טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentTrung-Anh Le
SubjectMolecularly Functionalized Hybrid Nanomaterials for
Photocatalytic Water Splitting
DepartmentDepartment of Chemistry
Supervisors Full Professor Gross Zeev
Professor Amirav Lilac
Full Thesis textFull thesis text - English Version


Abstract

Despite the intense global research, photocatalyst systems that are sufficiently stable and efficient for water splitting have not yet been realized. Therefore, an efficient visible light-driven photocatalyst system is still in great need. Recently, our group reported a photocatalyst system composed of Pt-tipped CdSe@CdS quantum rods (QRs). This system has given a perfect 100% quantum yield photon to hydrogen conversion under visible illumination for the water reduction half reaction. The next step would be introducing a cocatalyst onto the QR surface for promoting the water oxidation half reaction and therefore, completing the full water splitting cycle. Corroles, a family of molecular catalysts with superior properties for water oxidation, were chosen as the water oxidizing coordination catalysts.

Corroles were anchored via their metal centers onto the QR by L-cysteine and L-histidine dithiocarbamate linkers, which were expected to allow a direct charge transfer from the metal centers in corroles to QRs. The aggregation of photocatalysts, the desorption of immobilized corroles and the complete degradation of hybrid photocatalysts under illumination, have shown the inapplicability of this system for water splitting purpose.
? ? ? ? ? ? ? ? ? ? ?  Rather than having a direct charge transfer via linkers, the utilization of redox species as charge transfer shuttles was examined. Amphiphilic corroles of different transition metals were mixed with QRs at pH 14. Upon illumination, the QRs oxidize OH- to
??•?•OH radical, and this species can potentially oxidize the corroles, thus completing the charge transfer cycle. Even though the utilization of OH-/?•OH redox pair already proved itself efficient as it enabled the perfect quantum efficiency for H2 evolution from similar QRs. Only low activity towards photocatalytic hydrogen production, however, was obtained with this new system. Regardless of the metal nature in corroles and their relative amount, the photocatalyst system gave the same average quantum efficiency of 0.35%. The diffusion of highly reactive ?•OH radicals, therefore, was suspected to be the rate-determining step in this photocatalyst system.
? ? ? ? ? ? ? ? ? ? ?  The corrole organic skeleton was functionalized with three thiol-containing moieties to overcome the disadvantages of the two previous sections. These moieties might either promote a direct charge transfer or bring the two catalysts closer together to decrease the diffusion pathway of charge transfer shuttles. The new hybrid photocatalysts showed very good stability towards aggregation, desorption, photodegradation, and were found active for photocatalytic hydrogen production. Quantum efficiency of 4.3 and 0.19% was obtained for the hybrid photocatalysts containing one and two anchored corrole molecules respectively at neutral pH. This confirmed the group???s previous finding that for multi-electron reactions, the photocatalyst design should only include a single co-catalytic site per each segment of the semiconductor capable of light excitation. O2 was not detected during the experiments. Cobalt corroles were found to decompose at pH 14, and therefore other transition metals should be tested. Despite the relative success of this new hybrid photocatalysts, several questions still remain open for future study: what is the mechanism for charge transfer from corroles to QRs and how can it be improved? What is the source for O2 absence?