Anionic segregation of chloride and fluoride doped-nano ZnO: microstructure evolution, electrical conductivity and photocatalysis.

• Autor(a) principal: Fortes, Gustavo Mattos
• Data de Publicação: 2022
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Biblioteca Digital de Teses e Dissertações da USP
• Texto Completo: https://www.teses.usp.br/teses/disponiveis/3/3133/tde-17032023-105158/
• topic: Abatimento de CO2; Artificial photosynthesis; Cl--doping; CO2 abatement; Dopagem com Cl-; Dopagem com F-; Energia de interface; Excesso de interface; F--doping; Fotossíntese artificial; Interface energy; Interface excess; Lixiviação seletiva; Nanopartículas de ZnO; Selective lixiviation; ZnO nanoparticles

CO2 emission levels in the atmosphere are of increasing concern as it is associated with global warming. Thus, smart solutions such as artificial photosynthesis (AP) are proposed, because not only do these consume CO2, but they also can chemically store energy as fuels. Indeed, AP consists of converting CO2 and H2O into organic molecules, such as methane (CH4), methanol (CH3-OH), and ethanol (C2H5-OH). In this study, nano- ZnO was selected to perform APs photocatalytic reactions, since ZnO is an intrinsic ntype wide band gap (Eg = 3.37 eV) semiconductor. The ZnO surface, significantly increased due to the nanosized crystallites, presents a high affinity to adsorb CO2 and H2O for reduction/oxidation reactions. Further, chloride or fluoride doping was employed for nano-stabilization and to improve intergranular electrical conductivity through the grain boundary (GB), thus benefiting charge separation instead of recombination. The ZnO nanoparticles were prepared by polymeric precursor method and doped with chloride: 0, 1, 3, 4, and 6 mol%, while others with fluoride: 0, 1, 3, 5, and 7 mol%. Powder samples were characterized by XRD, XRF, BET (N2 adsorption for surface area measurement), He pycnometry, DRIFT, and TEM, while pressed pellets of the samples were tested in impedance spectroscopy to measure the electrical conductivity. Some doped samples exhibited a fine crystalline size of 23 nm, and a high specific surface area of 18 m2/g was achieved. These samples presented surface and GB dopant segregation, which demonstrated to enhance GB conductivity, but to hinder CO2 and H2O surface adsorption. Removing excess Cl from the surface by selective lixiviation decreased the electrical conductivity compared to pristine samples, but after lixiviation, samples still showed higher electrical conductivity than the undoped ZnO. Lixiviation affects mainly the surface segregated Cl ions that contribute to the charge transport, possibly by both Cl ionic species and electronic defects. On the other hand, the Cl segregated in the solid-solid interface is less affected by lixiviation. Thus, Cl-doped ZnO grain boundaries retain the dopant upon lixiviation and enhance electronic conduction. The photocatalytic activity of ZnO was improved with Cl-doping and its lixiviation, demonstrating a direct relationship between improved charge transport at the grain boundary and enhanced catalytic activity.