As Células a combustível são conversores de energia química em energia elétrica. As Células do tipo PEM que funcionam com metanol como combustível tem uma ampla variedade de aplicações. Os materiais utilizados como eletrocatalisadores nas células são responsáveis por uma grande parte do custo das mesmas. Outros problemas, que provocam diminuição da eficiência da célula, são a cinética lenta da reação de redução de oxigênio (RRO) e o potencialmisto gerado devido ao cruzamento de metanol através da membrana. Neste trabalho apresenta-se um estudo de catalisadores do tipo M-MxSy (M = Pt, Rh) para a RRO em meio ácido, com diferentes relações M:MxSy. Os materiais preparados a partir da modificação do método do ácido fórmico (MAF) com tiouréia (TU) foram caracterizados mediante XRD, XPS e XAS. Foi determinado mediante estas técnicas que os catalisadores consistem numa mistura de fases: Pt ou PtRh, PtS, RhxSy e PtS2. O efeito de um tratamento térmico em H2/Ar foi reduzir completamente o PtS2 e parcialmente o PtS. A fase de RhxSy mostrou ser mais estável nas condições do tratamento. Todos os materiais mostraram ter atividade para a RRO e alta seletividade na presença de metanol. Foi determinado que para que a RRO ocorra via 4 elétrons, deve haver sítios metálicos na superfície das nanopartículas. Determinou-se que os materiais contendo maior quantidade de fase MxSy podem ser ativados mediante tratamento térmico ou eletroquímico, melhorando a atividade catalítica frente a RRO e conservando a seletividade na presença de metanol. / Fuel cells are dispositives that convert chemical energy into electricity. The PEM fuel cell types that function with methanol as fuel have a wide variety of applications. The materials used as electrocatalysts in the cells are responsible for the major part of their cost. Other problems, that cause decrease in efficiency of the cell, are the slow kinetics of oxygen reduction reaction (ORR) and the mixed potential generated due to methanol crossover through the membrane. This thesis presents a study of M-MxSy-type catalysts (M = Pt, Rh) for the ORR in acid medium, with different M:MxSy ratios. The materials prepared from the modification of the formic acid method (FAM) with thiourea (TU) were characterized by XRD, XPS and XAS. It was determined by these techniques that the catalysts consist of a mixture of phases: Pt or PtRh, PtS, RhxSy and PtS2. The effect of heat treatment in H2/Ar atmosphere was to reduce PtS2 completely and PtS partially. The RhxSy phase proved to be more stable under the treatment conditions. All materials showed to have activity for the ORR and high selectivity in the presence of methanol. It was determined that for the ORR to occur via four electrons, there must be metallic sites at the surface of the nanoparticles. It was determined that the materials containing higher amount of MxSy phase can be activated by thermal or electrochemical treatment, improving the ORR catalytic activity and retaining the selectivity in the presence of methanol.

Yin, Ling = 銅基化合物薄膜太陽能電池 : 新透明導電電極與缺陷物理 / 尹苓. / Thesis Ph.D. Chinese University of Hong Kong 2015. / Includes bibliographical references. / Abstracts also in Chinese. / Title from PDF title page (viewed on 26, October, 2016). / Yin, Ling = Tong ji hua he wu bo mo tai yang neng dian chi : xin tou ming dao dian dian ji yu que xian wu li / Yin Ling.

No description available.

Current leading thin-film solar cell technologies, i.e., cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS), employ elements which are either toxic (Cd), or rare and/or expensive (In, Te, Ga, and Cd). The aim of this thesis is to investigate new, abundant, non-toxic p-type semiconductors for potential solar absorber application. Two ternary chalcogenides, Cu���PSe��� and CuTaS���, were selected for their attractive calculated optical absorption properties. Thin films of both materials were synthesized using physical vapor deposition (PVD) techniques in conjunction with post-deposition annealing. Cu���PSe��� appears promising for solar absorber applications, with a measured optical bandgap of 1.2 eV, an absorption coefficient (��) reaching 10��� cm�����, Hall mobilities of 19.8���30.3 cm��/V���s, and carrier concentrations of 3.3���4.9 10����� cm�����. Optical characterization of CuTaS��� thin-films showed a rapid turn-on of absorption, with �� exceeding 10��� cm����� within 0.5 eV of the bandgap. To date, reproducible synthesis of CuTaS��� thin films has been problematic. Moreover, these films are insulating and thus not yet appropriate for thin-film solar cell absorber applications. / Graduation date: 2013

Thermoelectric energy (TE) conversion can be used to create electricity from temperature gradients. Hence power can be generated from waste heat using TE materials, e.g. from the exhaust in automotives. This power in turn may lead to a reduction of gas consumption by reducing the alternator load on the engine. Because of the increasing demand and limited availability of energy sources, there is strong and renewed interest in advancing thermoelectric materials. Past research shows that the best TE materials are narrow band gap semiconductors composed of heavy elements, exhibiting a large Seebeck coefficient, S, combined with high electrical conductivity, σ, and low thermal conductivity, κ. Various research projects have been attempted during the past four years of my Ph.D. studies. These include the synthesis, crystal structure studies, electronic structure calculations and thermoelectric properties of transition metal oxides and thallium main group chalcogenides. Because of the good thermal stability, lack of sensitivity to the air, and non-toxicity, transition metal oxides are potential candidates for commercial thermoelectric applications. During the investigation of oxides for thermoelectric application, several interesting features of different transition metal oxides have been discovered: 1. A new quaternary layered transition-metal oxide, Na2Cu2TeO6, has been synthesized under air using stoichiometric mixtures of Na2CO3, CuO and TeO2. Na2Cu2TeO6 crystallizes in a new structure type, monoclinic space group C2/m with a = 5.7059(6) Å, b = 8.6751(9) Å, c = 5.9380(6) Å,  = 113.740(2)°, V = 269.05(5) Å3 and Z = 2, as determined by single crystal X-ray diffraction. The structure is composed of[Cu2TeO6] layers with the Na atoms located in the octahedral voids between the layers. Na2Cu2TeO6 is a green nonmetallic compound, in agreement with the electronic structure calculation and electrical resistance measurement. 2. An n-type narrow band gap semiconductor, LaMo8O14, exhibiting the high Seebeck coefficient of -94 μVK-1 at room temperature has been investigated. 3. Pb0.69Mo4O6 with a new modulated structure and stoichiometry was determined from single-crystal X-ray diffraction data. The compound crystallizes in the tetragonal super space group, P4/mbm(00g)00ss, with a = 9.6112(3) Å, c = 2.8411(1) Å, q = 0.25c*, which is different from the previously reported structure. As for the research of thermoelectric properties of thallium main group chalcogenides, three new ternary thallium selenides, Tl2.35Sb8.65Se14, Tl1.97Sb8.03Se13 and Tl2.04Bi7.96Se13, have been discovered. All three compounds crystallize in the same space group P21/m with different cell parameters, and in part different Wyckoff sites, hence different structure types. The three selenides with similar structures are composed of distorted edge-sharing (Sb,Bi)Se6 octahedra, while the distorted Tl/(Sb, Bi) sites are coordinated by 8 - 9 Se atoms. Electronic structure calculations and physical property measurements reveal they are semiconductors with high Seebeck coefficient but low electrical conductivity, and therefore not good thermoelectrics. On the other hand, our transport property measurements on the unoptimized Tl2SnTe3 sample show interesting thermoelectric properties of this known compound. Advanced thermoelectrics are dominated by antimonides and tellurides so far. The structures of the tellurides are mostly composed of NaCl-related motifs, hence do not contain any Te–Te bonds. All of the antimonide structures containing Sb–Sb bonds of various lengths are much more complex. The Sb atom substructures are Sb24– pairs in β-Zn4Sb3, linear Sb37– units in Yb14MnSb11, planar Sb44– rectangles in the skutterudites, e.g., LaFe3CoSb12, and Sb8 cubes interconnected via short Sb–Sb bonds to a three-dimensional network in Mo3Sb5Te2. The results of electronic structure calculations suggested that these interactions have a significant impact on the band gap size as well as on the effective mass around the Fermi level, which represent vital criteria for advanced thermoelectrics. The crystal structure and electronic structure investigation for the unique T net planar Sb–Sb interactions in Hf5Sb9 will be also presented, although Hf5Sb9 is metallic compound with poor thermoelectric performances.

Thermoelectric energy (TE) conversion can be used to create electricity from temperature gradients. Hence power can be generated from waste heat using TE materials, e.g. from the exhaust in automotives. This power in turn may lead to a reduction of gas consumption by reducing the alternator load on the engine. Because of the increasing demand and limited availability of energy sources, there is strong and renewed interest in advancing thermoelectric materials. Past research shows that the best TE materials are narrow band gap semiconductors composed of heavy elements, exhibiting a large Seebeck coefficient, S, combined with high electrical conductivity, σ, and low thermal conductivity, κ. Various research projects have been attempted during the past four years of my Ph.D. studies. These include the synthesis, crystal structure studies, electronic structure calculations and thermoelectric properties of transition metal oxides and thallium main group chalcogenides. Because of the good thermal stability, lack of sensitivity to the air, and non-toxicity, transition metal oxides are potential candidates for commercial thermoelectric applications. During the investigation of oxides for thermoelectric application, several interesting features of different transition metal oxides have been discovered: 1. A new quaternary layered transition-metal oxide, Na2Cu2TeO6, has been synthesized under air using stoichiometric mixtures of Na2CO3, CuO and TeO2. Na2Cu2TeO6 crystallizes in a new structure type, monoclinic space group C2/m with a = 5.7059(6) Å, b = 8.6751(9) Å, c = 5.9380(6) Å,  = 113.740(2)°, V = 269.05(5) Å3 and Z = 2, as determined by single crystal X-ray diffraction. The structure is composed of[Cu2TeO6] layers with the Na atoms located in the octahedral voids between the layers. Na2Cu2TeO6 is a green nonmetallic compound, in agreement with the electronic structure calculation and electrical resistance measurement. 2. An n-type narrow band gap semiconductor, LaMo8O14, exhibiting the high Seebeck coefficient of -94 μVK-1 at room temperature has been investigated. 3. Pb0.69Mo4O6 with a new modulated structure and stoichiometry was determined from single-crystal X-ray diffraction data. The compound crystallizes in the tetragonal super space group, P4/mbm(00g)00ss, with a = 9.6112(3) Å, c = 2.8411(1) Å, q = 0.25c*, which is different from the previously reported structure. As for the research of thermoelectric properties of thallium main group chalcogenides, three new ternary thallium selenides, Tl2.35Sb8.65Se14, Tl1.97Sb8.03Se13 and Tl2.04Bi7.96Se13, have been discovered. All three compounds crystallize in the same space group P21/m with different cell parameters, and in part different Wyckoff sites, hence different structure types. The three selenides with similar structures are composed of distorted edge-sharing (Sb,Bi)Se6 octahedra, while the distorted Tl/(Sb, Bi) sites are coordinated by 8 - 9 Se atoms. Electronic structure calculations and physical property measurements reveal they are semiconductors with high Seebeck coefficient but low electrical conductivity, and therefore not good thermoelectrics. On the other hand, our transport property measurements on the unoptimized Tl2SnTe3 sample show interesting thermoelectric properties of this known compound. Advanced thermoelectrics are dominated by antimonides and tellurides so far. The structures of the tellurides are mostly composed of NaCl-related motifs, hence do not contain any Te–Te bonds. All of the antimonide structures containing Sb–Sb bonds of various lengths are much more complex. The Sb atom substructures are Sb24– pairs in β-Zn4Sb3, linear Sb37– units in Yb14MnSb11, planar Sb44– rectangles in the skutterudites, e.g., LaFe3CoSb12, and Sb8 cubes interconnected via short Sb–Sb bonds to a three-dimensional network in Mo3Sb5Te2. The results of electronic structure calculations suggested that these interactions have a significant impact on the band gap size as well as on the effective mass around the Fermi level, which represent vital criteria for advanced thermoelectrics. The crystal structure and electronic structure investigation for the unique T net planar Sb–Sb interactions in Hf5Sb9 will be also presented, although Hf5Sb9 is metallic compound with poor thermoelectric performances.

Atomically thin group-VI transition metal dichalcogenides (TMDC) has been emerging as a family of intrinsic 2-dimensional (2D) crystals with a sizeable bandgap in the visible and near infrared range, satisfying numerous requirements for ultimate electronics and optoelectronics. This intrinsic 2D crystal also provides a perfect platform for physics study in 2D semiconductors. The characteristic inversion symmetry breaking presented in monolayer TMDCs leads to non-zero but contrasting Berry curvatures and orbital magnetic moments at K/K’ valleys located at the corners of the first Brillouin zone. These features provide an opportunity to manipulate electrons’ additional internal degrees of freedom, namely the valley degree of freedom, making monolayer TMDC a promising candidate for the conceptual valleytronics. Besides, the strong spin-orbit interactions and the subsequent spin-valley coupling demonstrated in 2D TMDCs open potential new routes towards quantum manipulation. In this thesis, I give a brief review on the background and our progress of the physics study in 2D TMDCs (MoS2, WS2) via optical spectroscopy. Particularly, our experimental approach on the excitonic effect, valley dependent circular dichroism, and the spin-valley coupling in monolayer and bilayer TMDCs are elaborated in individual chapters. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

No description available.

Reevaluation of organosulfur heats of formation resulted in the conclusion that the sulfur-sulfur bond of aliphatic disulfides is nearly 15 Kcal/mole stronger than the disulfide carbon-sulfur bond energy. Semi-empirical calculations using AM1, MNDO and MINDO/3 from the AMPAC program package confirmed these results and clarified their relative impact on organosulfide and disulfide properties. / Existing organoselenium bond energy data were also reevaluated and erroneous assumptions discovered. New bond energy estimates were made via a new procedure. It was concluded that the selenium-selenium bond is also stronger than the selenium-carbon of aliphatic diselenides. / The above results led to the conclusion that loss of molecular dichalcogen from molecules such as disulfides or diselenides is favored over the stepwise loss of a single chalcogen by about 40 Kcal/mol. Loss of molecular diselenium from dibenzyl diselenide is reported. The average carbon-selenium bond energy of the latter is calculated to be only 27 Kcal/mol. / The preparation of 2-thiatriselenides (RSeSSeR) and 2,3-dithiatetraselenides (RSeSSSeR) has been achieved from selenosilanes. The products were characterized by $ sp{77}$Se NMR and the trends of chemical shifts were analyzed. / The preparation of the first selenium transfer reagents, including the first unsymmetrical chalcogen transfer reagent, is reported. Their use to effect the synthesis of 2-selenatrisulfides (RSSeSR) is also reported. Low temperature $ sp{77}$Se NMR was used to elucidate the reaction mechanism and characterize several intermediates such as selenuranes, selenonium ions and azole selenides. In addition, 2,3-diselenatetrasulfides (RSSeSeSR) were characterized by $ sp{77}$Se NMR.

Thesis (Ph.D.) -- McMaster University, 1998. / Includes bibliographical references (leaves 281-296). Also available via World Wide Web.