The infiltration of transition metals into biopolymers by means of vapor phase processes has been shown to unusually change the bulk mechanical properties of those materials. Here for the first time a novel, single precursor infiltration process was applied to cellulose. The mechanical properties, as measured through uni-axial tensile testing, showed improvement as a function of the total number of infiltration cycles as well as the precursor used. For cellulose infiltrated with diethyl zinc with only four infiltration cycles the ultimate tensile strength was seen to nearly double from ~160 MPa to ~260 MPa. A significant increase is also seen in the elastic modulus with values increasing ~2.5X, from ~1.8 GPa to ~4.5 GPa. In contrast, cellulose infiltrated with trimethyl aluminum showed very little improvement in the mechanical properties. By choosing the appropriate precursor and/or number of cycles the mechanical properties become tunable. The chemical changes in the cellulose structure were measured with Raman spectroscopy and a novel semi in-situ x-ray photoelectron spectroscopy experiment. The results of both spectroscopic techniques were used to propose a reaction scheme.
A recent improvement in the design of dyesensitized solar cells has been the combination of lightabsorbing, electron-donating, and electron-withdrawing groups within the same sensitizer molecule. This dye architecture has proven to increase the energy conversion efficiency of the cells, leading to record efficiency values. Here we investigate a zinc(II)-porphyrin-based dye with triphenylamine donor groups and carboxyl linkers for the attachment to an oxide acceptor. The unoccupied energy levels of these three moieties are probed selectively by element-sensitive X-ray absorption spectroscopy at the K-edges of nitrogen and carbon. These results are complemented by ultraviolet/visible spectroscopy to obtain the optical band gap and the occupied molecular levels. Density functional theory and time-dependent density functional theory are employed to obtain a detailed understanding of the X-ray and optical absorption spectra. The attachment of electron-donating groups to the porphyrin ring significantly delocalizes the highest occupied molecular orbital (HOMO) of the molecule. This leads to a spatial separation between the HOMO and the lowest unoccupied molecular orbital (LUMO), with the HOMO having significant weight in the amine donors, while the LUMO remains localized in the porphyrin ring and the acceptor group. Such spatial separation of the frontier orbitals reduces the recombination rate of photoinduced electrons and holes, thus enhancing the energy conversion efficiency.
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