Neste trabalho foram avaliadas as atividades de catalisadores do tipo perovskita LaNi1-xCoxO3 frente à reação de reforma a vapor de etanol. Devido à baixa área superficial, característica de óxidos do tipo perovskita, esses foram suportados em SiO2, Al2O3 e ZrO2, a fim de verificar o efeito do suporte na atividade catalítica.<br /> Os catalisadores foram preparados pelo método da co-precipitação e caracterizados por espectrometria de emissão atômica por plasma induzido, difração de raios X pelo método do pó, adsorção de nitrogênio pelo método B.E.T, redução a temperatura programada e espectroscopia de absorção de raios X.<br /> Para estudar o processo de redução e a possibilidade de oxidação durante a reação catalítica, foram realizados estudos in situ da reação de reforma a vapor, através da espectroscopia de absorção de raios X. Estes dados foram comparados com os resultados de aplicação das técnicas de RTP e DRX às amostras parcial e totalmente reduzidas e foi proposto um mecanismo de redução do óxido do tipo perovskita durante o processo de ativação do catalisador.<br /> Todos os catalisadores mostraram-se ativos nas reações de reforma a vapor de etanol e a seletividade dos produtos foi dependente do tipo do catalisador avaliado. De acordo com os resultados obtidos, destaca-se o catalisador não suportado LaNiO3, com conversão de etanol de 99% e seletividade para H2, CO e CO2 de 4,8; 1,1 e 1,3, respectivamente. Além disso, o ensaio com dois catalisadores simultâneos (LaNiO3 + LaCoO3) foi o que apresentou melhor estabilidade na reação, com 100% de conversão de etanol e seletividade semelhante à obtida pelo catalisador LaNiO3. / In this work, the catalytic activity of perovskite oxides, LaNi1-xCoxO3, was evaluated in the ethanol steam reforming. Due to the low surface area, characteristic of perovskite oxides, these catalysts were supported on SiO2, AI2O3 and ZrO2 and the effect of the support was evaluated. The catalysts were prepared by the co-precipitation method and characterized by Atomic Induced Plasma Spectroscopy, X-Ray Powder Diffraction (XRD), Nitrogen adsorption by B.E.T. method, Temperature Programmed Reduction (TPR) and X-ray Absorption Near Edge Structure (XANES).<br /> The catalytic process was accompanied by XANES in situ to verify changes in the oxidation state of the active phase during the activation process with H2 and also during the process of steam reforming of ethanol. By relation of these results with TPR and XRD, for samples partially and fully reduced, it was proposed a mechanism for the reduction of the perovskite oxides during the conditions of activation.<br /> All catalysts showed activity for the ethanol steam reforming with the selectivity dependent of the catalyst evaluated. Summarizing, the results showed that the unsupported catalyst LaNiO3 presented the better performance, with the ethanol conversion of 99% and selectivity for H2, CO and CO2 of 4.8; 1.1 and 1.3, respectively. Moreover, the test using two simultaneous catalysts (LaNiO3 + LaCoO3), showed better stability in the reaction, presenting ethanol conversion of 100% and selectivity to H2, CO and CO2 similar to the LaNiO3 catalyst.

Lead halide perovskites (LHPs) have forcefully emerged as a promising materials class for next-generation solar cells. The high efficiencies of LHP-based photovoltaics are underpinned by their outstanding optoelectronic properties, including long carrier lifetimes, long carrier diffusion lengths, high radiative efficiencies, and long-lived hot carriers. In conventional semiconductors, high efficiencies are achieved by stringent control over defect densities; higher purity diminishes the number of carrier scattering events and yields better optoelectronic properties. Given the high defect densities of LHPs, these observed behaviors indicate that LHPs are defect-tolerant and disobey this paradigm via dynamic screening of charge carriers. In order to expand the library of defect-tolerant semiconductors, we must elucidate the carrier-lattice interactions that lead to dynamic screening. LHP lattices are highly anharmonic and dynamically disordered, which must play a role in this screening mechanism. This anharmonicity demands a departure from the conventional Fröhlich interaction, which considers the harmonic coupling of a carrier to one phonon, to a picture that incorporates anharmonic phonon-phonon couplings. The objective of this thesis is to investigate the ultrafast anharmonic lattice response associated with dynamic screening of charge carriers. We probe the formation of large polarons in CH3NH3PbBr3 and CsPbBr3 using time-resolved optical Kerr effect spectroscopy. We further investigate the coupling of phonon modes in a model system, CsPbBr3, in the presence of charge carriers using ultrafast coherent phonon spectroscopy.

The current energy and environmental scenario in the world demands acute attention on sustainable repurposing of waste CO2 to high value hydrocarbons that not only addresses the CO2 mitigation problem, but also provides pathways for a closed loop synthetic carbon cycle. Difference in the scales of global CO2 emissions (about 40 Gtpa, 2017) and the carbon capture and sequestration (CCS) facilities (estimated cumulative 40 Mtpa, 2018) provokes active research on this topic. Solar thermochemical (STC) and visible light photocatalysis are two of the most promising routes that have garnered attention for this purpose. While STC has the advantages of high CO2 conversion rates, it operates at high temperatures (more than 1000 °C) limiting its industrial implementation. Photocatalysis, on the contrary, is plagued by the poor quantum efficiency and conversion rates, although its exhibits the benefits of low temperature operation. Thus, any significant progress towards low temperature STC and visible light photocatalytic CO2 reduction is a giant leap towards a greener and sustainable energy solution. This dissertation is an effort towards improving both the STC and photocatalytic CO2 reduction. Reverse water gas shift - chemical looping (RWGS-CL) is a modified STC approach that has the potential for low temperature CO2 conversion. RWGS-CL process uses mixed metal oxides like perovskite oxides (ABO3) for the conversion to CO, a potential feedstock for subsequent hydrocarbon production. Generation of oxygen vacancy defects on these perovskite oxides is a key step of RWGS-CL and thus, oxygen vacancy formation energy has been found to be a key descriptor for this process. Using density functional theory based calculations, this intrinsic material property has been used towards rational design of better catalysts. Highest rate of CO2 conversion at the low temperatures of 450 °C was demonstrated by earth abundant perovskite oxide via RWGS-CL. This low temperature and stable CO2 conversion process enables thermal integration with subsequent Fischer Tropsch processes for the hydrogenation of CO to hydrocarbons. Parallel to the developments on materials discovery, another crucial parameter that deserves attention is the surface termination effects of the perovskite oxides. Hence, the site specificity of the bulk and surface oxygen vacancies have been probed in detail towards elucidating the CO2 conversion performance over these materials. In the view of recent progress on the growth of selective crystal facets and terminations, this study opens new avenues for enhanced CO2 conversion performance not only through bulk composition variation, but also via exposing desired crystal facets. Type-II semiconductor heterojunctions (staggered type) are promising candidates for efficient photocatalytic reactions, not only because of their capabilities of electronic density of states tuning, but also their ability to segregate the excited electrons and holes into different materials thereby restricting exciton recombination. Metal oxynitride heterojunctions have recently demonstrated promising activity on visible light water splitting. Elucidating the structure-function relationships for these materials can pave the way towards designing better CO2 conversion photocatalysts. This dissertation focuses on unravelling the roles of material composition, anion vacancy defects and lattice strain towards modulating the electronic density of states of lateral and vertical heterojunctions of (ZnO)X(AlN)1-X and (ZnO)X(GaN)1-X. The heterojunctions consist of periodic potential wells that allows for restricting interlayer charge transport. Increased ZnO concentration was explicitly shown to decrease the band gap due to N 2p and Zn-3d repulsion. Biaxial and vertical compressive strain effected increased band gap while tensile strain reduced the same. Oxygen vacancies was found to have different effect on the electronic state of the materials. When present in charged state (+2), it promotes mid gap state formation, while in neutral state it revealed increased electronic densities near the valence band and conduction band edges. These fundamental site specific material property tuning insights are essential for designing better photocatalysts for future.

Metal organic frameworks have gained much attention due to their tunable pore sizes and very high surface areas. With the discovery many of these type materials the need has raised to look into new applications of theses porous frameworks. This thesis focuses on the synthesis of a new perovskite-type metal organic framework and measurement of its thermal conductivity in search of its applicability as a thermoelectric material. The second part of this work focuses on the synthesis of a metal organic framework incorporated with a Lewis pair for the first time. The optimum loading amount of the Lewis pair into the framework was also investigated.

The discovery of ferroelectricity at the nanoscale has incited a lot of interest in perovskite ferroelectrics not only for their potential in device application but also for their potential to expand fundamental understanding of complex phenomena at very small size scales. Unfortunately, not much is known about the dynamics of ferroelectrics at this scale. Many of the widely held theories for ferroelectric materials are based on bulk dynamics which break down when applied to smaller scales. In an effort to increase understanding of nanoscale ferroelectric materials we use atomistic resolution computational simulations to investigate the dynamics of polar perovskites. Within the framework of a well validated effective Hamiltonian model we are able to accurately predict many of the properties of ferroelectric materials at the nanoscale including the response of the soft mode to mechanical boundary conditions and the polarization reversal dynamics of ferroelectric nanowires. Given that the focus of our study is the dynamics of ferroelectric perovskites we begin by developing an effective Hamiltonian based model that could simultaneously describe both static and dynamic properties of such materials. Our study reveals that for ferroelectric perovskites that undergo a sequence of phase transitions, such as BaTiO3. for example, the minimal parameter effective Hamiltonian model is unable to reproduce both static and dynamical properties simultaneously. Nevertheless we developed two sets of parameters that accurately describes the static properties and dynamic properties of BaTiO3 independently. By creating a tool that accurately models the dynamical properties of perovskite ferroelectrics we are able to investigate the frequencies of the soft modes in the perovskite crystal. The lowest energy transverse optical soft modes in perovskite ferroelectrics are known to be cause of the ferroelectric phase transition in these materials and affect a number of electrical properties. The performance of a ferroelectric device is therefore directly influenced by the dynamics of the soft mode. Interestingly, however, little study has been done on the effect of mechanical boundary conditions on the soft modes of perovskites. Understanding the effect of mechanical forces on the soft modes is critical to device applications as complicated growth structures often are the cause of pressures, stresses and strains. Using classical molecular dynamics we study the effect of hydrostatic pressure, uniaxial stress, biaxial stress and biaxial strain on the soft modes of the ferroelectric PbTiO3. The results of this study indicate the existence of Curie-Weiss laws for not only hydrostatic pressure, which is well known, but also for uniaxial stress, biaxial stress and biaxial strain. The mode frequencies are also seen to respond very differently to these mechanical forces and lead to a more complete picture of the behavior of nanoscale ferroelectrics. One nanoscale geometry of perovskite ferroelectrics is the pseudo one-dimensional nanowire. These structures have very unique properties that are highly attractive for use as interconnects, nanoscale sensors or more directly in computer memory devices. Perovskite nanowires have only recently been synthesized and the techniques are not well developed. While progress has been made towards consistently fabricating uniform, high quality nanowires experimental investigation of their properties is prohibitively difficult. Of immediate interest is the polarization reversal dynamics of ferroelectric nanowires. The reading and writing of bits of information stored in a wire's polarization state is done by switching the polarization. Again using classical molecular dynamics we study the polarization reversal dynamics in ferroelectric nanowires made of Pb(Ti1-xZrx)O3 disordered alloy. We find that there are two competing mechanisms for polarization reversal and that the interplay of these mechanisms is dependent on electric field strength. The dynamics in nanowires also sheds light on long standing theories about polarization reversal mechanisms in thin film and bulk geometries.

This work describes detailed convergent beam electron diffraction (CBED) studies of GdAlO3 and LaAlO3 perovskites. CBED patterns tilted away from major zone axes have been found to have high sensitivity to the presence of mirror or glide mirror symmetry. Such patterns confirm to high accuracy that the space group of GdAlO3 is orthorhombic, Pnma. Tilted patterns from this well characterised structure also serve as benchmarks against which similar patterns may be compared. In the case of LaAlO3, tilted patterns enable the space group to be confirmed as rhombohedral R3c, previously claimed to be cubic (Fm3c) by CBED. Furthermore, no evidence for the low symmetry (I2/a or F1) phases proposed for LaAlO3 has been observed. The LaAlO3 study also gives a careful assessment of the influence of tilted specimen surfaces on the CBED data. Within the qualitative scope of these experiments, no symmetry degrading effects could be observed. Some preliminary Quantitative CBED (QCBED) data from LaAlO3 is also presented. This shows it will be possible to make a detailed study of the bonding charge density (Δρ) in this material when combined with X-ray diffraction data. Also included is a brief CBED study of LaFeO3, a material that is isostructural with GdAlO3. Although this is restricted to exact zone axis patterns, it is noted that tilted patterns have significant potential to improve the quality of the symmetry determination.

No description available.

We report an intriguing giant dielectric and magnetodielectric (MD) response on double perovskite Pr2CoMnO6(PCMO) system. The Arrhenius plot indicates that the origin of giant dielectric is internal barrier layer capacitance. Meanwhile, at the highest applied magnetic field 9T, the giant dielectric constant around Tm ~ 150 K is enhanced almost ~ 20% (at 10 kHz frequency) compared with that at zero field. The observed positive MD effect is considered to be associated with the direct consequence of negative magnetoresistance changes (~ -20% at 150 K) which was calculated by temperature dependent impedance spectras. Concomitantly, a pronounced ferromagnetic ordering is observed near Tc ~ 150 K coinciding with Tm of £`¡¬(T). These experimental results suggest that the magnetoresistive and MD effect response is very strongly by magnetic property of PCMO.

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Phase-transformation-induced microstructures, including twin domains, anti-phase domains and inversion domains have been analyzed using the scanning and transmission electron microscopy for BaTiO3, BaCeO3 and CaTiO3 of the perovskite structure. Differential etching rate was taken to identify the ferroelectric domains in tetragonal (t-) BaTiO3. Space group Pbnm (No. 62) usually adopted for the orthorhombic crystals by materials scientists is assumed throughout this research to avoid confusion of the plane and direction indices. Traditional contrast analysis was adopted for determining dislocation Burgers vectors (b) and fault vectors (R) in deformed and phase-transformed perovskites, synthetic ceramics (BaTiO3, BaCeO3 and CaTiO3) as well as natural minerals (CaTiO3), polycrystalline (BaTiO3, BaCeO3 and CaTiO3) as well as single crystal (CaTiO3). Atomic images for the structures of twin boundaries and anti-phase boundaries were taken by high resolution technique and image contrast enhancement was performed using fast Fourier transform. Failure of Friedel¡¦s law is adopted for determining if the crystal belongs to non-centrosymmetric point groups. Whether the twins are £_-, £\- or £k-type (i.e. anti-phase domain boundaries) is analysed from the contrast of extreme fringe patterns. Tilting experiments were performed on selected area diffraction patterns containing un-split row of reflections to ensure that the twin boundaries are the reflection or rotation type. Transformation twinning in all perovskites studied here follows the prediction by the relation of point group symmetries between the high- and low-symmetry phases, assuming continuous, diffusionless, second-order transitions that obey the restrictions imposed by the Landau theory of phase transition. Although such predictions of transformation-induced twinning are only permitted when crystallographic group-subgroup relationship exists and structural coherence retains between the high- and low-symmetry phases, experimental observations for r (rhombohedral) ¡÷ o-BaCeO3 and t ¡÷ o in CaTiO3 that are not related by group-subgroup, c (cubic) ¡÷ t (tetragonal) in CaTiO3 and and c (cubic) ¡÷ t (tetragonal) in BaTiO3 that are related by group-subgroup, are all consistent with the predictions from loss of point group symmetry elements and change of unit cell volume. In order that the Landau theory is conformed, however, an intermediate phase of either the lowest common supergroup (cubic Pm m) or highest common subgroup (monoclinic C2/c), with phase transition experiencing multistage pathways suggested by Christy and assumption of non-disruption conditions proposed by Guymont, was identified to bridge between two structures, such as rhombohedral and orthorhombic that are not group-subgroup related. Both the 90o and 180o ferroelectric twin domains in t-BaTiO3 are the reflection type and have been identified in pressureless-sintered ceramics. Further, fault vectors (R = £`<110]) for such domain boundaries were determined, boundary planes of {110) for the former, {100) and {220) for the latter deduced accordingly. The polar c-direction between adjacent domains was determined by differential etching rate across domain boundaries, convergent beam electron diffraction was also adopted for identification and confirmation of the c-axis for two types of domains in t-BaTiO3. Plastic deformation resulting from the thermodynamic driving force for sintering (?p) intensified by a multiplication factor £p) was evidenced microstructurally from analysing dislocations in pressureless-sintered BaTiO3 where a Frank-Read source was observed. Slip systems are activated for the effective stress acting on the slip plane along the slip direction has exceeded the critical value of resolved shear stress (£nCRSS) and yielding occurs. It has contributed to densification, i.e. the overall system shrinkage of a green powder compact, although if such contribution is at all significant requires studies of sintering kinetics to ascertain. Dislocation dissociation into the scallop-shaped half partials according to the following reactions is determined from analysing corresponding Burgers vectors. [010] + [001] ¡÷ [011] [001] + [10 ] ¡÷ [100] [001] + [110] ¡÷ [111] Both transformation twins lying in {110) and {112) and anti-phase domain boundaries with R = 1/2<111> are detected in o-BaCeO3. For orthorhombic (o-) BaCeO3, fault vectors of the latter R = 1/2<111> determined by contrast analysis was confirmed by high-resolution imaging, but on the contrary, fault vectors the former R = £`<110] and £`<021], respectively, could not be determined from such images. Utilizing the technique of large-angle convergent beam electron diffraction, such fault vectors and dislocation Burgers vectors determined by traditional contrast analysis have been confirmed. Both twinning and dislocations were observed in hot-pressed CaTiO3 prepared in a multi-anvil apparatus. Such twins are deformation twins since hot-pressing was conducted in the orthorhombic stable phase field at 1000oC under 8 GPa. Since fault vectors R = £`<110] determined for {112) and {110) twins are different from the transformation-induced twins in o-CaTiO3, R = £`<021] determined for the {112) twinning in natural perovskite may serve as a diagnostic feature for the deformation twins. Plastic deformation in hot-pressured sample was contributed by both slip and twinning. Slip occurred via slip systems with dislocations of b = [110] gliding in (110) is therefore {110}o <1 0>o (equivalent to {100}pc <001>pc, where pc for pseudo-cubic) often found in perovskites deformed at high temperatures. Another set of dislocations with b = [001] in screw orientation was also determined. APB with R = 1/2<111> detected in natural minerals suggests that the phase transition sequence in CaTiO3 is better described by: (c) ¡÷ t (I4/mcm) ¡÷ o (Pbnm) and such APB are generated from loss of the lattice point at I-centre (1/2,1/2,1/2) in the absence of a second orthorhombic Cmcm between t-I4/mcm and o-Pbnm reported before from neutron and X-ray powder diffraction studies.