Structure, Stability, Vibrational, Thermodynamic, And Catalytic Properties Of Metal Nanostructures: Size, Shape, Support, And Adsorbate Effects

Recent advances in nanoscience and technology have provided the scientific community with new exciting opportunities to rationally design and fabricate materials at the nanometer scale with drastically different properties as compared to their bulk counterparts. A variety of challenges related to nanoparticle (NP) synthesis and materials characterization have been tackled , allowing us to make more homogenous, well defined, size- and shape-selected NPs, and to probe deeper and more comprehensively into their distinct properties. In this dissertation, a variety of phenomena relevant to nanosized materials are investigated, including the thermal stability of NPs and coarsening phenomena in different environments, the experimental determination of NP shapes, gaining insight into NP-support interactions, epitaxial relationships, and unusual thermodynamic and electronic properties of NPs, including the effect of adsorbates on the electron density of states of small clusters, and the chemical, and structural evolution of NPs under reaction conditions. In chapter 2, a general description of different characterization tools that are used in this dissertation is provided. In chapter 3, the details of two different methods used for NP synthesis, namely inverse micelle encapsulation and physical vapor deposition (PVD) are described. Chapter 4 describes the thermal stability and coarsening behavior of Pt NPs supported on TiO2(110) and γ-Al2O3 as a function of the synthesis method, support pretreatment, and annealing environment. For the Pt/TiO2(110) system, micellesynthesized NPs showed remarkable stability against coarsening for annealing temperatures up to 1060°C in vacuum, in contrast to PVD-grown NPs. When comparing v different annealing environments (H2, O2, H2O), Pt NPs on γ-Al2O3 annealed in O2 were found to be the least affected by coarsening, followed by those heated in H2O vapor. The largest NP growth was observed for the sample annealed in H2. The role of the PtOx species formed under oxidizing conditions will be discussed. In chapter 5, the shape of Pt and Au NPs and their epitaxial relationship with the TiO2(110) support was extracted from scanning tunneling microscopy (STM) measurements. Three main categories of NP shapes were identified, and through shape modeling, the contribution of facets with different orientations was obtained as a function of the number of atoms in each NP. It was also shown that the micellesynthesized Pt and Au NPs have an epitaxial relationship with the support, which is evident from the fact that they always have one symmetry axis parallel to TiO2(110) atomic rows in [001] directions. Chapter 6 describes how the presence of NPs on TiO2(110) surface affects its reconstruction upon high temperature annealing in vacuum. In contrast to NP-free TiO2(110) substrates, long and narrow TiO2 stripes are observed for Pt NP-decorated surfaces. This phenomenon is explained based on the stabilization of TiO2, induced by Pt NPs, which hinders the desorption of oxygen atoms in TiO2 to vacuum. In chapter 7, a systematic investigation of the thermodynamic properties of γ- Al2O3-supported Pt NPs and their evolution with decreasing NP size is presented. A combination of in situ extended x-ray absorption fine structure spectroscopy (EXAFS), ex situ transmission electron microscopy (TEM) measurements, and NP shape modeling is used to obtain the NPs shape, thermal expansion coefficient, and Debye vi temperature. The unusual thermodynamic behavior of these NPs such as their negative thermal expansion and enhanced Debye temperature are discussed in detail. Chapter 8 presents an investigation of the electronic properties of size-controlled γ-Al2O3-supported Pt NPs and their evolution with decreasing NP size and adsorbate (H2) coverage. The hydrogen coverage of Pt NPs at different temperatures was estimated based on XANES data and was found to be influenced by the NP size, and shape. In addition, correlations between the shift in the center of the unoccupied d-band density of states (theory) and energy shifts of the XANES spectra (experiment) upon hydrogen chemisorption as well as upon modification of the NP structure were established. Chapter 9 is dedicated to an operando study, describing the evolution of the structure and oxidation state of ZrO2-supported Pd nanocatalysts during the in-situ selective reduction of NO in H2 via EXAFS and XANES measurements

Looking inside the Black Box: The Role of Host Country Offices in Negotiating International Volunteering for Development

This research aims to investigate the significance of the Host Country Offices (HCOs) of International Volunteer Cooperation Organisations (IVCOs) in explaining the gap between the theory and practice of International Volunteering Service (IVS) in development. HCOs are operational arms of IVCOs that work in developing countries, but they are rarely studied in the literature and remain a ‘black box’. Focusing on five HCOs in Malawi, this study seeks to address the gaps in the literature by examining the different relationships between the HCOs and their multiple stakeholders: their head office, the key players involved in mobilising and implementing IVS in development projects (the government, the donors and the partner organisations), and the volunteers. The research uses the idea of ‘boundary managers’ to describe HCOs and their role in managing multiple mandates and negotiate relationships with different stakeholders. The conceptual framework is based on different kinds of trust and power relations to explore how HCOs negotiate these relationships and to demonstrate how other factors such as asymmetries in knowledge and information help explain the gap between the theory and practice of IVS. A cross-case study methodology is used that involves observation, document review and interviews with staff in head offices, HCOs, partners, beneficiaries and volunteers. The study shows that a development-focused IVS strategy, and fair and democratic organisational policies and structures that encourage staff contributions to knowledge, make more efficient use of human, physical and logistical resources, and can create trust-based relationships between the HCOs and their stakeholders leading to better processes, and potentially, to better and more consistent outcomes. The study makes empirical, theoretical, and policy contributions by identifying HCOs as influential actors in IVS, characterising their status as boundary managers, and making policy recommendations for HCOs to manage the relationships between different stakeholders more effectively

Method For Forming Thermally Stable Nanoparticles on Supports

This patent describes a new synthesis method for the large scale and low-cost production of size-selected nanoparticles with uniform 2D arrangement on a surface and nanowire patterned substrates with tunable width and interwire distance in a single preparation step. The nanoparticles prepared using our modified synthesis procedure exhibit an enhanced thermal stability and resistance against coarsening/sintering and desoreption at high temperature [at least 1060?C for Pt/TiO2(110)] thanks to the presence of a “polymeric glue” at the nanoparticle/support interface. Such thermally stable particles are can not only be stabilized on single crystal surfaces such as TiO2(110) but also on real-world catalytic supports such as nanocrystalline powders (anatase TiO2, CeO2, etc.). A second major advantage of our invention is the the possibility of using the strong nanoparticle/support interactions present in these low dimensional systems to create patterned surfaces at the nanoscale. As an exampl

Thermally Stable Nanoparticles on Supports

This patent describes a new synthesis method for the large scale and low-cost production of size-selected nanoparticles with uniform 2D arrangement on a surface and nanowire patterned substrates with tunable width and interwire distance in a single preparation step. The nanoparticles prepared using our modified synthesis procedure exhibit an enhanced thermal stability and resistance against coarsening/sintering and desoreption at high temperature [at least 1060?C for Pt/TiO2(110)] thanks to the presence of a “polymeric glue” at the nanoparticle/support interface. Such thermally stable particles are can not only be stabilized on single crystal surfaces such as TiO2(110) but also on real-world catalytic supports such as nanocrystalline powders (anatase TiO2, CeO2, etc.). A second major advantage of our invention is the the possibility of using the strong nanoparticle/support interactions present in these low dimensional systems to create patterned surfaces at the nanoscale. As an exampl

Enhanced thermal stability and nanoparticle-mediated surface patterning: Pt/TiO2(110)

This letter reports (i) the enhanced thermal stability (up to 1060 degrees C) against coarsening and/or desorption of self-assembled Pt nanoparticles synthesized by inverse micelle encapsulation and deposited on TiO2(110) and (ii) the possibility of taking advantage of the strong nanoparticle/support interactions present in this system to create patterned surfaces at the nanoscale. Following our approach, TiO2 nanostripes with tunable width, orientation, and uniform arrangement over large surface areas were produced

Advances in Peptide-Based Hydrogel for Tissue Engineering

The development of peptide-based materials has emerged as one of the most challenging aspects of biomaterials in recent years. It has been widely acknowledged that peptide-based materials can be used in a broad range of biomedical applications, particularly in tissue engineering. Among them, hydrogels have been attracting considerable interest in tissue engineering because they mimic tissue formation conditions by providing a three-dimensional environment and a high water content. It has been found that peptide-based hydrogels have received more attention due to mimicking proteins, particularly extracellular matrix proteins, as well as the wide variety of applications they are capable of serving. It is without a doubt that peptide-based hydrogels have become the leading biomaterials of today owing to their tunable mechanical stability, high water content, and high biocompatibility. Here, we discuss in detail various types of peptide-based materials, emphasizing peptide-based hydrogels, and then we examine in detail how hydrogels are formed, paying particular attention to the peptide structures that are incorporated into the final structure. Following that, we discuss the self-assembly and formation of hydrogels under various conditions, as well as the parameters to be considered as critical factors, which include pH, amino acid composi- tion within the sequence, and cross-linking techniques. Further, recent studies on the development of peptide-based hydrogels and their applications in tissue engineering are reviewed

Shape-Selection of Thermodynamically Stabilized Colloidal Pd and Pt Nanoparticles Controlled via Support Effects

Colloidal chemistry, in combination with nanoparticle (NP)/support epitaxial interactions is used here to synthesize shape-selected and thermodynamically stable metallic NPs over a broad range of NP sizes. The morphology of three-dimensional palladium and platinum NPs supported on TiO₂(110) was investigated using scanning tunneling microscopy. Well-defined Pd and Pt NPs were synthesized via inverse micelle encapsulation. The initially spherical NPs were found to become faceted and form an epitaxial relationship with the support after high-temperature annealing (e.g., 1100 °C). Shape selection was achieved for almost all Pd NPs, namely, a truncated octahedron shape with (111) top and interfacial facets. The Pt NPs were however found to adopt a variety of shapes. The epitaxial relationship of the NPs with the support was evidenced by the alignment of the cluster’s edges with TiO₂(110)-[001] atomic rows and was found to be responsible for the shape control. The ability of synthesizing thermally stable shape-selected metal NPs demonstrated here is expected to be of relevance for applications in the field of catalysis, since the activity and selectivity of NP catalysts has been shown to strongly depend on the NP shape

Orientation Distribution of Highly Oriented Type I Collagen Deposited on Flat Samples with Different Geometries

The structural arrangement of type I collagen in vivo is critical for the normal functioning of tissues, such as bone, cornea, tendons and blood vessels. At present, there are no established low-cost techniques for fabricating aligned collagen structures for applications in regenerative medicine. Here, we report on a straightforward approach to fabricate collagen films, with defined orientation distributions of collagen fibrillar aggregates within a matrix of oriented collagen molecules on flat sample surfaces. Langmuir Blodgett (LB) technology was used to deposit thin films of oriented type I collagen onto flat substrates exhibiting various shapes. By varying the shapes of the substrates (e.g. rectangles, squares, circles, parallelograms, and various shaped triangles) as well as their sizes, a systematic study on collagen alignment patterns was conducted. It was found that the orientation and the orientation distribution of collagen along these various shaped substrates is directly depending on the geometry of the substrate and the dipping direction of that sample with respect to the collagen/water subphase. An important factor in tissue engineering is the stability, durability and endurance of the constructed artificial tissue, and thus its functioning in regenerative medicine applications. By testing these criteria we found that the coated films and their alignments were stable for at least three months under different conditions and, moreover, that these films can withstand temperatures of up to 60°C for a short time. Therefore, these constructs may have widespread applicability in the engineering of collagen-rich tissues

Thermodynamic properties of Pt nanoparticles: Size, shape, support, and adsorbate effects

This study presents a systematic investigation of the thermodynamic properties of free and gamma-Al2O3-supported size-controlled Pt nanoparticles (NPs) and their evolution with decreasing NP size. A combination of in situ extended x-ray absorption fine-structure spectroscopy (EXAFS), ex situ transmission electron microscopy (TEM) measurements, and NP shape modeling revealed (i) a cross over from positive to negative thermal expansion with decreasing particle size, (ii) size- and shape-dependent changes in the mean square bond-projected bond-length fluctuations, and (iii) enhanced Debye temperatures (D-circle minus, relative to bulk Pt) with a bimodal size- dependence for NPs in the size range of similar to 0.8-5.4 nm. For large NP sizes (diameter d \u3e 1.5 nm) D-circle minus was found to decrease toward D-circle minus of bulk Pt with increasing NP size. For NPs \u3c = 1 nm, a monotonic decrease of D-circle minus was observed with decreasing NP size and increasing number of low-coordinated surface atoms. Our density functional theory calculations confirm the size- and shape-dependence of the vibrational properties of our smallest NPs and show how their behavior may be tuned by H desorption from the NPs. The experimental results can be partly attributed to thermally induced changes in the coverage of the adsorbate (H-2) used during the EXAFS measurements, bearing in mind that the interaction of the Pt NPs with the stiff, high-melting temperature gamma-Al2O3 support may also play a role. The calculations also provide good qualitative agreement with the trends in the mean square bond-projected bond-length fluctuations measured via EXAFS. Furthermore, they revealed that part of the D-circle minus enhancement observed experimentally for the smallest NPs (d \u3c = 1 nm) might be assigned to the specific sensitivity of EXAFS, which is intrinsically limited to bond-projected bond-length fluctuations