(Keynote) The Versatility of TiO₂ Nanotubes As Platform to Construct Photocathode or Photoanode for Clean Energy Production

The development of new technologies to convert solar energy into solar fuels through the synthesis of innovative photoactive materials is a topic of great interest nowadays. By manufacturing photo-electrodes designed with purpose to mimic the plant leaves, solar energy can be harvested and used to produce higher value-added compounds (methanol, ethanol, formic acid, methane, and others) from CO2 conversion or hydrogen from water splitting. So, the present work aims preparing, characterizing and testing semiconductor electrodes based on Ti/TiO2 nanotubes (TNT) modified with bismuth vanadate (BiVO4) or cuprous oxide (Cu2O) in the photoelectrocatalytic water splitting (water oxidation) and reduction of CO2, respectively. Considering that the photoelectrocatalytic process on the TNT surface is very small due to the lack of visible-light response and high recombination of electron-hole pair, that takes to low quantum efficiency of the process, its coupling with other semiconductors, such as BiVO4 or Cu2O can increase light absorption to the visible region and decrease the recombination of the electron-hole pair by the formation of heterojunction, therefore increasing both the processes. The TNT electrodes were prepared by electrochemical anodization of titanium sheets. BiVO4 was prepared by microwave-assisted hydrothermal method by using a commercial microwave synthesizer system [1]. Cu2O nanostructures with different morphologies were prepared as described by [2]. For water splitting, TNT electrodes were decorated with BiVO4 nanoparticles (TNT/BiVO4) by spin or spray coating method using Nafion® (Nafion® perfluorinated resin solution 5 wt. %) as an anchoring agent. On the other hand, for CO2 reduction, the Cu2O nanoparticles (cube, Nc; sphere, Ns; octahedron, No) layers on the catalysts were obtained using two sequential dip-coating procedures, after pre-coating the TNT surface with polydopamine (PDA). The water splitting was carried out using a PEC reactor homemade of Plexiglas and equipped with a quartz window. In the two compartments where placed the two electrodes acting as cathode (GDL-Pt) and photoanode (TNT/BiVO4), where the evolution of H2 and O2 evolution was monitored. The electrolyte in the anodic and cathodic compartment was 1 M NaOH and 0.5 M H2SO4, respectively. A Nafion® exchange membrane was used between the two electrodes configuration for the selective transport of protons towards the cathode for H2 production. It is noteworthy that for water oxidation no external bias was applied. Notwithstanding, CO2 reduction PEC sealed system (1 kgf cm−2), performed in low bias potential (0.2 V), using 0.1 M Na2SO4 electrolyte saturated with CO2 (pH 4.5) for 2h. The system was irradiated by either UV-Vis irradiation or solar simulator. The TNT/BiVO4 spray electrode presented a very low H2 production, maybe due to the excess of BiVO4 present on the surface of the TNT electrode. However, TNT/BiVO4 spin electrode produced 21.5 μmol of H2 after 125 min of irradiation, which is 57% higher when compared to the non-modified TNT electrode. Concerning the photoelectrocatalytic CO2 conversion, it was observed that the electrodes presented a significant performance in the conversion of CO2 to methanol, when compared with bare TNT electrode (1.5 mg L−1/1.2 mg L−1): 10 mg L−1/3.8 mg L−1 (Nc), 6.0 mg L−1/1.3 mg L−1 (No) and 5.4 mgL−1/1.7 mgL−1 (Ns) under UV-Vis radiation and solar simulator, respectively. The results demonstrated the facet-dependent performance of these nanostructures as photocatalysts and the use of PDA proved to be a good strategy to obtain p-n heterojunction semiconductors with the improvement of its response in visible light region. Therefore, these results demonstrated the potentiality of using TNT electrodes modified with nanoparticles of bismuth vanadate for water splitting, or cuprous oxide for CO2 reduction, contributing to the development of efficient technologies to produce solar fuels. References [1] S. Ulah, E.P. Ferreira-Neto, C. Hazra, R. Parveen, H.D. Rojas-Mantilla, M.L. Calegaro, Y.E. Serge-Correales, U.P. Rodrigues-Filho, S.J.L. Ribeiro, Appl. Cat. B: Environ., 2019, 243, 121–135. [2] F.A.C. Pastrián, A.G.M. da Silva, A.H.B. Dourado, A.P. de Lima Batista, A.G.S. de Oliveira-Filho, J. Quiroz, D.C. de Oliveira, P.H.C. Camargo, S.I. Córdoba de Torresi. ACS Catal., 2018, 8, 6265-6272.