4 research outputs found
Photochemical investigation of porphyrins for use in Dye Sensitized Solar Cells
As Dye Sensitized Solar Cells (DSSCs) have become a more prominent focus of the research
being published on alternative energy sources it is important to consider basic principles that
show promise for improving the energy conversion efficiency of these cells. By choosing and
building a dye system carefully it is possible to promote a process called triplet-triplet
annihilation (TTA) which involves the upconversion of two relatively low energy triplet excited
molecules to produce one highly excited singlet molecule and a ground state molecule (2 T1
Sn + S0 [n>1]). This process will give DSSCs the ability to take advantage of the photon-rich near
infrared portion of the solar spectrum making them inherently more efficient than systems
which cannot undergo TTA.
Two dye systems which show promise as TTA capable light harvesters are zinc
tetraphenylporphyrin (ZnTPP) and tin(IV)-dichloro,tetraphenylporphyrin (SnCl2TPP). The
photophysical properties of ZnTPP are studied in its standard form as well as in its ligated form.
Ligation of the central atom with nitrogen containing bidentate ligands (i.e. pyrazine, 4,4-
bipyridine etc…) can have a profound effect on both the properties of the monomer as well as
the aggregate behavior of solvated dyes and their interaction with each other. The properties of
SnCl2TPP are also examined and in particular methods for the formation of stacked molecular
columns as manipulation of interactions between the individual dyes can significantly impact
their ability to undergo TTA.
Spectral data indicate that ligation of the ZnTPP leads to the formation of a short lived charge
transfer state which greatly decreases the lifetime of the S2 state while having little effect on
the S1 decay kinetics. It also shows that there is little or no formation of six-coordinate ZnTPP
(i.e. doubly ligated) which makes the formation of stacked supramolecular structures very
difficult. Data concerning the electrochemical synthesis and chemical synthesis of a SnCl2TPP
based stack were inconclusive but show some promise that may give direction to future work
Electrochemical Studies of Capping Agent Adsorption Provide Insight into the Formation of Anisotropic Gold Nanocrystals
The ability of the 4-dimethylaminopyridine (DMAP) to stabilize and control the formation of anisotropic gold nanocrystals produced <i>via</i> the borohydride reduction of gold(III) salts is reported here. Electrochemical measurements of DMAP electrosorption on different low-index single crystal and polycrystalline electrodes is provided and shows a propensity for DMAP to preferentially adsorb on {100} facets. Measuring the electrochemical potential during nanocrystal formation shows that experimental conditions can easily be manipulated so that the growth of nanoseeds occurs at potentials that support preferential DMAP adsorption on {100} surfaces giving rise to highly anisotropic nanocrystals (nanorods, bipyramids, and nanopods). Nanopods with nearly 50 nm arm lengths are shown to form and produce a surface plasmon mode that extends well into the near IR (λ<sub>max</sub> ≈ 1350 nm). Evidence is provided of the slow, partial reduction of tetrachloroaurate to a DMAP stabilized Au<sup>I</sup> species. Shape control is achieved simply by varying the length of time, τ, that DMAP is allowed to partially reduce the Au<sup>III</sup> ions prior to the addition of the strong reducing agent, NaBH<sub>4</sub>. Thus the role of DMAP in producing anisotropic particle shapes is shown to be multifunctional. A mechanism accounting for the dependence of particle shape on τ is provided