Structure-directing ability of the kraft-lignin/cellulose carbon xerogel for the development of C-Nb2O5 sunlight-active photocatalysts

Abstract

This work explored the development of C-Nb2O5 materials through the use of kraft lignin/cellulose carbon xerogel as a structure-directing agent in a simple precipitation synthesis pathway. This strategy was based on xerogel’s low-cost and environmentally friendly nature, as well as the lignin's ability to promote structural changes through the chelation of metallic ions and stabilization of crystalline phases. The results showed that the addition of higher quantities of the kraft lignin/cellulose xerogel during the synthesis resulted in the formation of the hexagonal crystalline structure of niobium oxide, whereas the synthesis without the carbonaceous phase led to hexagonal K3NbO2F4 structure. The presence of the carbon xerogel also led to significant morphological changes, such as the formation of rod-like particles with smaller sizes and the augmentation of the specific surface area and pore volume. EDS and XPS show that the hexagonal Nb2O5 obtained was also doped with K and F atoms. The addition of the carbonaceous phase also led to the reduction of the bandgap energy of materials, whereas an increase in the calcination temperature caused a similar bandgap reduction. The material with the highest carbon content (Nb-0.25L) achieved the highest photoresponse under simulated solar light for the simultaneous photodegradation of methylene blue (MB) and photoreduction of Cr (VI), probably due to its lower bandgap energy, higher surface area, and enhanced methylene blue adsorption capacity. The effect of the calcination temperature implied that dye sensitization was an important factor for the Cr (VI) photoreduction, as faster MB degradation rates led to the suppression of Cr (VI) reduction. Finally, the study of the pH effect on the process showed that higher MB adsorption capacities are linked to higher MB removal rates, which coupled with mechanistic evaluation, proves that MB photodegradation is mainly linked to the direct oxidation reaction promoted by photogenerated vacancies.