Xien Liu

Pennsylvania State University, University Park, Maryland, United States

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Publications (3)18.45 Total impact

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    ABSTRACT: Enhancing the dielectric permittivity of organic semiconductors may open new opportunities to control charge generation and recombination dynamics in organic solar cells. The potential to tune the dielectric permittivity of organic semiconductors by doping them with redox inactive salts was explored using a combination of organic synthesis, electrical characterization, and time-resolved infrared spectroscopy. The addition of the salt, LiTFSI (lithium bis(trifluoro-methyl-sulfonyl)imide), to a conjugated polymer specifically designed to incorporate ions into its bulk phase increased the density of holes and enhanced the static dielectric permittivity of the polymer blend by more than an order of magnitude. The frequency and phase dependence of the real dielectric function demonstrates that the increase in dielectric permittivity resulted from dipolar motion of bound ion pairs or clusters of ions. Interestingly, the increases in the hole density and dielectric permittivity were associated with enhancement of the hole mobility by two orders of magnitude relative to the undoped polymer. The charge recombination lifetime also increased by an order of magnitude in the blend with a fullerene electron acceptor when ions were added to the polymer. The findings indicate that ion-doping enables organic semiconductors with large increases in low frequency dielectric permittivity and that these changes result in improved charge transport and suppressed charge recombination on the microsecond time scale.
    The Journal of Physical Chemistry B 09/2013; · 3.61 Impact Factor
  • Xien Liu, Fengying Wang
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    ABSTRACT: The study of catalytic water oxidation continues to be one of the most active areas of research across many sub-disciplines of chemistry. From efforts toward developing artificial photosynthetic assemblies to the exploration of nanoscale materials to be used as a photoanode for splitting water, a detailed understanding of the mechanistic details of water oxidation in photosystem II (PSII) is paramount for the rational design of an artificial model. In addition, insight into the model's mechanism of molecular-level water-oxidation catalysis will provide us with a unique opportunity to elucidate the mechanistic pathways of water oxidation in the oxygen-evolving complex (OEC) of PSII.In this review, the proposed mechanisms, catalytic activities and reaction kinetics of catalytic water oxidation with transition metal complexes in homogeneous systems published from 1979 to 2010, with an emphasis on the last decade, are discussed. These metal complexes include mononuclear, dinuclear and multinuclear manganese, ruthenium, iridium, iron and cobalt complexes. Electrodeposited cobalt complexes are a type of heterogeneous water-oxidation catalyst; however, these complexes are discussed herein because they are topological analogs of the Mn cluster in the OEC. MVO species (M = Mn, Ru) as the key active species in homogeneous catalytic evolution of O2 are common feature in many catalysts, in which formation of the OO bond can be achieved either by intramolecular elimination of dioxygen from two MVO groups or by nucleophilic attack of OH− species on MVO groups. Another common feature appears from tetranuclear ruthenium complex 71, manganese complex 10 and cobalt complex, all of these cubane structural complexes have self-repair properties similar to OEC–protein complex in nature.
    Coordination Chemistry Reviews 06/2012; 256(s 11–12):1115–1136. · 11.02 Impact Factor
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    ABSTRACT: Nanostructured WO(3) has been developed as a promising water-splitting material due to its ability of capturing parts of the visible light and high stability in aqueous solutions under acidic conditions. In this review, the fabrication, photocatalytic performance and operating principles of photoelectrochemical cells (PECs) for water splitting based on WO(3) photoanodes, with an emphasis on the last decade, are discussed. The morphology, dimension, crystallinity, grain boundaries, defect and separation, transport of photogenerated charges will also be mentioned as the impact factors on photocatalytic performance.
    Physical Chemistry Chemical Physics 04/2012; 14(22):7894-911. · 3.83 Impact Factor