Development of ternary ZnO/Bi2O3/carbon xerogel composites as photocatalysts for the degradation of persistent organic pollutants

Author(s)

Nícolas Perciani de Moraes

• Document type: Doctoral Thesis
• Press: Lorena.
• Institution: Universidade de São Paulo (USP). Ciência e Engenharia dos Materiais (IFS/EES)
• Defense date: 2022-04-29

Examining board members

Liana Alvares Rodrigues; Larissa de Freitas; Luiz Claudio Pardini; Inês Conceição Roberto

• Advisor: Liana Alvares Rodrigues

Abstract

In this work, the development of high efficiency photocatalysts based on zinc oxide, bismuth oxide and carbon xerogel was explored for application in sustainable photocatalytic processes using simulated sunlight and visible light, aiming at the degradation of persistent organic pollutants (4-chlorophenol and bisphenol-A) in aquatic effluents. The use of ZnO and Bi2O3 semiconductors is justified by the formation of p-n heterojunctions, while the choice of carbon xerogel as co-catalyst is justified by its high electrical conductivity, high surface area, controllable porosity and possibility of synthesis using precursors obtained from renewable and low-cost sources, such as tannin and lignin. For the photocatalysts, the effect of the ratios between carbon xerogel/ZnO and ZnO/Bi2O3, calcination temperature, synthesis medium (aqueous or alcoholic), synthesis route (acid or basic) and different carbon precursors (tannin, lignin and resorcinol) in their structural and photocatalytic properties was studied. Regarding the materials ZnO/Bi2O3/carbon xerogel synthesized using tannin as a precursor for carbon xerogel, it was determined that the calcination temperature is of paramount importance for the photocatalytic activity obtained, due to its influence on the final structure of the composite. The highest photocatalytic efficiency was verified for the material calcined at 600 °C, due to the formation of β-Bi2O3 and Bi0 phases, which concomitantly with the carbon xerogel, promoted an increase in the separation of photogenerated charges due to the formation of heterojunctions between the different phases present in the structure of the material. Among the ZnO/Bi2O3 ratios evaluated (1-10%), the intermediate proportion of bismuth oxide in the material structure (XC/ZnO-Bi2O3 5%) resulted in a greater efficiency of photodegradation of 4-chlorophenol and bisphenol-A, under both studied radiations (solar and visible). It was also verified that the syntheses in alcoholic medium or by acid route did not lead to the improvement of the photocatalytic activity of the material XC/ZnO-Bi2O3 5%, due to the unfavorable structural and morphological changes observed in the resulting materials, such as the decrease in surface area and modification in the ratios between the β-Bi2O3 and Bi0 phases. For the materials synthesized using lignin, it was observed that an increase in the calcination temperature to 700 ºC resulted in materials with higher photocatalytic activity, and the material XCL/ZnO-Bi2O3 5% presented the highest efficiency for the degradation of the evaluated pollutants. Again, the calcination temperature was a critical factor for the formation of bismuth-based crystalline phases, responsible for the formation of heterojunctions in the materials structure. The addition of lignin also resulted in morphological changes in the composite materials, such as the formation of plate-shaped particles and an increase in the specific surface area. Finally, photocatalysts produced using resorcinol as carbon precursor were studied, since this synthetic molecule is commonly used for the production of carbon xerogels. It was verified that the ternary materials did not obtain good photocatalytic efficiency for the degradation of the evaluated molecules, since there was no appropriate synergy between the composite components, leading to low formation of crystalline phases based on bismuth and low specific surface area. (AU)

• FAPESP's process: 18/16360-0 - Development of ZnO/Bi2O3/Carbon xerogel ternary composites as photocatalysts for the degradation of persistent organic pollutants
• Grantee: Nicolas Perciani de Moraes
• Support Opportunities: Scholarships in Brazil - Doctorate