Wanbiao Hu

Colossal Dielectric Behavior of Ga+Nb Co-Doped Rutile TiO2

Stimulated by the excellent colossal permittivity (CP) behavior achieved in In+Nb co-doped rutile TiO2, in this work we investigate the CP behavior of Ga and Nb co-doped rutile TiO2, i.e., (Ga(0.5)Nb(0.5))(x)Ti(1-x)O2, where Ga(3+) is from the same group as In(3+) but with a much smaller ionic radius. Colossal permittivity of up to 10(4)-10(5) with an acceptably low dielectric loss (tan δ = 0.05-0.1) over broad frequency/temperature ranges is obtained at x = 0.5% after systematic synthesis optimizations. Systematic structural, defect, and dielectric characterizations suggest that multiple polarization mechanisms exist in this system: defect dipoles at low temperature (∼10-40 K), polaronlike electron hopping/transport at higher temperatures, and a surface barrier layer capacitor effect. Together these mechanisms contribute to the overall dielectric properties, especially apparent observed CP. We believe that this work provides comprehensive guidance for the design of new CP materials.

Atomic-scale control of TiO 6 octahedra through solution chemistry towards giant dielectric response

The structures of many important functional oxides contain networks of metal-oxygen polyhedral units i.e. MOn. The correlation between the configurations and connectivities of these MOn to properties is essentially important to be well established to conduct the design, synthesis and application of new MOn-based functional materials. In this paper, we report on an atomic-scale solution-chemistry approach that for the first time enables TiO6 octahedral network control starting from metastable brookite TiO2 through simultaneously tuning pH values and interfering ions (Fe3+, Sc3+, and Sm3+). The relationship between solution chemistry and the resultant configuration/connectivity of TiO6 octahedra in TiO2 and lepidocrocite titanate is mapped out. Apart from differing crystalline phases and morphologies, atomic-scale TiO6 octahedral control also endows numerous defect dipoles for giant dielectric responses. The structural and property evolutions are well interpreted by the associated H+/OH− species in solution and/or defect states associated with Fe3+ occupation within TiO6 octahedra. This work therefore provides fundamental new insights into controlling TiO6 octahedral arrangement essential for atomic-scale structure-property design.

Colossal permittivity with ultralow dielectric loss in In + Ta co-doped rutile TiO2

Colossal permittivity (CP) materials have many important applications in electronics but their development has generally been hindered due to the difficulty in achieving a relatively low dielectric loss. In this work, we report an In + Ta co-doped TiO2 material system that manifests high dielectric permittivity and low dielectric loss based on the electron-pinned defect-dipole design. The dielectric loss can be reduced down to e.g. 0.002 at 1 kHz, giving high performance, low temperature dependent dielectric properties i.e. er > 104 with tanδ < 0.02 in a broad temperature range of 50–400 K. Density functional theory calculations coupled with the defect analysis uncover that electron-pinned defect dipoles (EPDDs), in the form of highly stable triangle-diamond and/or triangle-linear dopant defect clusters with well-defined relative positions for Ti reduction, are also present in the host material for the CP observed. Such a high-performance dielectric material would thus help for practical applications and points to further discovery of promising new materials of this type.

Electric-field-induced AFE-FE transitions and associated strain/preferred orientation in antiferroelectric PLZST.

Electric-field-induced, antiferroelectric-ferroelectric (AFE-FE) phase transitions are common for AFE materials. To date, the strain and preferred orientation evolution as well as the role of the intermediate FE state during the successive AFE-FE-AFE phase transitions has not been clear. To this end, we have herein studied a typical AFE Pb0.97La0.02(Zr0.56Sn0.33Ti0.11)O3 (PLZST) material using in-situ neutron diffraction. It is striking that the AFE-FE phase transition is not fully reversible: in the electric-field-induced FE state, the induced strain exhibits an elliptical distribution, which in turn leads to significant preferred orientation in the final AFE state after withdrawal of the applied electric-field. The ω-dependent neutron diffraction patterns show clear evidence of the induced strain distribution and associated preferred orientation arising from the AFE-FE phase transition. The current work also provides an explanation for several temperature and electric-field dependent dielectric anomalies as well as unrecovered strain change which appear in AFE materials after exposure to sufficiently high electric fields.

The Formation of Defect-Pairs for Highly Efficient Visible-Light Catalysts

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.

Colossal permittivity behavior and its origin in rutile (Mg 1/3 Ta 2/3 ) x Ti 1-x O 2

This work investigates the synthesis, chemical composition, defect structures and associated dielectric properties of (Mg2+, Ta5+) co-doped rutile TiO2 polycrystalline ceramics with nominal compositions of (Mg2+1/3Ta5+2/3)xTi1−xO2. Colossal permittivity (>7000) with a low dielectric loss (e.g. 0.002 at 1 kHz) across a broad frequency/temperature range can be achieved at x = 0.5% after careful optimization of process conditions. Both experimental and theoretical evidence indicates such a colossal permittivity and low dielectric loss intrinsically originate from the intragrain polarization that links to the electron-pinned

Electron-pinned defect-dipoles for high-performance colossal permittivity materials

{\bf{M}}{{\bf{g}}}_{{\bf{T}}{\bf{i}}}^{{\prime}{\prime} }+{{\bf{V}}}_{{\bf{O}}}^{\bullet \bullet }+{\bf{2}}{\bf{T}}{{\bf{a}}}_{{\bf{T}}{\bf{i}}}^{\bullet }+{\bf{2}}{\bf{T}}{{\bf{i}}}_{{\bf{T}}{\bf{i}}}^{\prime}

Synthesis and chemical modifications of in-situ grown anatase TiO2 microspheres with isotropically exposed {0 0 1} facets for superhydrophobic and self-cleaning properties

MgTi′′+VO••+2TaTi•+2TiTi′ defect clusters with a specific configuration, different from the defect cluster form previously reported in tri-/pent-valent ion co-doped rutile TiO2. This work extends the research on colossal permittivity and defect formation to bi-/penta-valent ion co-doped rutile TiO2 and elucidates a likely defect cluster model for this system. We therefore believe these results will benefit further development of colossal permittivity materials and advance the understanding of defect chemistry in solids.