Hybrid ZnO Complexes From a Simple Thiol Modification Process: Understanding Electro-Optical Properties

Metal oxide nanoparticles, nanotubes, nanorods, nanowires and whiskers are finding applications ranging from catalysis to optoelectronics.  In particular, nano-crystalline zinc oxide (nano-ZnO) is an important electro-optical material due to its wide bandgap and bimodal photoluminescence (PL) spectrum.  Nano-ZnO represents a promising semiconductor for future hybrid devices; however, use has been limited due to inefficient charge transport.  Towards creating more efficient ZnO hybrid organic-inorganic complexes, a simple thiol modification approach has been developed.  Organothiol surface modifiers are covalently attached to the surface of ZnO nanoparticles (rods and spheres) by stirring an ethanolic nanoparticle/organothiol mixture for one hour.  Stirring under ambient conditions creates a thiol monolayer, while elevated temperatures result in complete encapsulation of the ZnO nanoparticles, as determined by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis.  While previous studies have demonstrated that alkanethiols adsorb on ZnO surfaces and nanoparticles, the present work is the first to demonstrate encapsulation of ZnO nanoparticles.  The encapsulation layer consists of a thick (e.g., 100-500 Å) organic shell comprised of a 1:2 Zn:thiol complex.  The thickness and morphology of the encapsulating layer is controllable by choice of thiol and preparation conditions.  Furthermore, because a large selection of functionalized thiols is available, it is possible to surround ZnO nanoparticles with a variety of chemical functional groups for subsequent interactions.  The effect of thiol monolayer and encapsulating layer formation on the inherent PL of nano-ZnO has also been analyzed, demonstrating the ability to induce unique changes in the PL, when compared to other surface modifiers. 

For organic-inorganic hybrid devices, it is often desirable to covalently attach organic molecules to metal oxide surfaces such that electrons and holes may be transported across the inorganic-organic interface.  Here, we discuss the creation of ZnO organic-inorganic and metal-metal oxide hybrid complexes via thiol linkages using the facile thiol modification approach described above.  One such hybrid, formed from nano-ZnO and a potential photovoltaic dye, was created.  A thiol-derivatized ruthenium-based dye was synthesized and directly attached through covalent interaction of the thiol groups and the surface of nano-ZnO under monolayer adsorption conditions while flushing with argon gas.  Detailed understanding of the electronic structure of the hybrid complex, which has previously been lacking, was generated through XPS, ultraviolet photoelectron spectroscopy (UPS), and absorbance spectroscopy studies.  An energy level diagram was constructed, indicating that the lowest unoccupied molecular orbital (LUMO) of this dye is lower in energy than the ZnO conduction band edge, providing minimal enthalpic driving force for photovoltaic electron injection.  Toward the goal of tailoring the optical properties of nano-ZnO, the influence of dye adsorption on the inherent PL emission of nano-ZnO was determined.  Adsorption of the dye caused complete quenching of the nano-ZnO inherent visible emission intensity, while the excitonic UV emission intensity remained unaltered.  Creation of this complex using the simple thiol modification approach confirms the possibility of using thiol-terminated dyes for ZnO-based DSSC devices, and understanding the electronic structure will facilitate future optimization of the ZnO:dye hybrid complex. 

Another hybrid system, ZnO nanorods tethered to gold nanoparticles (AuNPs) via dithiol linkers, has also been synthesized.  Although ZnO:AuNP complexes have previously been created, this is the first successful attempt to covalently attach AuNPs to the surface of ZnO using dithiol chemistry and to understand the influence on electronic transfer processes at the interface.  AuNPs functionalized with octanethiol were tethered to the nanorod surface using the thiol monolayer absorption approach by mixing octanethiol-protected AuNPs and the nanorods in the presence of p-terphenyl dithiol.  One end of the dithiol linker bonds to AuNPs via a ligand place-exchange reaction, and the other end attaches to ZnO via Zn-S bonding.  Hybrid complexes of varying AuNP surface density were created by adjusting the AuNP:ZnO ratio and dithiol concentration.  XPS and TEM confirm attachment of gold nanoparticles to the nanorod surface, and fluorescence spectroscopy was employed to measure the influence of the proximal metal on the photoluminescence of the ZnO.  As the AuNP surface coverage increases, the visible emission band, centered at ~520 nm, and the UV emission band decrease in intensity.  Furthermore, the UV emission band is increasingly blue shifted as the AuNP surface density increases.  The electronic processes that enable control of the nano-ZnO optical properties upon AuNP adsorption were also investigated in-depth.  The information derived will facilitate formation of future ZnO:AuNP complexes with increasing control of the electro-optical properties, enabling design of a series of hybrid materials with specific optical responses. The creation of hybrid organic:inorganic and metal: metal oxide complexes through the described simple thiol modification approach could have significant implications on future chemical sensors, as well as photovoltaic devices.