(377b) 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.