Synthesis and characterization of semiconducting nanocrystals for sensing and photoelectrochemical water splitting applications

Semiconductor nanocrystals or quantum dots (QDs) are usually II-VI or III-V materials whose size is in the range of 1--100 nm, and exhibit optical and electronic properties differing from their bulk counterparts; when the size of the nanoparticle is smaller than the bulk exciton radius quantum. The property of quantum confinement in QDs has caused increased interest in their use in solar energy to conversion applications. As well as quantum confinement properties, their broad absorption in the blue due to a large density of states has made semiconductor nanocrystals optimal for sensing since multiple particle sizes can absorb a single wavelength and emit at different wavelengths. Two sets of aqueous CdTe QDs were synthesized with different capping ligands, 2-mercaptoethlyamine and thioglycolic acid, and fluorescent quenching and enhancement was observed respectively when KCl was added. Fluorescence sensing of ions in solution is a unique application of QDs, but more importantly the interaction of QDs with ions in solution will give more information on the stability of QDs when used in biological applications in the presence of buffers.;QDs are also used as sensitizers to increase the visible light absorption of TiO2 for solar cells and photoelectrochemical water splitting. Nitrogen doped TiO2 and sensitized TiO2, has been studied in solar to energy conversion materials to increase the visible light absorption and efficiency in TiO2 devices. CdSe QD sensitization of nitrogen doped TiO2 was combined for the first time in our lab and shown to have a synergistic effect. CdSe QD sensitized and nitrogen doped TiO 2, nanoparticle films and nanowire arrays were shown to have increased photocurrent for hydrogen generation when compared to CdSe quantum dot sensitized undoped TiO2. This enhancement is due to the increase in hole transport from the valence band of the CdSe to the oxygen vacancy states in the nitrogen doped TiO2. The mechanism for enhancement has been investigated by static and time-resolved fluorescence spectroscopy. Studying a model system of aqueous CdTe QDs linked to TiO2:N nanoparticles using thioglycolic acid, has show similar quenching and PL decay rates compared to QDs linked to TiO2 alone.