No description available.

A deposição em regime de subtensão (DRS) tem sido amplamente utilizada para a formação de filmes semicondutores, mas muito pouco utilizada para estudar a dopagem de filmes eletrodepositados. Logo, neste trabalho é discutido o uso da DRS como técnica de dopagem de filmes de selênio. Para isso, estudou-se as condições de deposição de um filme de selênio trigonal sobre substrato de ouro (f-Se) e sua posterior modificação com a DRS de Bi (f-Se_Bi), Pb (f-Se_Pb) e Cu (f-Se_Cu). Estudos empregando voltametria cíclica, cronoamperometria, microscopia óptica, microscopia de varredura eletrônica e difração de raio-X evidenciaram a produção de um filme altamente cristalino, homogêneo e aderente formado por estruturas hexagonais em forma de microbastões com diâmetros entre 300 e 600 nm. Contudo, o filme com as características citadas é formado somente com a deposição a 80 ºC em HNO3 0,1 mol L-1 contendo SeO2 0,02 mol L-1, com polarização do substrato em -0,45 V (vs SCE), sob iluminação com lâmpada de halogênio 100 W, irradiância de 200 mW cm-2, agitação magnética e tempos de deposição ≥ 600 segundos. Essas estruturas são mantidas mesmo após a DRS dos metais, a qual foi caracterizada empregando a microbalança eletroquímica de cristal de quartzo (MECQ). A partir do perfil massométrico constatou-se a ocorrência de difusão dos metais para o interior da fase de selênio, visto que não se observou a saturação da superfície do filme. Além disso, os metais apresentam cinética de difusão diferentes, onde o metal com maior difusão é o cobre, sendo o único a apresentar dopagem efetiva da matriz de selênio. A caracterização óptica dos filmes determinou um band gap de aproximadamente 1,87 ±0,03 eV para f-Se, f-Se_Pb e f-Se_Bi, entretanto, o f-Se_Cu apresentou band gap de 3,19 eV. Ainda assim, ao avaliar a atividade fotoeletroquímica dos semicondutores constatou-se a obtenção de fotocorrentes distintas entre eles. O f-Se_Bi produz fotocorrente 3 vezes maior que o f-Se_Pb e 35 vezes mais elevada que os demais, embora o selênio puro e os filmes dopados com DRS de Bi e Pb apresentem densidade de portadores de carga similares, aproximadamente 6,0 x1015 cm-3, enquanto o f-Se_Cu exibe um aumento de 4 ordens de grandeza para o mesmo parâmetro. Logo, a elevada fotocorrente do filme f-Se_Bi está relacionada com a minimização da recombinação de cargas na interface semicondutor-eletrólito, enquanto a baixa fotocorrente exibida pelo f-Se_Cu se deve à elevada energia de band gap do filme. De qualquer forma, a dopagem de filmes semicondutores com a DRS mostrou ser uma maneira simples e barata de dopagem de filmes relativamente espessos. / The underpotential deposition (UPD) have been widely used to production of semiconductor films, but not applied to search the doping of electrodeposited films. Therefore, here is discussed the use of UPD as a technique to doping selenium films. Then, it was studied the conditions to attain deposits of trigonal selenium on gold substrate (f-Se) and after its modification with Bi UPD (f-Se_Bi), Pb UPD (f-Se_Pb) and Cu UPD (f -Se_Cu). Studies using cyclic voltammetry, chronoamperometry, optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD) showed the production of a highly crystalline, homogeneous and adherent selenium film formed by hexagonal structures with microrod shape and diameters between 300 and 600 nm. However, the trigonal selenium with those features is synthezised only on the deposition into HNO3 0.1 M containing SeO2 0.02 M at 80 °C, -0.45 V (vs. SCE), under illumination with halogen lamp 100 W, irradiance of 200 mW cm-2, magnetic stirring and deposition time ≥ 600 seconds. These structures are maintained even after the UPD of the metals, which was characterized using electrochemical quartz crystal microbalance (EQCM). From massogram profile was possible to observe the occurrence of diffusion of the metals into the selenium phase, since was not verified the surface saturation of the film. Furthermore, the metals exhibited different diffusion kinetics, where the metal with higher diffusion was copper, which was the only one to show effective doping of selenium film. The optical characterization of the films determined a band gap average of 1.87 ±0,03 eV for f-Se, f-Se_Pb and f-Se_Bi, although, the f-Se_Cu presented band gap of 3.19 eV. Moreover, when it is studied the photoelectrochemical activity of the semiconductors films was noted different photocurrents between them. The f-Se_Bi produces photocurrent 3 times greater than the f-Se_Pb and 35 times higher than the other, but the pure selenium and those doped with Bi and Pb UPD present similar charge-carriers density, approximately 6.0 x1015 cm-3, while the f-Se_Cu shows an increase of 4 orders of magnitude for the same parameter. Therefore, the high photocurrent to f-Se_Bi is associated with the minimization of charge recombination at semiconductor-electrolyte interface, while the low photocurrent exhibited by f-Se_Cu is due to the higher energy band gap of the film. Anyway, the doping of the semiconductor film with the UPD proved to be a simple and inexpensive way to doping relatively thick films.

A deposição em regime de subtensão (DRS) tem sido amplamente utilizada para a formação de filmes semicondutores, mas muito pouco utilizada para estudar a dopagem de filmes eletrodepositados. Logo, neste trabalho é discutido o uso da DRS como técnica de dopagem de filmes de selênio. Para isso, estudou-se as condições de deposição de um filme de selênio trigonal sobre substrato de ouro (f-Se) e sua posterior modificação com a DRS de Bi (f-Se_Bi), Pb (f-Se_Pb) e Cu (f-Se_Cu). Estudos empregando voltametria cíclica, cronoamperometria, microscopia óptica, microscopia de varredura eletrônica e difração de raio-X evidenciaram a produção de um filme altamente cristalino, homogêneo e aderente formado por estruturas hexagonais em forma de microbastões com diâmetros entre 300 e 600 nm. Contudo, o filme com as características citadas é formado somente com a deposição a 80 ºC em HNO3 0,1 mol L-1 contendo SeO2 0,02 mol L-1, com polarização do substrato em -0,45 V (vs SCE), sob iluminação com lâmpada de halogênio 100 W, irradiância de 200 mW cm-2, agitação magnética e tempos de deposição ≥ 600 segundos. Essas estruturas são mantidas mesmo após a DRS dos metais, a qual foi caracterizada empregando a microbalança eletroquímica de cristal de quartzo (MECQ). A partir do perfil massométrico constatou-se a ocorrência de difusão dos metais para o interior da fase de selênio, visto que não se observou a saturação da superfície do filme. Além disso, os metais apresentam cinética de difusão diferentes, onde o metal com maior difusão é o cobre, sendo o único a apresentar dopagem efetiva da matriz de selênio. A caracterização óptica dos filmes determinou um band gap de aproximadamente 1,87 ±0,03 eV para f-Se, f-Se_Pb e f-Se_Bi, entretanto, o f-Se_Cu apresentou band gap de 3,19 eV. Ainda assim, ao avaliar a atividade fotoeletroquímica dos semicondutores constatou-se a obtenção de fotocorrentes distintas entre eles. O f-Se_Bi produz fotocorrente 3 vezes maior que o f-Se_Pb e 35 vezes mais elevada que os demais, embora o selênio puro e os filmes dopados com DRS de Bi e Pb apresentem densidade de portadores de carga similares, aproximadamente 6,0 x1015 cm-3, enquanto o f-Se_Cu exibe um aumento de 4 ordens de grandeza para o mesmo parâmetro. Logo, a elevada fotocorrente do filme f-Se_Bi está relacionada com a minimização da recombinação de cargas na interface semicondutor-eletrólito, enquanto a baixa fotocorrente exibida pelo f-Se_Cu se deve à elevada energia de band gap do filme. De qualquer forma, a dopagem de filmes semicondutores com a DRS mostrou ser uma maneira simples e barata de dopagem de filmes relativamente espessos. / The underpotential deposition (UPD) have been widely used to production of semiconductor films, but not applied to search the doping of electrodeposited films. Therefore, here is discussed the use of UPD as a technique to doping selenium films. Then, it was studied the conditions to attain deposits of trigonal selenium on gold substrate (f-Se) and after its modification with Bi UPD (f-Se_Bi), Pb UPD (f-Se_Pb) and Cu UPD (f -Se_Cu). Studies using cyclic voltammetry, chronoamperometry, optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD) showed the production of a highly crystalline, homogeneous and adherent selenium film formed by hexagonal structures with microrod shape and diameters between 300 and 600 nm. However, the trigonal selenium with those features is synthezised only on the deposition into HNO3 0.1 M containing SeO2 0.02 M at 80 °C, -0.45 V (vs. SCE), under illumination with halogen lamp 100 W, irradiance of 200 mW cm-2, magnetic stirring and deposition time ≥ 600 seconds. These structures are maintained even after the UPD of the metals, which was characterized using electrochemical quartz crystal microbalance (EQCM). From massogram profile was possible to observe the occurrence of diffusion of the metals into the selenium phase, since was not verified the surface saturation of the film. Furthermore, the metals exhibited different diffusion kinetics, where the metal with higher diffusion was copper, which was the only one to show effective doping of selenium film. The optical characterization of the films determined a band gap average of 1.87 ±0,03 eV for f-Se, f-Se_Pb and f-Se_Bi, although, the f-Se_Cu presented band gap of 3.19 eV. Moreover, when it is studied the photoelectrochemical activity of the semiconductors films was noted different photocurrents between them. The f-Se_Bi produces photocurrent 3 times greater than the f-Se_Pb and 35 times higher than the other, but the pure selenium and those doped with Bi and Pb UPD present similar charge-carriers density, approximately 6.0 x1015 cm-3, while the f-Se_Cu shows an increase of 4 orders of magnitude for the same parameter. Therefore, the high photocurrent to f-Se_Bi is associated with the minimization of charge recombination at semiconductor-electrolyte interface, while the low photocurrent exhibited by f-Se_Cu is due to the higher energy band gap of the film. Anyway, the doping of the semiconductor film with the UPD proved to be a simple and inexpensive way to doping relatively thick films.

No description available.

No description available.

No description available.

The synthesis and characterization of Pt core/ Cu shell (Pt@Cu) dendrimer-encapsulated nanoparticles (DENs) having full and partial Cu shells deposited via electrochemical underpotential deposition (UPD) is described. Pt DENs containing averages of 55, 147, and 225 Pt atoms immobilized on glassy carbon electrodes served as the substrate for the UPD of a Cu monolayer. This results in formation of Pt@Cu DENs. Evidence for this conclusion is based on results from the analysis of cyclic voltammograms (CVs) for the UPD and stripping of Cu on Pt DENs, and from experiments showing that the Pt core DENs catalyze the hydrogen evolution reaction before Cu UPD, but that after Cu UPD this reaction is inhibited. Results obtained by in-situ electrochemical X-ray absorption spectroscopy (XAS) confirm the core@shell structure. Calculations from density functional theory (DFT) show that the first portion of the Cu shell deposits onto the (100) facets, while Cu deposits lastly onto the (111) facets. The DFT-calculated energies for Cu deposition on the individual facets are in good agreement with the peaks observed in the CVs of the Cu UPD on the Pt DENs. Finally, structural analysis of Pt DENs having just partial Cu shells by in-situ XAS is consistent with the DFT-calculated model, confirming that the Cu partial shell selectively decorates the (100) facets. These results are of considerable significance because site-selective Cu deposition has not previously been shown on nanoparticles as small as DENs. In summary, the application of UPD as a synthetic route and characterization tool for core@shell DENs having well defined structures is established. A study of the degradation mechanism and degradation products of Pd DENs is provided as well. These DENs consisted of an average of 147 atoms per dendrimer. Elemental analysis and UV-vis spectroscopy indicate that there is substantial oxidation of the Pd DENs in air-saturated solutions, less oxidation in N₂-saturated solution, and no detectable oxidation when the DENs are in contact with H₂. Additionally, the stability improves when the DEN solutions are purified by dialysis to remove Pd²⁺-complexing ligands such as chloride. For the air- and N₂-saturated solutions, most of the oxidized Pd recomplexes to the interiors of the dendrimers, and a lesser percentage escapes into the surrounding solution. The propensity of Pd DENs to oxidize so easily is a likely consequence of their small size and high surface energy. Calculations from density functional theory (DFT) show that the first portion of the Cu shell deposits onto the (100) facets, while Cu deposits lastly onto the (111) facets. The DFT-calculated energies for Cu deposition on the individual facets are in good agreement with the peaks observed in the CVs of the Cu UPD on the Pt DENs. Finally, structural analysis of Pt DENs having just partial Cu shells by in-situ XAS is consistent with the DFT-calculated model, confirming that the Cu partial shell selectively decorates the (100) facets. These results are of considerable significance because site-selective Cu deposition has not previously been shown on nanoparticles as small as DENs. In summary, the application of UPD as a synthetic route and characterization tool for core@shell DENs having well defined structures is established. A study of the degradation mechanism and degradation products of Pd DENs is provided as well. These DENs consisted of an average of 147 atoms per dendrimer. Elemental analysis and UV-vis spectroscopy indicate that there is substantial oxidation of the Pd DENs in air-saturated solutions, less oxidation in N2-saturated solution, and no detectable oxidation when the DENs are in contact with H2. Additionally, the stability improves when the DEN solutions are purified by dialysis to remove Pd2+-complexing ligands such as chloride. For the air- and N2-saturated solutions, most of the oxidized Pd recomplexes to the interiors of the dendrimers, and a lesser percentage escapes into the surrounding solution. The propensity of Pd DENs to oxidize so easily is a likely consequence of their small size and high surface energy. / text

The ability to fabricate a charge-driven water pump is a crucial step toward mimicking the catalytic ability of natural enzyme systems. The first step towards making this water pump a reality is the ability to make a carbon nanotube (CNT) membrane with uniform, 0.8 nm pore diameter. Proposed in this work is a method for synthesizing these carbon nanotubes via VPI-5 zeolite templated, transition metal catalyzed pyrolysis. Using a membrane composed of these CNTs, it is possible to get water molecules to flow single file at a high flow rate, and to orient them in such a way that would maximize their ability to be catalyzed. Additionally, using the ability to plate a monolayer of precious metal catalyst molecules around the exit to the membrane, catalyst efficiency can be maximized by making every catalyst atom come into contact with a substrate molecule. In this work, we also demonstrate the ability to plate a monolayer of precious metal catalyst atoms onto an insulating, mesoporous, support material. By combining these two chemical processes, it is possible to mimic the catalytic efficiency of natural enzyme systems.

Nanoparticle-functionalized electrodes have attracted attention in areas such as energy production and storage, sensing, and electrosynthesis. The electrochemical properties of these electrodes depend upon the nanoparticle properties, e.g., core size, core morphology, surface chemistry, as well as the structure of the nanoparticle/electrode interface, including the coverage on the electrode surface, choice of electrode support, and the interface between the nanoparticle and the electrode support. Traditionally used methods of producing nanoparticle-functionalized electrodes lack sufficient control over many of these variables, particularly the nanoparticle/electrode interface. Tethering nanoparticles to electrodes with molecular linkers is a strategy to fabricate nanoparticle-functionalized electrodes that provides enhanced control over the nanoparticle/electrode structure. However, many existing tethering methods are done on catalytically active electrode supports, which makes isolating the electrochemical activity of the nanoparticle challenging. Furthermore, previous work has focused on larger nanoparticles, yet smaller nanoparticles with core diameters less than 2.5 nm are of interest due to their unique structural and electronic properties. This dissertation addresses both of these gaps, exploring small nanoparticle electrocatalysts that are molecularly tethered to catalytically inert electrodes. This dissertation first reviews and compares the methods of fabricating nanoparticle-functionalized electrodes with a defined molecular interface in the context of relevant attributes for electrochemical applications. Next, a new platform approach to bind small gold nanoparticles to catalytically inert boron doped diamond electrodes through a defined molecular interface is described, and the influence of the nanoparticle/electrode interface on the electron transfer properties of these materials is evaluated. The next two studies build upon this platform to evaluate molecularly tethered nanoparticles as oxygen electroreduction catalysts. The first of these two describes the systematic study of atomically precise small gold clusters, highlighting the influence of atomic level differences in the core size and the electrode support material on the catalytic properties. The second study extends the platform approach to study small bimetallic silver-gold nanoparticles produced on the electrode surface and highlights the influence of the structural arrangement of the metals on the catalytic activity. Finally, future opportunities for the field of molecularly tethered nanoparticle-functionalized electrodes are discussed. This dissertation includes previously published and unpublished co-authored material. / 2019-01-27

abstract: Palladium metal in its various forms has been heavily studied for many catalytic, hydrogen storage and sensing applications and as an electrocatalyst in fuel cells. A short review on various applications of palladium and the mechanism of Pd nanoparticles synthesis will be discussed in chapter 1. Size dependent properties of various metal nanoparticles and a thermodynamic theory proposed by Plieth to predict size dependent redox properties of metal nanoparticles will also be discussed in chapter 1. To evaluate size dependent stability of metal nanoparticles using electrochemical techniques in aqueous media, a synthetic route was designed to produce water soluble Pd nanoparticles. Also, a purification technique was developed to obtain monodisperse metal nanoparticles to study size dependent stability using electrochemical methods. Chapter 2 will describe in detail the synthesis, characterization and size dependent anodic dissolution studies of water soluble palladium nanoparticles. The cost associated with using expensive metal catalysts can further decreased by using the underpotential deposition (UPD) technique, in which one metal is electrodeposited in monolayer or submonolayer form on a different metal substrate. Electrochemically, this process can be detected by the presence of a deposition peak positive to the bulk deposition potential in a cyclic voltammetry (CV) experiment. The difference between the bulk deposition potential and underpotential deposition peak (i.e. the UPD shift), which is a measure of the energetics of the monolayer deposition step, depends on the work function difference between the metal pairs. Chapter 3 will explore how metal nanoparticles of different sizes will change the energetics of the UPD phenomenon, using the UPD of Cu on palladium nanoparticles as an example. It will be shown that the UPD shift depends on the size of the nanoparticle substrate in a way that is understandable based on the Plieth model. High electrocatalytic activity of palladium towards ethanol oxidation in an alkaline medium makes it an ideal candidate for the anode electrocatalyst in direct ethanol based fuel cells (DEFCs). Chapter 4 will explore the poisoning of the catalytic activity of palladium in the presence of halide impurities, often used in synthesis of palladium nanoparticles as precursors or shape directing agents. / Dissertation/Thesis / Doctoral Dissertation Chemistry 2016