nanostructured multijunction semiconductors for solar energy conversion: case of cuox@pt/tio2
Résumé
Photocatalysis for solar energy storage is a growing research field. In particular, photocatalytic CO2 reduction or H2 production by photocatalytic alcohol reforming or water splitting are widely studied. These reactions need the development of modified photocatalytic materials by adding co-catalysts permitting either proton or CO2 reduction and water oxidation. Nanostructured multijunction semiconductors are interesting materials for photocatalytic water splitting or to mimic photosynthesis processes.
Indeed, the formation of heterojunctions leads to an efficient separation of electron-hole pairs which enhance photocatalytic activity. The objective of this research is to synthesize a composite material made of two different semiconductors TiO2 and Cu2O separated by Pt nanoparticles to create SC2-M-SC1 heterojonctions. This photocatalyst, by reaching the suitable potentials, should be able to reduce CO2 and oxidize H2O under artificial or natural light as described on the Figure 1. The first efforts were focused on the synthesis of the composite material CuOx@Pt/TiO2. The main challenge was to cover selectively platinum nanoparticles deposited on TiO2 by Cu2O. The synthesis of such composite materials was tried by various techniques such as co-precipitation, sol-gel method or impregnation [1], [2]. However, the selective coverage of metal does not seem to be really proved. To achieve a selective coverage of metallic particles, we build our material by successive photodeposition of platinum and copper. Specific in situ XAS experiments were performed during the synthesis of the materials to understand mechanisms occurring during deposition of Pt and Cu. A kinetic mechanism has been established and different parameters were optimised. In particular, the versatility of CuOx particles has been demonstrated. Then different characterizations were conducted to verify the specific architecture of the obtained composite materials. XPS and XANES analyses confirm the reduction of Cu2+ into Cu+ but also into metallic copper Cu on the catalyst surface. TEM images certify the presence of CuOx@Pt/TiO2 structure as shown in the Figure 2. The presence of heterojunctions, apparently responsible for the specific electronic structure, has been established by UPS spectroscopy and will be discussed. Finally, a specific photocatalytic activity of such CuOx@Pt/TiO2 has been demonstrated for 2-propanol reforming.