Browsing by Author "Frankcombe, TJ"

Now showing 1 - 6 of 6
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    Collective nonlinear electric polarization via defect-driven local symmetry breaking
    (Royal Society of Chemistry, 2019-05-17) Dong, W; Cortie, DL; Lu, T; Sun, QB; Narayanan, N; Hu, WB; Jacob, L; Li, Q; Yu, DH; Chen, H; Chen, AP; Wei, XY; Wang, G; Humphrey, MG; Frankcombe, TJ; Liu, Y
    In this work, we report the defect-mediated, abnormal non-linear polarization behavior observed in centrosymmetric rutile TiO2 where less than 1 at% of sterically mismatched Mg2+ ions are introduced to create ferroelectric-like polarization hysteresis loops. It is found that the Image ID:c9mh00516a-t2.gif defect cluster produces a dipole moment exceeding 6 Debye, with a rotatable component. Such a polarization is further enhanced by the displacement of neighboring Ti4+ ions. The coupling between such defect-driven symmetry-breaking regions generates a collective nonlinear electrical polarization state that persists to high temperatures. More importantly, an observation of abnormal bias shift of polarization hysteresis suggests an antiparallel alignment of certain dipoles frozen relative to the external poling electric field, which is associated with oxygen vacancy hopping. This result challenges the long-standing notion of parallel alignment of dipoles with the external electric field in ferroelectrics. This work also reveals an unexpected new form of non-linear dielectric polarization (non-ferroelectricity) in solid-state materials. © Royal Society of Chemistry 2024
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    The formation of defect-pairs for highly efficient visible-light catalysts
    (Wiley, 2017-01-23) Sun, QB; Cortie, DL; Zhang, SY; Frankcombe, TJ; She, GW; Gao, J; Sheppard, LR; Hu, WB; Chen, H; Zhuo, SJ; Chen, DH; Withers, RL; McIntyre, GJ; Yu, DH; Shi, WS; Liu, Y
    Highly efficient visible-light catalysts are achieved through forming defect-pairs in TiO2 nanocrystals. This study therefore proposes that fine-tuning the chemical scheme consisting of charge-compensated defect-pairs in balanced concentrations is a key missing step for realizing outstanding photocatalytic performance. This research benefits photocatalytic applications and also provides new insight into the significance of defect chemistry for functionalizing materials. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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    Highly efficient visible light catalysts driven by Ti3+-VO-2Ti4+-N3− defect clusters
    (Wiley, 2018-10-13) Sun, QB; Zhang, SY; Cortie, DL; Langley, J; Cox, N; Frankcombe, TJ; Gao, J; Chen, H; Withers, RL; Kremer, F; Yu, DH; Brink, F; Shi, WS; Liu, Y
    Local defect structures play significant roles on material properties, but they are seriously neglected in the design, synthesis, and development of highly efficient TiO2-based visible light catalysts (VLCs). Here, we take anatase TiO2 nanocrystals that contain (Ti3+, N3−) ions and have the complicated chemical formula of (Ti1-x4+Tix3+)(O2-2-y-zNy3-□z) as an example, and point out that the formation of Ti3+-VO-2Ti4+-N3− local defect clusters is a key missing step for significantly enhancing VLC properties of host TiO2 nanocrystals. Experimental and theoretical investigations also demonstrate the emergent behaviors of these intentionally introduced defect clusters for developing highly efficient VLCs. This research thus not only provides highly efficient visible light catalysts for various practical applications but also addresses the significance of local defect structures on modifying material properties. © 2019 Wiley-VCH Verlag GmbH & Co.
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    Highly efficient visible light catalysts driven by Ti3+‐VO‐2Ti4+‐N3− defect clusters
    (Wiley, 2018-10-13) Sun, QB; Zhang, SY; Cortie, DL; Langley, J; Cox, N; Frankcombe, TJ; Gao, J; Chen, H; Withers, RL; Kremer, F; Yu, DH; Brink, F; Shi, WS; Liu, Y
    Local defect structures play significant roles on material properties, but they are seriously neglected in the design, synthesis, and development of highly efficient TiO2‐based visible light catalysts (VLCs). Here, we take anatase TiO2 nanocrystals that contain (Ti3+, N3−) ions and have the complicated chemical formula of ()(□z) as an example, and point out that the formation of Ti3+‐VO‐2Ti4+‐N3− local defect clusters is a key missing step for significantly enhancing VLC properties of host TiO2 nanocrystals. Experimental and theoretical investigations also demonstrate the emergent behaviors of these intentionally introduced defect clusters for developing highly efficient VLCs. This research thus not only provides highly efficient visible light catalysts for various practical applications but also addresses the significance of local defect structures on modifying material properties. © 1999-2024 John Wiley & Sons, Inc or related companies.
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    Role of a-site molecular ions dynamics in the polar functionality of perovskite metal-organic framework
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Lu, T; Cortie, DL; Li, Z; Narayanan, N; Liu, Z; Sun, QB; Frankcombe, TJ; McIntyre, GJ; Yu, DH; Liu, Y
    Recent studies on organic-inorganic hybrid perovskites (OIHPs) and ferroelectric metal-organic framework perovskites (MOFPs) reveal their superb performance as highly efficient photovoltaics and promising ferroelectrics. This has enabled a new generation of optic electronic-mechanical devices based on green chemistry. However, the ultimate strategies to optimize these polarization-related functionalities are not yet clear, leading to confused reports in the literature. In this work, we investigate a rationally selected series of molecular ions within Mg(HCOO)3– frameworks to form [CH3NH3]Mg(HCOO)3 (MAMOF),[(CH3)2NH2]Mg(HCOO)3 (DMAMOF), and [C(NH2)3]Mg(HCOO)3 (GUAMOF). Single-crystal X-ray diffraction, inelastic neutron spectroscopy and ab initio molecular dynamics are used to achieve detailed structural pictures of three MOFPs. Intriguingly, our study reveals that the alignments of protonated amines are highly dependent on the directional hydrogen bonds that link N-H units to the surrounding MgO6 octahedra. The alignments of different amines and their dynamics are therefore determined by the acceptor O provided by the distortive frameworks. We successfully assigned the alignments of the A-site ions associated with different polar behavior to the dielectric properties for three MOFPs and propose that the configuration of the A-site molecular ions and potential hydrogen bonds are critical to enable the design of polarization-related functionalities in both MOFPs and OIHPs.
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    Role of a-site molecular ions in the polar functionality of metal–organic framework perovskites
    (American Chemical Society (ACS), 2021-12-28) Lu, T; Cortie, DL; Li, ZX; Narayanan, N; Liu, Z; Sun, QB; Frankcombe, TJ; McIntyre, GJ; Yu, D; Liu, Y
    Recent studies on organic–inorganic hybrid perovskites (OIHPs) and ferroelectric metal–organic framework perovskites (MOFPs) reveal their superb performance as highly efficient materials for photovoltaics and ferroelectrics. This has enabled the development of a new generation of optic-electronic-mechanical devices based on green chemistry. However, the fundamental understanding of these polarization-related functionalities is not yet clear, which has hindered the progress in further designing and developing materials with expected properties. In this work, we investigate three MOFPs that have the same Mg(HCOO)3– frameworks with different molecular ions: [CH3NH3][Mg(HCOO)3] (MA-MOF), [(CH3)2NH2][Mg(HCOO)3] (DMA-MOF), and [C(NH2)3][Mg(HCOO)3] (GUA-MOF). Single-crystal and powder X-ray diffraction, inelastic neutron spectroscopy, and ab initio molecular dynamics simulations are combined to achieve a detailed description of the three MOFPs’ static and dynamic structures as a function of temperature. Intriguingly, our study reveals that the alignments and motions of the guest molecular ions are highly dependent on the directional hydrogen bonds that link N–H units to the surrounding MgO6 octahedra through the O acceptor from the frameworks. At the same time, the size, dynamic behavior, and alignments of the A-site molecular ions influence the distortive framework structures and their temperature-dependent deformation. Therefore, the mutual interaction between the guest and the framework determines the overall functionalities of the MOFPs. This study indicates that the configuration of the A-site molecular ions and the potential hydrogen bonds are critical to design the polar functionalities in both MOFPs and OIHPs. © 2021 American Chemical Society