The topic:
Semiconductor nanowires have diameters of about 10 billionths of a meter, or ten thousand
times smaller than a human hair. Owing to their thinness, such
nanowires are expected to have special optical and electronic properties.
For example, they can efficiently transport electrons with a specific energy from one place
to another, a property useful for solar cell applications. Fullerenes, on the other hand, are
carbon cage molecules, among which C60, the
Buckminsterfullerene, has the most symmetrical soccer ball structure. These fullerene molecules
have so far turned out to be the most efficient electron acceptors in organic polymer solar cells.
One of the stumbling blocks in developing this type of solar cells is the
difficulty in controlling the morphology of the mixture of polymers
and fullerenes. To solve this problem, a hybrid structure of inorganic nanowires and fullerenes
has been proposed in this project. The goal was to create inorganic nanowires coated with fullerene
molecules and study their photoelectric properties.
Methodology used:
The first step was to develop various bottomup chemical methods including direct solution
reactions, electrochemical reactions, and insitu interfacial reactions to synthesize inorganic
nanowires. Then fullerenes were attached to the surfaces of the nanowires using different
strategies including fullerene polymerization, chemical functionalization, and
ligand-binding. Finally photoelectrochemistry measurements
were conducted to uncover the properties of photoinduced charge separation, transport and
collection.
Research findings:
The successful development of a unique copper(I)-assisted fullerene-polymerization method for preparing novel
cuprous oxide-fullerene[60] core-shell nanowires was
the highlight of this project. Nanoparticles were also
studied for comparisons. For this work, we started from a
Cu2O nanowire array obtained using our in-situ growth method. The fullerene
coating was based on a reaction of C60 and
ethyl isocyanoacetate to form polymerized fulleropyrolines, catalyzed by and thus seamlessly
coated on the Cu2O nanomaterials. Such a Cu(I)-
assisted C60-polymerization process combines the
new solution chemistry and surface chemistry of C60. Interestingly, when
Cu2O nanocubes were used as a core for coating fullerenes,
the Cu2O cores in the composite nanocubes
could be removed by acidic etching, yielding monodispersed
C60 nanoboxes.
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In addition, we have surface-functionalized ZnO
Nanotetrapods, a type of branched nanowires, with uniform monolayers of
C60 molecules functionalized with carboxylate groups. We have
achieved chemical conjugation of fullerenes to CdSe nanocrystals using dithiocarbamate ligands
by exchanging TOPO ligands on the CdSe nanocrystals with
C60-bound dithiocarbamate ligands. This technique can be easily adapted to
the fullerene functionalization of CdSe nanowires.
Encouragingly, the above composite
nanomaterials have shown enhanced photoinduced charge separation, transport and
collection. For example, we have demonstrated the enhanced photocurrent of the
Cu2O@C60 nanoribbons arrayed on Cu foil compared
with that of the Cu2O nanoribbons. Similarly,
photoelectrochemistry of a film cast from the (C60)8-CdSe conjugate revealed a significantly enhanced
photocurrent compared with the film of CdSe-TOPO nanocrystals as well as that of
C60 alone, suggesting that our conjugation strategy is viable
for solar cell applications.
Implications to the related area or to the society:
This project has created nanocomposites containing semiconductor nanostructures and
fullerenes; the former are electron donors and the latter are electron acceptors. The synthesis
methods developed in this project can be extended to the synthesis of other composite
nanomaterials. Combining electron donors and acceptors at the nanoscale is an important
endeavor to effectively harvest solar energy, which is of huge interest globally. The proof-of-concept
for designing the materials basis for the nanoscale charge separation has been demonstrated. To
utilize the photoinduced charge separation in real device applications, we have
already started to implement the nanostructures as photoanodes for
excitonic solar cells.
Prof Shihe YANG
Department of Chemistry
The Hong Kong University of Science and Technology
chsyang@ust.hk
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