Synthesis and investigation of hybrid nanomaterials for photocatalytic and spectroanalytical applications

This thesis is divided into two parts where the first part of the thesis deals with the design and synthesis of heterostructures for efficient photocatalysis e.g. for water remediation while the second part deals with the design of effective substrates suitable for sensitive surface-enhanced Raman scattering (SERS) analysis. Investigating different hybrid nanomaterials which can serve as efficient photocatalysts, the wide band gap semiconductors ZnO and ZnS have been combined with vertically aligned carbon nanotubes (VACNTs) to form ZnO@CNT and ZnS@CNT nanocomposite heterostructures which are able to make use not only from the ultraviolet (UV) region of light but from most of the solar spectrum. Through atomic layer deposition (ALD) ZnO nanoparticles are directly deposited on VACNTs which could be converted in a gas phase sulfidation process to ZnS@CNT nanocomposites. Depending on the gas phase conversion temperature different ZnS/ZnO@CNT, sphalerite-based ZnS@CNT and wurtzite-based ZnS@CNT nanocomposites could be obtained where at high conversion temperatures defects could be even induced in the ZnS nanoparticles. This study revealed that wurtzite-based ZnS@CNT nanocomposites with induced defects show the highest photocatalytic activity towards the degradation of methyl orange which was used as model pollutant under simulated sunlight. Furthermore, the photocatalytic properties of two-dimensional titanium chalcogenides have been investigated. These materials possess in contrast to traditional titanium dioxide a small band gap and a layered structure which is advantageous for photocatalytic applications. The different titanium chalcogenides were prepared by a chemical vapor transport method whereby tuning of the ratio of the starting elements and the reaction temperature a series of different titanium chalcogenides (TiS3, TiS2, TiS, TiSe2 and TiTe) could be synthesized. The investigation of titanium sulfides, TiS3 and TiS which is defect-rich TiS2 showed promising results concerning their photocatalytic activity. Non-stoichiometric titanium disulfide shows a large number of defects which are responsible for a high photocatalytic and thermocatalytic activity. Excellent recyclability of these materials was also found and was attributed to the spontaneous formation of titanium sulfide/titanium oxide heterostructures due to the surface oxidation with time further increasing the photocatalytic activity of the material. New approaches for the preparation of efficient SERS substrates with high enhancement factors were investigated as they are crucial for trace analysis e.g. of bioactive compounds. First, a facile plasma-assisted approach for the preparation of SERS substrates has been developed. Different plasma treatments using different plasma gases and different parameters have been investigated on thin transparent silver films of 10 nm thickness and thick non-transparent silver films of 200 nm thickness. Hydrogen, nitrogen and argon plasma were found to increase the surface roughness of these sputtered silver films, thus increasing their SERS enhancement factors significantly. Subsequent oxidation-reduction plasma treatment of the 200 nm thick silver sputtered films through oxidation with oxygen plasma followed by reduction with either hydrogen, nitrogen or argon plasma enabled the formation of complex three-dimensional porous silver films showing high SERS enhancement factors. Finally, aluminum/anodic aluminum oxide/silver (Al/AAO/Ag) substrates have been investigated to elucidate the different factors which could increase the SERS response obtained from such substrates. It was found that a possible chemical enhancement from iodine species introduced in barrier-type anodic aluminum oxide by iodine oxoacid electrolytes could effectively increase the SERS enhancement factors of these substrates compared to other dense barrier-type anodic aluminum oxides obtained from citrate buffer and porous anodic aluminum oxide (PAOX) films. These findings could be valuable for the preparation of more effective SERS substrates based on Al/AAO/Ag thin layer compositions

Synthesis of Anisotropic Metal Oxide Nanoparticles via Non‑Aqueous and Non-Hydrolytic Routes

Due to their low cost, high stability and low toxicity, metal oxide nanomaterials are widely used for applications in various fields such as electronics, cosmetics and photocatalysis. There is an increasing demand thereby for nanoparticles with highly defined properties, in particular a narrow particle size distribution and a well-defined morphology. Such products can be obtained under high control via bottom-up synthesis approaches. Although aqueous processes are largely found in literature, they often lead to particles with low crystallinity and broad size distribution. Thus, there has been a growing trend towards the use of non-aqueous and non-hydrolytic synthesis routes. Through variation of the reaction medium and the use of adequate additives, such non-aqueous systems can be tuned to adapt the product properties, and especially to yield anisotropic nanoparticles with peculiar shapes and even complex architectures. Anisotropic particle growth enables the exposure of specific facets of the oxide nanocrystal, leading to extraordinary properties such as enhanced catalytic activity. Thus, there is an increasing demand for anisotropic nanoparticles with tailored morphologies. In this review, the non-aqueous and non-hydrolytic synthesis of anisotropic metal oxide nanoparticles is presented, with a particular focus on the different parameters resulting in anisotropic growth to enable the rational design of specific morphologies. Furthermore, secondary phenomena occurring during anisotropic particle growth, such as oriented attachment mechanisms, will be discussed

Controlling surface morphology and sensitivity of granular and porous silver films for surface-enhanced Raman scattering, SERS

The design of efficient substrates for surface-enhanced Raman spectroscopy (SERS) for large-scale fabrication at low cost is an important issue in further enhancing the use of SERS for routine chemical analysis. Here, we systematically investigate the effect of different radio frequency (rf) plasmas (argon, hydrogen, nitrogen, air and oxygen plasma) as well as combinations of these plasmas on the surface morphology of thin silver films. It was found that different surface structures and different degrees of surface roughness could be obtained by a systematic variation of the plasma type and condition as well as plasma power and treatment time. The differently roughened silver surfaces act as efficient SERS substrates showing greater enhancement factors compared to as prepared, sputtered, but untreated silver films when using rhodamine B as Raman probe molecule. The obtained roughened silver films were fully characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron (XPS and Auger) and ultraviolet–visible spectroscopy (UV–vis) as well as contact angle measurements. It was found that different morphologies of the roughened Ag films could be obtained under controlled conditions. These silver films show a broad range of tunable SERS enhancement factors ranging from 1.93 × 102 to 2.35 × 105 using rhodamine B as probe molecule. The main factors that control the enhancement are the plasma gas used and the plasma conditions, i.e., pressure, power and treatment time. Altogether this work shows for the first time the effectiveness of a plasma treatment for surface roughening of silver thin films and its profound influence on the interface-controlled SERS enhancement effect. The method can be used for low-cost, large-scale production of SERS substrates

Lrfd Flexural Provisions For Prestressed Concrete Bridge Girders Strengthened With Carbon Fiber-Reinforced Polymer Laminates

The behavior and design of prestressed concrete (PSC) bridge girders flexurally strengthened with carbon fiber-reinforced polymer (CFRP) laminates are discussed. A fiber section model that accounts for inelastic material behavior as well as the construction sequence including transfer, composite action between the cast-in-place deck and girder, and bonding of CFRP laminates, is developed. The model is verified and is then used to conduct thousands of Monte Carlo simulations of a number of bridges designed according to the 1998 AASHTO LRFD. The bridge designs address a broad range of design parameters including span length, ratio of dead load to live load, and amount of CFRP strengthening. The numerical simulations are used to develop cross-sectional resistance models from which the flexural reliability of the designed bridges is calculated using the first-order reliability method. An equation for the flexural strength reduction factor for PSC bridge girders strengthened with CFRP laminates is proposed

Considerations For Opening New Access Holes In Curved Box Girders

Access hatches (holes) in curved-box-girder bridges are usually provided in the bottom flange immediately before or after an expansion joint. If additional access hatches are required after the bridge is built, they must be placed in such a way that (1) they satisfy such important practical constraints as feasibility, accessibility, water leakage, traffic impact, and unauthorized access, and (2) they do not adversely affect the structural behavior of the bridge - i.e., their installation should not impair serviceability or decrease ultimate strength or fatigue life. This paper discusses both of these issues and proposes approaches that are suitable for identifying appropriate locations for access hole placement. The proposed approaches are used to investigate seven curved-box-girder bridges located in the state of Florida. One of the bridges is chosen for further study using a detailed finite-element model. The numerical model is used to confirm the proposed methods and to further investigate the effects of adding access holes. © ASCE

Static And Fatigue Analyses Of Rc Beams Strengthened With Cfrp Laminates

Extensive testing has shown that externally bonded carbon fiber reinforced polymer (CFRP) laminates are particularly suited for improving the short-term behavior of deficient reinforced concrete beams. Accelerated fatigue tests conducted to date confirm that fatigue response is also improved. This paper describes an analytical model for simulating the static response and accelerated fatigue behavior of reinforced concrete beams strengthened with CFRP laminates. Static and fatigue calculations are carried out using a fiber section model that accounts for the nonlinear time-dependent behavior of concrete, steel yielding, and rupture of CFRP laminates. Analysis results are compared with experimental data from two sets of accelerated fatigue tests on CFRP strengthened beams and show good agreement. Cyclic fatigue causes a time-dependent redistribution of stresses, which leads to a mild increase in steel and CFRP laminate stresses as fatigue life is exhausted. Based on the findings, design considerations are suggested for the repair and/or strengthening of reinforced concrete beams using CFRP laminates