Search

The spectral coverage of a light-harvesting polymer largely determines the maximum achievable photocurrent in organic photovoltaics, and therefore constitutes a crucial parameter for improving their performance. The D–A1–D–A2 copolymer motif is a new and promising design strategy for extending the absorption range by incorporating two acceptor units with complementary photoresponses. The fundamental factors that promote an extended absorption are here determined for three prototype D–A1–D–A2 systems through a combination of experimental and computational methods. Systematic quantum chemical calculations are then used to reveal the intrinsic optical properties of ten further D–A1–D–A2 polymer candidates. These investigated polymers are all predicted to exhibit intense primary absorption peaks at 615–954 nm, corresponding to charge-transfer (CT) transitions to the stronger acceptor, as well as secondary absorption features at 444–647 nm that originate from CT transitions to the weaker acceptors. Realization of D–A1–D–A2 polymers with superior spectral coverage is thereby found to depend critically on the spatial and energetic separation between the two distinct acceptor LUMOs. Two promising D–A1–D–A2 copolymer candidates were finally selected for further theoretical and experimental study, and demonstrate superior light-harvesting properties in terms of significantly extended spectral coverage. This demonstrates great potential for enhanced light-harvesting in D–A1–D–A2 polymers via multiple absorption features compared to traditional D–A polymers. [ABSTRACT FROM AUTHOR]

The commercial heating element Kanthal Super ER (Mo(Si,Al) 2 -based composite) was oxidized at 1580 °C for 24 h up to 1 year using two methods: (1) a furnace set at the desired temperature, (2) passing an alternating current (AC) through the material. Initially, the oxidation behaviors appeared similar for the AC- and furnace-exposed materials. However, at longer exposure times, the level of Al depletion was more pronounced in the furnace-exposed material. A dense alumina scale formed with the same thickness during the two types of exposures, although a porous outer alumina scale was also formed on the composite that was heated in the furnace. The bulk microstructures were also different, in that dark regions in a circle were noted on the AC-exposed material, while the dark regions were evenly distributed on the furnace-exposed material. These dark regions consisted of agglomerated Mo 5 (Si,Al) 3 and Al 2 O 3 . The AC-exposed material also has “bands” where Al 2 O 3 have agglomerated. [ABSTRACT FROM AUTHOR]

Abstract: Surface interactions of water with nano-TiO2 and structurally related group IV metal dioxide nanoparticles have been investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. The investigated clusters include 46 and 92 unit models of TiO2 (i.e., (TiO2)46 and (TiO2)92) of the three low-energy polymorphs of TiO2: anatase, rutile, and brookite. The investigation also includes studies of corresponding rutile-type clusters of three structurally related forms of group IV oxides: SnO2, GeO2, and SiO2. The calculations demonstrate the strong influence of water surface adsorption on several important nanoparticle properties including structure, charge distribution, and electronic structure for all investigated materials. Significant differences between the various materials are also revealed by the calculations, including an unusual stability of the electronic structure of TiO2 in the environment of particular relevance for its photoelectrochemical and photocatalytic applications. [Copyright &y& Elsevier]

Density functional theory (DFT) and time-dependent DFT calculations have been performed on a set of 34 titanium dioxide clusters ((TiO2) n with n ≤ 125) to investigate structural and electronic properties of nanostructured TiO2 (nano-TiO2) materials. The investigated clusters include models of the three low-energy polymorphic forms of TiO2 anatase, rutile, and brookite. A systematic comparison of clusters of increasing size show clear trends for emerging bulk properties in the investigated systems as the surface-to-bulk ratio changes from small clusters dominated by undercoordinated surface atoms to more realistic model nanocrystals with significant bulk components. Differences and similarities in terms of atomic coordination, structural stability, and electronic properties for the three different polymorphic forms of nano-TiO2 are discussed. The calculations provide evidence for emerging polymorphism with increasing cluster sizes so that the different TiO2 forms can be clearly distinguished based on structural characteristics associated with the local bonding environment of the constituent atoms. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Abstract: Time-dependent density functional theory (TD-DFT) calculations have been used to investigate low-energy singlet and triplet excited state potential energy surfaces (PES) of two prototype RuII-bistridentate complexes: [RuII(tpy)2]2+ (tpy is 2,2’:6’,2’’-terpyridine) and [RuII(dqp)2]2+ (dqp is 2,6-di(quinolin-8-yl)pyridine). Solvent effects were considered using a self-consistent reaction field scheme. The calculations provide information about the excited state manifold along pathways for activated decay of metal-to-ligand charge-transfer (MLCT) excited states via metal-centered (MC) states for the two complexes. Significant differences in the energy profiles of the investigated PESs are explained through characterization of the electronic properties of the involved states calculated by the TD-DFT calculations. Finally, implications of the computational results for the design of octahedral metal complexes utilizing ligand field splitting (LFS) strategies for efficient light-harvesting in photochemical applications such as artificial photosynthesis are discussed. [Copyright &y& Elsevier]