Preparation and characterisation of Pt-Ru/C catalysts for direct methanol fuel cells

The direct methanol fuel cell (DMFC) is identified as a promising fuel cell for portable and micro fuel cell applications. One of the major benefits is that methanol is an energy dense, inexpensively manufactured, easily stored and transported, liquid fuel (Hamann et al., 2007). However, the DMFC's current efficiency and power density is much lower than theoretically possible. This inefficiency is predominantly due to the crossover of methanol from the anode to the cathode, Ru dissolution and Ru crossover from the anode to the cathode. In addition, the DMFC has a high manufacturing cost due to expensive catalyst costs and other materials. Catalyst expenses are further increased by catalyst loading due to low activity at the anode of the DMFC (Zhang, 2008). Hence, with increasing activity and stability of the Pt-Ru/C catalyst, catalyst expenditure will decrease due to a decrease in catalyst loading. In addition, performance will increase due to a reduction in ruthenium dissolution and crossover. Therefore, increasing the activity and stability of the Pt-Ru/C catalyst is paramount to improving the current DMFC performance and viability as an alternative energy conversion device. Pt-Ru/C catalyst synthesis method, precursors, reduction time and temperature play a role in the activity for methanol electro-oxidation and stability since these conditions affect structure, morphology and dispersivity of the catalyst (Wang et al., 2005). Metal organic chemical deposition methods have shown promise in improving performance of electro-catalysts (Garcia & Goto, 2003). However, it is necessary to optimise deposition conditions such as deposition time and temperature for Pt(acac)₂ and Ru(acac)₃ precursors. This study focuses on a methodical approach to optimizing the chemical deposition synthesis method for Pt-Ru/C produced from Pt(acac)₂ and Ru(acac)₃ precursors. Organo-metallic chemical vapour deposition (OMCVD) involved the precursor's vapourisation before deposition and a newly developed method which involved the precursors melting before deposition. An investigation was conducted on the effects of precursor's phase before deposition. The second investigation was that of the furnace operating temperature, followed by an exploration of the furnace operating time influence on methanol electro-oxidation, CO tolerance and catalyst stability. Lastly, the exploration of the Pt:Ru metal ratio influence was completed. It was found that the catalyst produced via the liquid phase precursor displayed traits of a high oxide content. This led to an increased activity for methanol electro-oxidation, CO tolerance and catalyst stability despite the OMCVD catalyst producing smaller particles with a higher electrochemically active surface area (ECSA)

Aerosolimassaspektrien tilastollinen analyysi : kemometria ja kemiallinen luokittelu

Climate change and degraded air quality rank among the most severe environmental problems that humanity currently faces. Atmospheric aerosols - the fine particles suspended in air – play a significant role in both of them. Aerosols affect Earth’s radiation balance via their interactions with radiation (scattering and absorption of light) and atmospheric water vapour (cloud formation, lifetime, brightness and rainfall). Overall, aerosols have a cooling effect on climate. When inhaled, aerosols can cause a range of adverse health effects, from allergies to lung disease and increased rates of cancer. Air pollution is estimated to result in millions of premature deaths each year in polluted environments. All of these aerosol effects and interactions significantly depend on their chemical compositions. This thesis focuses on aerosol chemical composition, measured by using mass spectrometry. To make sense of the vast diversity of chemical compounds present in the atmospheric aerosol particles, we rely on chemical classifications (speciations). In this work, different types of traditional aerosol mass spectrometric chemical speciations are evaluated by their ability to explain measured aerosol physicochemical properties, such as volatility, hygroscopicity and optical properties. While the parameter estimates for aerosol physicochemical properties derived from traditional chemical classifications do correlate with experimental data, the estimates fail to adequately capture the observed variabilities. Whether this is due to deficiencies in speciation schemes, flaws in assumptions or models or measurement uncertainties, is not apparent. However, it seems evident that there is room for improvement in all of the above-mentioned fields. During the course of this work, chemometric methods, (i.e. the application of mathematical and statistical methods to the analysis of chemical measurements) were implemented in the analysis of aerosol mass spectra. The results clearly show the value of statistics-based, machine learning oriented methods for feature extraction and chemical classification. Such chemometric approaches maximise the amount of information available for an analyst of aerosol mass spectrometric results, and enable making better informed interpretations. Combining methods such as data factorisation and clustering can overcome typical limitations of these methods when used as stand-alone techniques. Chemometrics-based, machine-learning-like methods thus show considerable potential for improving aerosol chemical analysis. Finally, chemometric tools were shown to be capable of producing new, comprehensive, mathematically and statistically robust chemical classifications, which again contribute to our understanding of the atmospheric aerosol properties, interactions and effects.Ilmastonmuutos ja ilmansaasteet ovat ihmiskunnan tämänhetkisiä suurimpia ympäristöongelmia. Ilmakehän pienhiukkaset, aerosolit, vaikuttavat niistä molempiin. Aerosolit paitsi vaikuttavat valon ja lämpösäteilyn kulkuun ilmakehässä, myös vuorovaikuttavat ilmakehän vesihöyryn kanssa, vaikuttaen pilvenmuodostukseen ja pilvien ominaisuuksiin, kuten kirkkaus, elinaika ja satavuus. Aerosolien kokonaisvaikutus ilmastoon on viilentävä. Hengitettynä aerosolit voivat aiheuttaa monia terveyshaittoja allergioista aina keuhkosairauksiin ja kohonneisiin syöpäriskeihin. Ilmansaasteiden on arvioitu johtavan miljooniin ennenaikaisiin kuolemiin vuosittain, erityisesti saastuneissa ympäristöissä. Kaikki yllä luetellut pienhiukkasten vaikutukset riippuvat voimakkaasti niiden kemiallisesta koostumuksesta. Tämä väitöskirja keskittyy aerosolien kemiallisen koostumuksen mittaukseen aerosolimassaspektrometrian menetelmin. Aerosolit voivat sisältää kymmeniä tuhansia erilaisia kemiallisia yhdisteitä, jolloin koostumuksen kuvaukseen tarvitaan käytännössä näiden yhdisteiden kemiallista luokittelua. Tässä työssä vertaillaan erilaisia luokittelujärjestelmiä ja niiden hyödyllisyyttä aerosolien ominaisuuksien ennustamisessa. Tällaisia tärkeitä fysikaalisia ja kemiallisia ominaisuuksia ovat mm. haihtuvuus, kyky sitoa vettä sekä optiset ominaisuudet. Vaikka perinteisillä kemiallisilla luokittelumenetelmillä saadut ennusteet aerosolien ominaisuuksille vastaavat jossain määrin mitattuja arvoja, ne eivät selitä mittaustuloksia riittävän hyvin. Toistaiseksi ei ole selvää johtuvatko nämä puutteellisuudet kemiallisista luokittelujärjestelmistä, muiden tarvittavien fysikaalisten mallien epätäydellisyydestä vai mittausepätarkkuuksista. Vaikuttaa silti ilmeiseltä, että kaikilla edellä mainituilla osa-alueilla on parantamisen varaa. Tässä työssä käytettiin kemometrisiä menetelmiä eli kemiallisten mittausten analyysiä edistynein tilastollisin ja matemaattisin menetelmin. Tulokset osoittavat tilastollisten ja koneoppimismenetelmien hyödyllisyyden kemiallisessa luokittelussa ja tärkeiden kemiallisten piirteiden tunnistamisessa massaspektreistä. Kemometristen menetelmien käyttö helpottaa massaspektrien tulkintaa ja tulosten ymmärtämistä. Yhdistelemällä useita tilastollisia ja matemaattisia menetelmiä voidaan lisäksi välttää yksittäisiin menetelmiin liittyviä ongelmia ja tyypillisiä puutteita. Kemometriaan ja koneoppimiseen pohjautuvat menetelmät vaikuttavatkin hyvin lupaavilta aerosolikemiallisten tulosten analyysiin. Kemometrisilla menetelmillä tuotetut aerosolien uudet, kemialliset luokittelut havaittiin kattaviksi ja matemaattisesti hyvin perustelluiksi. Niiden odotetaankin lisäävän ymmärrystämme ilmakehän pienhiukkasten koostumuksesta, ominaisuuksista ja vaikutuksista

The synthesis of novel precursors for the CVD of antimony sulphide and antimony sulpho-iodide thin films

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Atomic layer deposition and metal organic chemical vapour deposition of materials for photovoltaic applications

In this thesis, the development of thin films and nanostructures prepared with chemical vapour techniques are investigated for applications in photovoltaics. The deposition of both p-type and n-type oxides are investigated as a means of preparing all oxide p-n junctions. Both CVD and ALD precursors and processes have been developed. Zinc oxide nanowires are of interest as an n-type absorber layer with high surface area. In this thesis, the crystal structures of DEZn and DMZn were revisited and a new understanding of conventional zinc CVD precursors is presented. For DEZn a single structure was isolated and characterised with single crystal XRD. In the case of DMZn two temperature dependant structures were identified: namely α and β at 200K and 150K respectively. The DMZn precursor was subsequently exploited in a series of adduct-based precursors of the notation [DMZn.L] (where L = 1,2-dimethoxyethane, 1,4-dioxane and 1,4-thioxane). The crystal structures of these precursors were determined, and they were subsequently used to grow ZnO and sulphur doped ZnO across a range of CVD growth conditions. The microstructure and electronic properties of the nanowires have been characterised with electron microscopy, x-ray diffraction, Raman spectroscopy and photoluminescence. The II:VI ratio and substrate temperatures were both confirmed as playing a significant role in determining the microstructure of the nanowires. It has been demonstrated that the use of [DMZn.L] can avoid the pre-reaction between DMZn and oxygen. The studies with the thioxane adduct suggests the involvement of the ligand and hence sulphur incorporation in the nanowires. Two copper precursors were selected as the basis of p-type copper oxide film studies. The first Cu(hfac)(COD) has been used previously to deposit copper oxide by conventional CVD. In this thesis it is demonstrated for the first time that a pulsed LI-ALD approach can be exploited to deposit CuO with ozone as the co-reagent. An unexpected outcome of the research was the successful growth of electrically conductive copper metal films with a sheet resistance of 0.83Ω/□ when the precursor was thermally decomposed. The second copper precursor, namely CpCu(tBuNC) was used in atomic layer deposition to successfully deposit CuO or Cu2O with oxygen plasma and water respectively. Having identified that the β-diketonate compound yielded copper, the cyclopentadienyl based precursor was investigated as a route for the deposition of conductive copper metal films. Both thermal decomposition and a hydrogen plasma ALD process have been shown to deposit copper. With the plasma process, deposition of copper was demonstrated as low as 75˚C with a sheet resistance of only 0.55Ω/□. This thesis has demonstrated novel deposition routes for p- and n-type oxide materials which have potential future applications in thin film or nanostructured photovoltaic technology

Group 13 metal oxide and pnictide thin films: developing novel single-source precursors for the deposition of functional materials

This thesis describes the synthesis and characterisation of molecular, single-source precursors for the deposition of group 13 metal oxides and pnictides and their subsequent application in the formation of thin films of those materials by Aerosol Assisted Chemical Vapour Deposition (AACVD). A range of tetradentate β-ketoimine ligands have been developed and subsequently employed in the formation and isolation of complexes of group 13 metals aluminium, gallium and indium through a variety of synthetic routes. In particular, the ligand systems have been successfully employed in the stabilisation of rare gallium mono-hydride species, which exhibit considerable thermal stability. The thermal decomposition of the group 13 β-ketoiminate complexes was investigated by thermogravimetric analysis and subsequently the deposition of gallium oxide was carried out on glass substrates in an AACVD process. The resulting films were amorphous as-deposited, however crystalline gallium oxide was obtained in some cases through the addition of a post-deposition annealing step. The single-source deposition of InAs by AACVD has been investigated, including an investigation of the alkane elimination reaction between trimethyl indium and tert-butyl arsine in the formation of InAs cluster complexes. InAs was successfully deposited, representing the first report of the deposition of this material by AACVD. Thin films were analysed and characterised by multiple common techniques, including glancing angle powder X-ray diffraction, energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy and scanning electron microscopy, confirming the formation of InAs, though film quality was hampered by varying degrees of indium oxide formation within the films, in particular at the surface. Hall effect measurements provided an initial investigation of the electrical properties of the films, showing the formation of conductive, n-type InAs. However, electron mobility was poor by comparison to literature suggesting that optimisation of the deposition conditions is required in order to yield device-quality materials

Deposition of zinc oxide by spray pyrolysis

The objective of this work was to grow doped ZnO by spray pyrolysis at low temperature. This was achieved via the initial objective of growing ZnO in unhoped-for over a wide range of conditions, in order to understand the growth behaviour of ZnO from Zn(acac)2.H(_2)O and to establish the optimum growth procedure at high and low temperature. Various techniques were employed to characterize the films and thus determine the optimum growth conditions (i.e measurement of film thickness, resistivity, Hall coefficient. X-ray diffraction and reflection electron diffraction to name but a few).With the growth of undoped films it was found that the film properties varied with temperature and moisture content and that the optimum conditions for low temperature growth in a dry ambient were at 200ºC and for high temperature growth in a wet ambient at 300ºC.An analysis of the growth behaviour of ZnO was carried out and it was suggested that there were at least four mechanisms leading to the decomposition of the precursor used (Zn(acac)2.H(_2)O). They were decomposition by miramolecular, mtermolecular, thermolysis and hydrothermolysis processes. A kinetic analysis demonstrated that evaporation was the dominant process which reduced the efficiency of utilization of Zn(acac)(_2).H(_2)O. The growth of doped ZnO in glass and plastic at low temperature using InChl(_3) as a dopant yielded conducting films. The results also showed that films obtained using solutions with low concentrations of Zn(acac)(_2).H(_2)O and high concentrations of InCl(_3) were even more conducting ( p ≈ 10(^-5)Ωm). The morphology of film growth was dominated by the presence of dopant. High temperature growth of doped ZnO in a wet ambient using InCl(_3) also yielded conducting films and these were compared with indium, aluminium and gallium doped films where alternative dopant matericds such as In(acac)(_3), Al(OPr)(_3), AICL(_3) and Ga(acac)(_3) at a variety of different solution concentrations had been used. This was undertaken to discover whether these materials functioned as well as InCl(_3) in producing low resistivity ZnO. Doped films were characterised using the same techniques as before. Elemental analysis, photoluminescence and optical measurements were also carried out on these films. The main conclusions were that : (1) The growth rate of ZnO from Zn(acac)2 .H2O is heavily influenced by the growth temperature and other conditions. (2) The film resistivity was influenced by growth temperature. A minimum in the film resistivity was observed when a growth temperature of 300ºC was used. (3) Undoped ZnO films grown below 200ºC had a different preferred order to those grown above 200ºC.(4) The best high temperature conditions for the deposition of undoped conducting adherent ZnO lay in the region of 276 - 306ºC. (5) The best low temperature conditions for the growth of undoped conducting ZnO were in the region of 200ºC. (6) The low temperature growth (175-200ºC) of doped ZnO produced films with resistivities of the order of l-5xl0(^-5)Ωm and a visible transmittance of 80%. This compares favourably with ZnO:Al and SnO(_2) which have also been grown at low temperature

Self-propagating metathesis preparations of inorganic materials.

The potential of self-propagating reactions, with reagents such as lithium nitride, calcium nitride, sodium arsenide and magnesium silicide, in the production of inorganic materials has been investigated. Reactions were performed with anhydrous d-block and rare earth metal chlorides and can be described by the following generic equation where M is a Group 3-12 metal, Alk is a Group 1 or 2 element and E is Si, N, P, As, Sb, Bi or O. MClm + xAlknE → yMαE + xAlkClβ + zEγ Crude products were obtained normally as fused masses of material consisting of the products coated in the alkali chloride co-products. Grinding followed by washing with an appropriate solvent yielded the pure products with low levels of contamination from the other elements present in the reaction flux. The phases produced include rare earth and transition metal nitrides, metals and alloys, d-block phosphides, arsenides and antimonides, metal silicides and d-block oxides. The products were variously characterised by X-ray powder diffraction, scanning electron microscopy, energy dispersive X-ray analysis, magnetic susceptability, X-ray photoelectron spectroscopy, microanalysisand solid state (magic angle spinning) nuclear magnetic resonance spectroscopy. Thermocouple experiments, differential scanning calorimetry, photography and constant pressure calculations were used to examine the thermal aspects and timescales of reactions. Dilution with inert solids was used to reduce voracity of reactions and to control crystallinity of products. Liquid chlorides (TiCl4 and VCl4) were successfully employed to make high quality ternary phases such as Ti0.5V0.5E (E= N, P, As). Such reactions can progress via ionic or elemental mechanisms and evidence for either of these was gathered. Examples were found for both mechanisms which supported that the process was occurring. These conclusions were based on end- product analysis since the reaction conditions and timescales precluded the use of other techniques

Plasma assisted thin film formation

Thin films of fluorocarbon-based polymers can be deposited by plasma assisted polymerisation of various perfluorocarbons. The chemical natures of plasma polymers of hexafluoropropene and perfluorohexane were examined as a function of power, flow rate and position in reactor. Polymerised hexafluoropropene displayed increased fluorine contents at high powers; this is at odds with perfluorohexane which demonstrated lower fluorine contents. Differing reaction mechanisms between saturated and unsaturated perfluorocarbons were proposed to explain this. Both perfluorocarbons were found to give increased CF(_2) contents out of the plasma glow region. This was demonstrated to be a function of distance from the monomer inlet, and was ascribed to the production of long lived polymer forming species in the gas phase. Plasma oxidation of low density polyethylene, polystyrene and poly (ether ether ketone) with oxygen and carbon dioxide was modelled by corresponding photooxidation reactions. Correlations between the structure of the polymer, the treatment used, and the final products were drawn. Aliphatic components tended to give carbon-oxygen single bonds, phenyl rings were oxidised to carbonyl and acid groups, and carbonyl groups to acids. Metal-containing polymeric thin films were produced from plasmas of zinc acetylacetonate and aluminium tri-sec -butoxide. The products from each monomer were different, with the zinc compound resulting in a high proportion of zinc carboxylate and the aluminium compound giving the oxide or hydroxide. Incorporation of these compounds into a perfluorohexane plasma resulted in the formation of metal fluoride containing thin films