Application Of Molecular Catalyst In Photoelectrodes And Characterization Of Piezoelectric 2D Materials

The continuous use of fossil fuels to fulfill human energy requirements is not only leading us to a global energy crisis,but also expeling massive amounts of carbon dioxide(CO2)and other green house gases to the atmosphere,increasing the air and water pollution,raising the global average temperature,and overall destroying the natural environment.Therefore,looking for clean energy sources and technologies is the primary task.In this context,technologies based on nanostructured materials can provide an effective solution for several energy problems.In this thesis,I will present our recent progress on the synthesis and characterization two different nanomaterials based energy technologies:?)molecular catalysts for water-splitting solar cells,and ?)layered materials for piezoelectric energy harvesters.Molecular catalysts for water-splitting solar cells.In this section we progress towards the construction of an active photoanode for water oxidation based on n-type silicon and molecular water oxidation catalysts.Different metal oxides(TiO2,Al2O3 and ZnO)with various thicknesses(2,5 and 10 nm)have been studied as silicon protective and catalyst layer,which also served as anchoring platform.The results show that the interface between the semiconductor and the catalyst,as well as the electrolyte highly influence the performance of the final water-splitting device.The conductivity and thickness of the metal oxide at the interface are critical factors influencing the catalytic activity of the cell.Three types of molecular water oxidation catalyst were successfully anchored onto the silicon semiconductor having either metal oxide layers or graphene.Carboxylate,sulfonate or pyrene groups were used as anchoring motifs.Two of the resulting photoanodes showed improved current density when compared to analogous photoanodes without the molecular catalyst.Despite this exciting proof-of-concept work,we find that the stability molecular-catalysts-based cells lower than that provided by metal-insulator-semiconductor cells.Future efforts in the field should focus on finding stronger binding groups that guarantee a stable semiconductor-catalyst interaction.Layered materials for piezoelectric energy harvesters.In this section,we synthesize piezoelecric 2D MoS2 sheets for energy harvesters and self-powered devices using a cost-effective and scalable approach,namely liquid phase exfolication(LPE).The piezoelectric capability of a 2D material is for the first time mapped with nanoscale precission via conductive atomic force microscopy(CAFM).Our experiments demonstrate the presence of electrical current densities above 100 A cm-2(in the absence of bias)when the flakes are strained,and the current increases proportional to the strain.Simultaneously collected topographic and current maps reveal that the edges of stepped mutilayer MoS2 flakes are the most piezo-active location of the materials.Density functional theory calculations are consistent with these observations.Similar experiments performed in graphene didn't show piezo-currents generation,but just noise current.Our results pave the way to the design of piezoelectric devices using layered materials.