Linker-assisted attachment of CdSe quantum dots to TiO2: Time- and concentration-dependent adsorption, agglomeration, and sensitized photocurrent.

We have characterized the concentration and time dependences of the attachment of colloidal CdSe quantum dots (QDs) to 16-mercaptohexadanoic acid (MHDA)-functionalized nanocrystalline TiO2 thin films. The amount of QDs and the extent of their agglomeration on MHDA-functionalized TiO2 films were characterized by transmission- and reflectance-mode UV/vis absorption spectroscopy and scanning electron microscopy. Optically transparent films with spatially homogeneous coloration and minimal agglomeration of QDs were prepared from 2.2 and 5.0 µM toluene dispersions of QDs at short reaction times (<5 h). In contrast, prolonged exposure of MHDA-functionalized TiO2 films to 22 µM dispersions of QDs yielded relatively opaque QD-functionalized films with spatially inhomogeneous coloration and substantial agglomeration of QDs. Agglomeration of QDs decreased the absorbed photon-to-current efficiencies of QD-sensitized solar cells (QDSSCs) by almost 3-fold. These results highlight the profound influence of agglomeration on the optical properties and interfacial electron-transfer reactivity of QD-functionalized TiO2 films prepared by in situ linker-assisted assembly as well as the photoelectrochemical performance of QDSSCs incorporating such films.

Sensitization of ZnO single crystal electrodes with CdSe quantum dots.

CdSe quantum dots (QDs) were attached to single crystal ZnO(0001) and ZnO(1100) substrates using capping groups, 4-mercaptobenzoic acid, 2-mercaptoacetic acid, 3-mercaptopropionic acid, 8-mercaptooctanoic acid, and 11-mercaptoundecanoic acid, as bifunctional linker molecules. The spectral response and photosensitization yields of the adsorbed QDs were studied with photocurrent spectroscopy. Atomic force microscopy (AFM) was used to verify the surface structure of the ZnO crystals and to examine the coverage and arrangement of the QDs on the single crystal surface. The inner-sphere aqueous redox couple Sx(2-)/S(2-), often used as a regenerator for chalcogenide-based QDs, as well as outer-sphere redox couples such as ferrocene, were able to regenerate the photoexcited CdSe QDs and suppress their photocorrosion. Differences in the binding of the QDs to different ZnO crystal faces are also reported.

Influence of dispersion forces and ordering on the compositions of mixed monolayers of alkanoic acids on nanocrystalline TiO2 films.

Lateral dispersion forces induce the ordering of n-alkanoic acids on nanocrystalline TiO2 films and cause the compositions of mixed monolayers to change. The equilibrium formation of single-component monolayers of n-alkanoic acids and 6-bromohexanoic acid (Br6A) on TiO2 was well-modeled by the Langmuir adsorption isotherm. Surface adduct formation constants were 10(3)-10(4) M(-1), and saturation amounts of adsorbates per projected surface area of TiO2 were on the order of 10(-7) mol cm(-2). 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 coverages, n-alkanoic acids with 14 or more methylene groups formed ordered, crystalline monolayers, as evidenced by shifts of asymmetric and symmetric CH2 stretching bands in IR spectra. The extent of ordering was similar to that of self-assembled monolayers of alkanethiols on gold. The formation of ordered monolayers was 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 TiO2 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 TiO2. We propose that these compositional changes were driven by the stabilization of 21A via cohesive lateral dispersion forces. Our results reveal that mixed monolayers on nanocrystalline TiO2 films are dynamic and that noncovalent intermolecular interactions can profoundly influence their compositions and properties.

Influence of solvation and the persistence of adsorbed linkers on the attachment of CdSe quantum dots to TiO2 via linker-assisted assembly.

CdSe quantum dots (QDs) were attached to the surfaces of nanocrystalline TiO(2) films functionalized with 16-mercaptohexadecanoic acid (MHDA). The solvent from which MHDA was adsorbed to TiO(2) determined the amount of adsorbed MHDA, the extent of ordering within monolayers, and the lability of MHDA-TiO(2) interactions, which in turn dictated the quality of QD-functionalized TiO(2) films. When MHDA was adsorbed to TiO(2) from dichloromethane, toluene, or heptane, it desorbed into toluene dispersions of CdSe QDs, causing the flocculation of QDs and the formation of opaque, nonuniform QD-functionalized TiO(2) films overcoated with thick (0.1-0.5 µm) layers of agglomerated CdSe. When MHDA was adsorbed to TiO(2) from tetrahydrofuran, it persisted upon exposure to toluene dispersions of QDs. The resulting QD-functionalized TiO(2) films were transparent with a 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; we propose that MHDA adsorbed to CdSe QDs and decreased their dispersibility in nonpolar solvents. The susceptibility of QDs to MHDA-induced flocculation and agglomeration increased with postsynthesis purification. Our results highlight the influence of solvation on the persistence of linker-substrate interactions, which is of central importance in optimizing the linker-assisted assembly of interfaces.

Synthesis and Characterization of Ultrathin Silver Sulfide Nanoplatelets.

We report the synthesis of ultrathin silver sulfide (Ag2S) nanoplatelets (NPLs) synthesized via a one-pot method in ethylene glycol with 3-mercaptopropionic acid serving as both the sulfur precursor and the platelet ligand. The colloidally synthesized nanoplatelets are exceptionally thin, with a thickness of only 3.5 ± 0.2 Å and a 1S exciton Bohr diameter to confinement ratio of ∼12.6. The NPL growth is shown to be quantized by layer thickness using absorption and photoluminescence (PL) spectroscopy. Transmission electron microscopy, atomic force microscopy, and X-ray diffraction analyses of the NPLs show that they correspond to the (202) plane of the ß-Ag2S structure. The PL quantum yield of these NPLs is ∼30%, suggesting their potential use in biomedical imaging. Optoelectronic properties were evaluated via sensitized photocurrent spectroscopy with the resulting spectra closely matching the distinctive absorption spectral shape of the Ag2S NPLs.