Photocatalytic patterning of monolayers for the site-selective deposition of quantum dots onto TiO₂ surfaces

A novel photochemical mechanism is reported for the site-selective deposition of quantum dots onto nanocrystalline TiO₂ films. The patterning mechanism involves the combination of surfactant-mediated self-assembly and monolayer photolithography. In the self-assembly process, CdS and CdSe quantum dots were attached to TiO₂ surfaces through bifunctional mercaptoalkanoic acid (MAA) linkers. MAAs were adsorbed to the TiO₂ surface as the deprotonated carboxylates, primarily through monodentate coordination to Ti⁴⁺ sites. CdSe quantum dots were bound to the terminal thiol groups of surface-adsorbed MAAs, with a surface adduct formation constant, K_ad, of (2.1 ± 0.7) × 10⁴ M^-1. The color and optical density of the quantum dot-functionalized TiO₂ films were tunable. Monolayer photopatterning involved the TiO₂-catalyzed oxidative degradation of surface-adsorbed mercaptohexadecanoic acid (MHDA). A mechanism is proposed wherein MHDA degradation occurs through both oxidative decarboxylation and the formation of interchain disulfides. These MHDA photodegradation processes regulate the extent to which CdSe quantum dots adsorb onto the TiO₂ surface. Illumination through a photomask yielded optically patterned, quantum dot-functionalized TiO₂ films that were characterized by scanning electron microscopy and energy-dispersive X-ray analysis.