Investigations of the surface functionalization of titania: Dispersion-induced effects within mixed monolayers, linker-assisted assembly of quantum dots, and photovoltaic device performance
Kern, Meghan Elizabeth
MetadataShow full item record
The research presented in this dissertation focuses on the surface chemistry of nanomaterials, specifically the study of the parameters influencing the surface functionalization of nanocrystalline TiO 2 thin films with adsorbates and CdSe quantum dots (QDs) and their influence on the photovoltaic device performance of quantum dot-sensitized solar cells (QDSSCs). Mixed monolayers will be the subject of discussion for the first half of this dissertation and the remaining portion will focus on the discussion of materials assembly. Lateral dispersion forces induce the ordering of n-alkanoic acids on nanocrystalline TiO 2 films and cause the compositions of mixed monolayers to change. The equilibrium formation of singlecomponent monolayers of n-alkanoic acids and 6-bromohexanoic acid (Br6A) on TiO 2 was wellmodeled by the Langmuir adsorption isotherm. The adsorption of n-heneicosanoic acid (21A) followed Langmuir kinetics, whereas the net rates of adsorption of shorter n-alkanoic acids and Br6A were slower than predicted by simple Langmuir kinetics, suggesting that desorption was non-negligible. At high surface coverage, n-alkanoic acids with 14 or more methylene groups formed ordered, crystalline monolayers, as evidenced by shifts of asymmetric and symmetric CH 2 stretching bands in IR spectra. The formation of ordered monolayers were well-modeled by an idealized mechanism, in which adsorption and desorption followed Langmuir kinetics and ordering was first-order with respect to the fractional surface coverage of adsorbates. Dispersion forces and ordering affected the compositions of mixed monolayers of 21A and Br6A on TiO 2 films that remained in contact with mixed coadsorption solutions. When the fractional surface coverage of 21A was sufficiently high to induce ordering, it displaced Br6A from TiO 2 . We propose that these compositional changes were driven by the stabilization of 21A via cohesive lateral dispersion forces. CdSe QDs were attached to surfaces of nanocrystalline TiO 2 films functionalized with 16- mercaptohexadecanoic acid (MHDA). The solvent from which MHDA was adsorbed to TiO 2 xix determined the amount of adsorbed MHDA, the extent of ordering within monolayers, and the lability of the MHDA-TiO 2 interactions, which in turn dictated the quality of QD-functionalized TiO 2 films. When MHDA linkages to TiO 2 were labile, it desorbed into toluene dispersions of CdSe QDs, causing flocculation of QDs and the formation of opaque, non-uniform QD-functionalized TiO 2 films overcoated with thick (0.1-0.5 μm) layers of agglomerated CdSe. When MHDA linkages to TiO 2 were inert, it persisted upon exposure to toluene dispersions of QDs. The resulting QD-functionalized TiO 2 films were transparent with uniform loading of QDs and without an agglomerated overlayer. Control experiments revealed that flocculation and the formation of low-quality films were correlated with the presence of MHDA in dispersions of QDs. The susceptibility of QDs to MHDA-induced flocculation and agglomeration increased with post-synthesis purification. The morphology of QD-MHDA-TiO 2 interfaces was found to be dependent upon both the concentration of the QD suspension used and the amount of time MHDA-functionalized TiO 2 films were immersed in the QD suspension. QD suspensions with concentrations of ~5μM or less yielded films with little agglomeration of QDs on the TiO 2 surface but low surface loadings of QDs regardless of adsorption time. QD-MHDA-TiO 2 films with high loadings of QDs and minimal agglomeration of QDs were formed with ~22μM QD suspensions when MHDA-functionalized TiO 2 were immersed in QD suspensions for ~2 h; adsorption times greater than 2 h led to the agglomeration of QDs and the formation of an overlayer of agglomerated QDs on the TiO 2 surface. The photoelectrochemical performance of QD-sensitized solar cells was evaluated as a function of interfacial chemistry and morphology, enabling us to correlate electron-injection reactivity with the quality of QD-linker-TiO 2 interfaces. The absorbed photon-to-current efficiency (APCE) for QDMHDA- TiO 2 electrodes exhibiting minimal agglomeration and lower loadings of QDs on the TiO 2 surface was ~2-fold greater than for those containing large agglomerates and a thick overlayer of QDs despite increased light absorption from higher loadings of QDs.