Intentionality in Media
No full textBook review of Torben Grodal (ed.): Visual Author-ship: Creativity and Intentionality in Media Northern Lights, vol. 3, 2004, Museum Tusculanum Press/University of Copenhage
Novel redox couples for dye-sensitized solar cells
No full textDye-sensitized solar cells (DSCs) are regarded to be a promising technology with the ability to convert sun light directly into electricity at competitive costs. Unlike conventional solar cells, these devices are not made from high purity semiconductor junctions but from an assembly of functional materials, which under illumination undergo a series of charge transfer reactions leading to charge separation and the generation of electricity. In the typical assembly, a mesoporous wide band gap semiconductor is sensitized by a dye. This sensitized working electrode is in direct contact with an electrolyte containing a redox couple. The electrolyte layer is positioned between the working electrode and the counter electrode which is usually a conductive glass sheet coated with a thin layer of a catalyst. The benefit of the DSC is that each function in the solar cell is carried out by a different material, leading to lower requirements of the materials and allowing convenient fine tuning of each individual component. While most components of the DSC have been thoroughly optimized, the electrolyte still is the limiting factor when it comes to up scaling and commercialization of this technology. In operation the redox couple in the electrolyte has a dual role. It is required to reduce the photooxidized dye and facilitate charge transport between the electrodes to allow for the continuous generation of electricity. To date, most DSC designs make use of the iodide-triiodide redox couple. While this leads to relatively high efficiencies, the complicated two electron redox chemistry of this couple causes intrinsic efficiency losses, limiting the achievable power conversion efficiency. Furthermore, the corrosive nature of I-/I3- complicates the design of durable, large scale solar cells, since cost reducing metal substrates and charge collection grids are dissolved by these electrolytes. As a result, DSCs were frequently efficient in the laboratory, but devices with the commercially sufficient dimensions were often comparatively inefficient and expensive. At the beginning of this work, relatively little was known about the requirements for the ‘perfect’ redox couple to replace I-/I3-. A few alternatives had been published, but all the devices showed comparatively low efficiency. In this thesis, we have investigated iron complexes as DSC redox mediators. The ferrocene / ferrocenium redox couple had been tried previously but the resulting DSCs showed very poor energy conversion efficiency. Ferrocene (biscyclopentadienyl iron) is the widely accepted reference redox couple for the work in non-aqueous solutions and one of the best characterized compounds in the field of electrochemistry. The ferrocene unit has been shown to be highly tunable in its electrochemical properties by derivatization. Furthermore, a wide range of ferrocene derivatives is commercially available. Due to the combination of being well characterized and the availability of a vast compound library, this group of complexes was considered ideal when attempting to define the properties of the ideal DSC redox couple. In this work, high efficiency DSCs are reported that use a ferrocene / ferrocenium redox couple. It was identified that the decomposition of the ferrocenium ion in air leads to rapid device degradation and low efficiencies. After developing a technique that allowed the DSCs to be filled and sealed under the exclusion of oxygen, and optimizing the DSC design for the use with this new electrolyte, conversion efficiencies of up to 7.5 % were achieved. The reference I-/I3- solar cell reached 6.2 %. A series of ferrocene derivatives was then studied to investigate the influence of the redox potential of the redox couple on the charge transfer kinetics in the solar cell. It was identified that a threshold exists until which an increased redox potential directly results into an increased voltage and efficiency. After this threshold, the charge transfer reaction between the photooxidized dye and the redox couple becomes sluggish, leading to increased losses and lower efficiencies. An in depth study, utilizing a range of sensitizers and a library of ferrocene derivatives, allowed us to investigate this crucial charge transfer process. Measurements of the reaction kinetics by nanosecond pulsed laser transient absorption spectroscopy allowed us to estimate that a driving force of 0.25 eV is necessary to achieve quantitative dye regeneration (yield > 99.9 %) for typical electrolyte concentrations. With the knowledge gained from this work, any particular sensitizer can be matched with an appropriate redox couple, reducing energy losses in the solar cell due to the poor alignment of redox potentials. A further focus was the work with aqueous electrolytes containing the ferricyanide / ferrocyanide redox system. The use of water instead of organic solvents has clear benefits due to the abundance of this solvent and environmental considerations. Ferrocyanide is a standard redox couple for aqueous solutions, having a well defined and documented redox chemistry under a wide range of conditions. In this work, we present the first efficient DSCs utilizing this system. Water as an electrolyte solvent for DSCs has been rarely investigated. The influence of parameters, such as pH and buffers, on device performance will have to be studied and the ferricyanide / ferrocyandide redox system forms a solid base for such future work. In summary, a range of iron complexes has been investigated for the use as redox couples for dye-sensitized solar cells resulting in high efficiencies. Certain requirements of the ideal redox couple have been established, allowing to match sensitizers with a redox couple that optimizes the system and minimizes internal energy losses.Awards: Winner of the Mollie Holman Doctoral Medal for Excellence, Faculty of Science, 2012
Novel redox couples for dye-sensitized solar cells
No full textDye-sensitized solar cells (DSCs) are regarded to be a promising technology with the ability to convert sun light directly into electricity at competitive costs. Unlike conventional solar cells, these devices are not made from high purity semiconductor junctions but from an assembly of functional materials, which under illumination undergo a series of charge transfer reactions leading to charge separation and the generation of electricity. In the typical assembly, a mesoporous wide band gap semiconductor is sensitized by a dye. This sensitized working electrode is in direct contact with an electrolyte containing a redox couple. The electrolyte layer is positioned between the working electrode and the counter electrode which is usually a conductive glass sheet coated with a thin layer of a catalyst. The benefit of the DSC is that each function in the solar cell is carried out by a different material, leading to lower requirements of the materials and allowing convenient fine tuning of each individual component.
While most components of the DSC have been thoroughly optimized, the electrolyte still is the limiting factor when it comes to up scaling and commercialization of this technology. In operation the redox couple in the electrolyte has a dual role. It is required to reduce the photooxidized dye and facilitate charge transport between the electrodes to allow for the continuous generation of electricity. To date, most DSC designs make use of the iodide-triiodide redox couple. While this leads to relatively high efficiencies, the complicated two electron redox chemistry of this couple causes intrinsic efficiency losses, limiting the achievable power conversion efficiency. Furthermore, the corrosive nature of I-/I3- complicates the design of durable, large scale solar cells, since cost reducing metal substrates and charge collection grids are dissolved by these electrolytes. As a result, DSCs were frequently efficient in the laboratory, but devices with the commercially sufficient dimensions were often comparatively inefficient and expensive.
At the beginning of this work, relatively little was known about the requirements for the ‘perfect’ redox couple to replace I-/I3-. A few alternatives had been published, but all the devices showed comparatively low efficiency. In this thesis, we have investigated iron complexes as DSC redox mediators. The ferrocene / ferrocenium redox couple had been tried previously but the resulting DSCs showed very poor energy conversion efficiency.
Ferrocene (biscyclopentadienyl iron) is the widely accepted reference redox couple for the work in non-aqueous solutions and one of the best characterized compounds in the field of electrochemistry. The ferrocene unit has been shown to be highly tunable in its electrochemical properties by derivatization. Furthermore, a wide range of ferrocene derivatives is commercially available. Due to the combination of being well characterized and the availability of a vast compound library, this group of complexes was considered ideal when attempting to define the properties of the ideal DSC redox couple.
In this work, high efficiency DSCs are reported that use a ferrocene / ferrocenium redox couple. It was identified that the decomposition of the ferrocenium ion in air leads to rapid device degradation and low efficiencies. After developing a technique that allowed the DSCs to be filled and sealed under the exclusion of oxygen, and optimizing the DSC design for the use with this new electrolyte, conversion efficiencies of up to 7.5 % were achieved. The reference I-/I3- solar cell reached 6.2 %.
A series of ferrocene derivatives was then studied to investigate the influence of the redox potential of the redox couple on the charge transfer kinetics in the solar cell. It was identified that a threshold exists until which an increased redox potential directly results into an increased voltage and efficiency. After this threshold, the charge transfer reaction between the photooxidized dye and the redox couple becomes sluggish, leading to increased losses and lower efficiencies. An in depth study, utilizing a range of sensitizers and a library of ferrocene derivatives, allowed us to investigate this crucial charge transfer process. Measurements of the reaction kinetics by nanosecond pulsed laser transient absorption spectroscopy allowed us to estimate that a driving force of 0.25 eV is necessary to achieve quantitative dye regeneration (yield > 99.9 %) for typical electrolyte concentrations. With the knowledge gained from this work, any particular sensitizer can be matched with an appropriate redox couple, reducing energy losses in the solar cell due to the poor alignment of redox potentials.
A further focus was the work with aqueous electrolytes containing the ferricyanide / ferrocyanide redox system. The use of water instead of organic solvents has clear benefits due to the abundance of this solvent and environmental considerations. Ferrocyanide is a standard redox couple for aqueous solutions, having a well defined and documented redox chemistry under a wide range of conditions. In this work, we present the first efficient DSCs utilizing this system. Water as an electrolyte solvent for DSCs has been rarely investigated. The influence of parameters, such as pH and buffers, on device performance will have to be studied and the ferricyanide / ferrocyandide redox system forms a solid base for such future work.
In summary, a range of iron complexes has been investigated for the use as redox couples for dye-sensitized solar cells resulting in high efficiencies. Certain requirements of the ideal redox couple have been established, allowing to match sensitizers with a redox couple that optimizes the system and minimizes internal energy losses. Awards: Winner of the Mollie Holman Doctoral Medal for Excellence, Faculty of Science, 2012.</div
Fabrication et caractérisation de couches ultra-minces de composés de gallium pour la photonique intégrée hybride
No full textLes matériaux 2D sont apparus comme des candidats prometteurs pour l'amélioration de la photonique du silicium. Avec leur structure atomiquement fine, leur mobilité élevée des porteurs et leur forte interaction lumière-matière, les matériaux 2D offrent la possibilité d'une émission, d'une modulation et d'une détection efficaces de la lumière dans la photonique du silicium. Leur compatibilité avec les techniques de traitement du silicium et leur capacité à s'intégrer dans les plates-formes photoniques au silicium existantes les rendent attrayants pour la réalisation de dispositifs photoniques compacts, à haute performance et à faible consommation d'énergie. Notre approche exploite principalement la technique LMC, qui permet une intégration plus facile des matériaux 2D sur les dispositifs photoniques par rapport aux méthodes traditionnelles descendantes et ascendantes. Notre processus de fabrication comporte deux étapes : l'impression de Ga2O3 à base de métal liquide, suivie d'une réaction de nitruration améliorée par plasma. Ce processus en deux étapes permet de contrôler la composition de la couche de GaOxNy de quelques millimètres d'épaisseur qui en résulte et d'obtenir finalement du GaN stœchiométrique. Les propriétés structurelles et la composition élémentaire de ces matériaux 2D sont caractérisées par AFM, TEM, XPS et spectroscopie Raman. Notre processus de fabrication donne accès à une gamme de composés GaOxNy avec des propriétés optiques distinctes, qui peuvent être adaptées entre celles de Ga2O3 et GaN, comme le démontrent les mesures ellipsométriques et la comparaison avec les simulations DFT. En outre, nous avons démontré avec succès l'intégration de ces matériaux dans un MZI et effectué des mesures linéaires avant et après nitruration. Nos résultats élargissent les connaissances sur les composés ultraminces de gallium, qui ont été peu étudiés, et représentent une étape essentielle vers l'intégration de ces matériaux 2D dans les puces photoniques. Ce travail offre de nouvelles possibilités d'améliorer les performances des dispositifs optoélectroniques hybrides.2D materials have emerged as promising candidates for enhancing silicon photonics. With their atomically thin structure, high carrier mobility, and strong light-matter interaction, 2D materials offer the potential for efficient light emission, modulation, and sensing in silicon photonics. Their compatibility with silicon processing techniques and ability to integrate into existing silicon photonic platforms make them attractive for achieving compact, high-performance, and energy-efficient photonic devices.In this context, we introduced ultrathin Ga2O3 and GaN synthesized using the liquid metal chemistry technique. Our approach primarily exploits the LMC technique, which allows for easier integration of 2D materials onto photonic devices compared to traditional top-down and bottom-up methods. Our fabrication process involves a two-step procedure: liquid metal-based printing of Ga2O3, followed by plasma-enhanced nitridation reaction. This two-step process enables control over the composition of the resulting nm-thick GaOxNy layer and the eventual achievement of stoichiometric GaN. The structural properties and elemental composition of these 2D materials are characterized using AFM, TEM, XPS, and Raman spectroscopy.Our fabrication process grants access to a range of GaOxNy compounds with distinct optical properties, which can be tailored between those of Ga2O3 and GaN, as demonstrated by ellipsometry measurements and comparison with DFT simulations. Additionally, we successfully demonstrated the integration of these materials into a MZI and performed linear measurements before and after nitridation. Our findings expand the knowledge of ultra-thin gallium compounds, which have been poorly studied, and represent an essential step toward integrating such 2D materials into photonic chips. This work offers new opportunities to improve the performance of hybrid optoelectronic devices
Rh and Pt Cocatalysts in Photocatalytic Overall Water Splitting and Advanced Applications of TiN film in Attenuated Total Reflection Surface–Enhanced Infrared Absorption Spectroscopy (ATR–SEIRAS)
No full textPhotocatalytic overall water splitting (OWS) is widely recognized as the ideal route for producing green hydrogen, a key component of the global transition to sustainable energy. This thesis focuses on exploring the photophysical differences between Rh and Pt cocatalysts in photocatalytic OWS for hydrogen production. By employing advanced spectroscopic techniques, this work aims to elucidate the underlying mechanisms that contribute to the distinct performances of these noble metal cocatalysts. The study not only highlights their comparative behavior in photocatalytic OWS systems but also leverages operando spectroscopic analysis to provide detailed insights into their photophysical properties under working conditions. This research bridges material science and spectroscopy to advance our understanding of catalyst behavior in sustainable hydrogen production. The work is organized into four chapters, each addressing a specific aspect of this multidisciplinary research.
The first chapter introduces the importance of hydrogen in the global energy transition, examining production pathways and emphasizing the significance of electrochemical and photocatalytic OWS for sustainable energy solutions. Additionally, this chapter highlights the importance of operando studies for advancing the understanding of materials used in these processes. Special focus is placed on spectroelectrochemical techniques, such as Attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR–SEIRAS), which provide real-time insights into material behaviors and reaction mechanisms at electrode-electrolyte interfaces.
The second chapter investigates the catalytic performance of Rh and Pt cocatalysts on SrTiO3 and Al:SrTiO3 photocatalysts. The chapter highlights differences in activity, emphasizing Rh’s superior activity compared to Pt.
The third chapter shows the mechanisms underlying Rh’s superior performance compared to Pt, despite Pt being the benchmark for HER in electrochemical systems. ATR–SEIRAS is utilized to study band alignments and interfacial processes under working conditions. The chapter highlights the use of TiN films as a buffer layer in ATR–SEIRAS studies, enabling the successful characterization of SrTiO3 and Al:SrTiO3 electrodes with Rh and Pt cocatalysts.
The final chapter discusses the role of TiN films as a novel buffer layer in ATR-SEIRAS. This buffer layer enables the study of materials that can be deposited at high temperatures and facilitates investigations under harsh conditions, such as high potential and alkaline media
Nucleation and growth of polyaniline nanofibers onto liquid metal nanoparticles
No full textLiquid metals can play an essential role in the generation of electrically conductive composites for electronic devices and environmental sensing and remediation applications. Here, a method for growing a polyaniline nanofibrous network at liquid metal nanoparticle interfaces is demonstrated for generating hybrid liquid metal-polymer nanocomposites. The investigation shows that an initial functionalization step of the liquid metal nanoparticles with a polymerization enhancer is essential for providing stable and specific nucleation points for the formation of the polyaniline nanofibrous network. The acidity and mechanical agitation conditions are carefully adjusted to control the fibrous polyaniline. The embedded gallium elements form an initial seeding layer around the liquid metal nanoparticles. The novel nanocomposites offer synergistic properties for environmental sensing and molecular separation applications. This study provides a road map for the direct synthesis of long organic molecular chains at the dynamic interfaces of liquid metals.</p
Oxygen-Induced Doping of Spiro-MeOTAD in Solid-State Dye-Sensitized Solar Cells and Its Impact on Device Performance
No full textSolid state dye-sensitized solar cells (sDSCs) employing the hole conductor 2,2'7,7'-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene (spiro-MeOTAD) require the presence of oxygen during fabrication and storage. In this paper, we determine the concentrations of oxidized spiro-MeOTAD within devices under different operating and storage conditions by UV-vis spectroscopy. Relative concentrations of spiro-MeOTAD(+) were found to be greater than 10% after illumination for standard sDSCs, where no chemical dopant had been used in the solar cell fabrication but oxygen and lithium ions were present. We suggest that oxidized spiro-MeOTAD is created as a byproduct of oxygen reduction at the TiO(2) surface during cell illumination. Furthermore, we studied the effect of light soaking under different conditions and associated changes in spiro-MeOTAD(+) concentration on the solar cell measurements. Our findings give insights to photochemical reactions occurring within sDSCs and provide guidelines for which doping levels should be used in device fabrication in absence of oxygen.</p
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