Hierarchically Architected Titania Nanostructures for Photocatalytic Activity

In this work, we report the findings of our research activities focused on the study of the functional properties of architected materials by utilizing the concept of nanoarchitectonics for catalysis. The effect of shape and dimensions of titania nanostructures, as well as loading of a co-catalyst and quantum dot sensitizers on the photocatalytic reduction of carbon dioxide (CO₂) and photocatalytic hydrogen evolution was studied. Titania nanotubes and nanopowders, of rutile and anatase phases, acting as photocatalyst were loaded with platinum (Pt) nanoparticles as co-catalyst, and sensitized with II-VI semiconductor quantum dots,and characterized for their structure, morphology, optical absorption and photocatalytic activity. Ordered Titania nanotubes arrays (TNTA, grown on a titanium foil by anodic oxidation) and free nanotubes in powder form were compared to each other, and to rutile and anatase phases of titania nanopowders for their photocatalytic activity. The concentration of Pt nanoparticles loaded onto the titania nanostructures was varied to study the photocatalytic response. The quantum dots as sensitizers were grown directly over the TNTAs by Successive Ionic Layer Adsorption And Reaction (SILAR) process (Cadmium Sulfide, CdS) and also grown separately by hot injection process to be further deposited by spin coating (Cadmium Selenide, CdSe). The concentration of CdS and CdSe quantum dots onto TNTAs was varied by changing the number of SILAR cycles and spin coating cycles, respectively, whereas for nanotubes powder and nanopowders, stirring process was employed. The photocatalytic reduction of CO₂ and photocatalytic hydrogen evolution was optimized based on the shape of nanostructures, concentration of Pt nanoparticles and of II-VI semiconductor sensitizers. TNTAs showed higher rate of photocatalytic activity in comparison to titania nanopowders for volume of hydrogen generation, rate of hydrogen generation, reducing CO₂ to methane and methanol. Furthermore, the height of TNTAs plays significant role in dominating the photocatalytic response with 7µm as the optimized dimension. This work is important because, as of now, 80% of the total world’s energy supply is mainly met by fossil fuels, leading to excessive emission of carbon dioxide which is a major contributor to the greenhouse effect, and tackling it has been listed as one of the agendas on the Horizon Europe and European Union climate action plan, in addition to production and use of clean fuels, such as hydrogen, for energy generation.