Browsing by Author "Hu, D"

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    Antiferroelectric-to-Ferroelectric Switching in CH3NH3PbI3 Perovskite and Its Potential Role in Effective Charge Separation in Perovskite Solar Cells
    (American Physical Society, 2016) Sewvandi, GA; Hu, D; Chen, C; Ma, H; Kusunose, T; Tanaka, Y; Nakanishi, S; Feng, Q
    Perovskite solar cells (PSCs) often suffer from large performance variations which impede to define a clear charge-transfer mechanism. Ferroelectric polarization is measured numerically using CH3NH3PbI3 (MAPbI3) pellets to overcome the measurement issues such as pinholes and low uniformity of thickness, etc., with MAPbI3 thin films. MAPbI3 perovskite is an antiferroelectric semiconductor which is different from typical semiconducting materials and ferroelectric materials. The effect of polarization carrier separation on the charge-transfer mechanism in the PSCs is elucidated by using the results of ferroelectric and structural studies on the perovskite. The ferroelectric polarization contributes to an inherent carrierseparation effect and the I–V hysteresis. The ferroelectric and semiconducting synergistic chargeseparation effect gives an alternative category of solar cells, ferroelectric semiconductor solar cells. Our findings identify the ferroelectric semiconducting behavior of the perovskite absorber as being significant to the improvement of the ferroelectric PSCs performances in future developments.
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    Controlling dye coverage instead of addition of organic acid to reduce dye aggregation in dye-sensitized solar cells
    (Elsevier, 2020) Sewvandi, GA; Kakimoto, M; Chen, C; Hu, D; Abeygunawardhana, PKW; Feng, Q
    The photo-generated electron injection yield of dye-sensitized solar cells (DSSCs) based on donor-acceptor conjugated dyes is lowered by the aggregation of surface adsorbed organic dyes that pose a low-photoenergy conversion efficiency. Coadsorbates used to prevent the aggregation cause to decompose or detach the dye molecules anchored on the TiO2 surface. In this study, the effect of coadsorption of organic acid and organic dyes on photovoltaic performances was systematically scrutinized by means of adsorption isotherms and photovoltaic measurements. Our laboratory synthesized {0 1 0}-faceted TiO2 (PA TiO2) and P25 TiO2 were used as mesoporous nanocrystals, D149 organic dye was used as a sensitizer and cheno-deoxycholic acid, CDA, was used as a coadsorbate. The coadsorption of CDA reduces the adsorption parameters, maximum adsorption density (Qm) and adsorption constant (Kad), and the reduction depended on the type of TiO2. The photovoltaic performance indicates that the D149 dye has the best dye coverage at around 70% for the effective photovoltaic energy conversion. The coadsorption of CDA increased the photovoltaic performances of DSSCs based on P25 TiO2 but, CDA decreased the photovoltaic performances of DSSCs based on PA TiO2 due to the reduction of the dye coverage below 70%. The results suggest that the coadsorption of organic acids is not necessary if the particular TiO2 can maintain its coverage at the best coverage. Thus, the requirement of coadsorbates to reduce the dye aggregation depends on the type of TiO2 used in DSSCs.
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    One-Dimensional Piezoelectric BaTiO3 Polycrystal of Topochemical Mesocrystal Conversion from Layered H2Ti4O9·H2O Single Crystal
    (American Chemical Society, 2018) Hu, D; Miao, L; Zhang, Z; Li, L; Wang, Y; Hualei, C; Feng, Q; Fan, M; Zhao, L; Sewwandi, GA
    A one-dimensional (1D) piezoelectric BaTiO3(BT) polycrystal with a tetragonal system was controllably prepared using a one-step hydrothermal soft chemical process. A layered H2Ti4O9·H2O (HTO) single crystal was used as the precursor. Interestingly, the obtained polycrystalline BT constructed from oriented BT nanoparticles shows a set of single-crystalline diffraction points, implying that the BT polycrystal is a mesocrystal. The [010] and [001] directions of the obtained mesocrystalline BT correspond to the [001] and [010] directions of the original HTO crystal, respectively, and the [010] direction of the mesocrystalline BT corresponds to the direction along the length of a 1D rod matrix. An in situ topochemical mesocrystal conversion of mesocrystalline BT grown on the HTO substrate occurred not only on the substrate surface but also in the interlayers owing to the unique TiO6 octahedral layers of HTO. In addition, the piezoelectric response of the 1D mesocrystal was captured for the first time. The piezoelectric property of BT mesocrystals hinges on its nanostructure. It is a challenge to develop a special process to prepare highly 1D anisotropic perovskite titanate polycrystals with high crystallinity and piezoelectric response. This novel strategy can be utilized to develop piezoelectric mesocrystalline materials.
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    Rod-like incipient ferroelectric SrTiO3 polycrystal with crystal-axis orientation
    (Elsevier, 2020) Zhang, Z; Miao, L; Yao, M; Li, L; Zhao, F; Gu, H; Han, Y; Galhenage, AS; Feng, Q; Yang, D; Wang, X; Hu, D
    Strontium titanate (SrTiO3, ST) is an incipient ferroelectric material. Generally, a one-dimensional (1D) ST polycrystal is difficult to obtain because of its high aspect ratio and directional orientation. We prepared a 1D ST polycrystal from a layered H2Ti4O9·H2O (HT) single crystal via a solvothermal soft chemical process. The 1D ST polycrystal fabricated from ST nanocrystals has a [010] crystal axis, implying that it is a mesocrystal. The ST nanocrystals were grown not only on the surfaces but also in the interlayers of the original HT matrix. The formation mechanism of the ST mesocrystal is based on a topochemical mesocrystal conversion process. ST ceramics fabricated from ST mesocrystal powder present a lossy capacitor response and excellent energy-storage capabilities owing to the presence of locally oriented ST nanocrystals with a lattice mismatch in the rod-like ST mesocrystals. The investigations performed in this study can set a new benchmark for the development of new electric ST-based materials.
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    Solvothermal reaction and piezoelectric response of oriented KNbO3Polycrystals
    (ACS Publications, 2021) Yang, D; Wang, Y; Li, L; Yao, M; Zhang, W; Gu, H; Zhang, S; Fan, M; Sewvandi, GA; Hu, D
    KNbO3 (KN) piezoelectric polycrystals were prepared by a two-step solvothermal reaction process with the managed organic solvents as reaction mediums at a low temperature for a short time. In the solvothermal reaction system, the formation mechanism of polycrystalline KN is mainly the dissolution–deposition mechanism. The influences of alkalinity, viscosity, and the polarity for reaction mediums on the formation of the niobates were investigated. The chemical reaction mechanisms of niobate products and formation mechanism of niobate crystals from the precursor were clarified. The regulating and controlling mechanism of the phase compositions, the morphologies, and the lattice constants for the niobates obtained in varied reaction mediums were revealed. The obtained KN piezoelectric polycrystals are constructed from oriented KN nanocrystals. Piezoelectric hysteresis loops of cuboid KN polycrystals were detected for the first time. A prepared cuboid KN polycrystal shows an average d33* value of 32 pm/V. The study provides a strategy for the development of oriented KN piezoelectric materials to apply the orientation engineering.
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    Topological relations and piezoelectric responses of crystal-axis-oriented BaTiO3/CaTiO3 nanocomposites
    (Royal Society of Chemistry, 2017) Hu, D; Niu, X; Ma, H; Zhang, W; Sewvandi, GA; Yang, D; Wang, X; Wang, H; Kong, X; Feng, Q
    2D crystal-axis-oriented mesocrystalline BaTiO3/CaTiO3 (BT/CT) nanocomposites with high-density heteroepitaxial interfaces were synthesized by a two-step solvothermal soft chemical process. The nanostructures, formation mechanism, topological relations between the BT and CT, and piezoelectric responses of the nanocomposites were investigated. The mesocrystalline nanocomposites are polycrystals constructed from crystal-axis-oriented BT and CT nanocrystals with the same crystal-axis orientation, respectively. The directions of the [001] and [1−10] of crystalline BT correspond to the directions of the [0−10] and [100] of crystalline CT, respectively. The mesocrystalline nanocomposites were formed via an in situ topochemical mesocrystal conversion mechanism. The density of the artificial BT/CT heteroepitaxial interface in these mesocrystalline nanocomposites can be adjusted by regulating the fraction of BT and CT in the nanocomposites. The mesocrystalline BT/CT nanocomposite with the composition close to BT/CT = 1/1 presents a large piezoelectric response owing to the lattice strain derived from its heteroepitaxial interfaces with the high density in the nanocomposite.