The impact of charge transfer and structural disorder on the thermoelectric properties of cobalt intercalated TiS2

Realization of an Ultrahigh Power Factor and Enhanced Thermoelectric Performance in TiS2 via Microstructural Texture Engineering

Layered 1T-type TiS₂ powders were pretreated by an ethanol-based shear pulverization process, which showed outstanding effectiveness in reducing the average grain size and narrowing the size distribution while maintaining high crystallinity and plate-shaped morphology. The resulting bulk ceramics densified by spark plasma sintering possessed a highly (00l)-oriented texture and pronounced anisotropy. They showed a noticeably increased σ and an unaffected S in the in-plane direction due to the increased carrier mobility μ and the constant carrier concentration n, which resulted in a significant enhancement of the in-plane power factor, optimally to an unprecedented high level of 1.6-1.8 mW m^-1 K^-2 in a range of 323-673 K. Meanwhile, the lattice thermal conductivity was reduced by approximately 20% due to the intensified grain boundary phonon scattering that overwhelmed the effect due to texturing. These effects not only demonstrated the powder shear pulverization pretreatment as a facial and reliable route toward a high-textured TiS₂ but also enabled a remarkable increase of ZT record for TiS₂-based thermoelectrics (TEs) to approximately 0.7 at 673 K, indicating clearly the significant effect of texture engineering on TE performance.

Distinct anisotropy and a high power factor in highly textured TiS2 ceramics via mechanical exfoliation

Polycrystalline bulk of TiS2 with a remarkable enhancement of the texture degree was obtained by densifying powders refined by a liquid-based mechanical exfoliation process. As compared to the pristine TiS2, the in-(a-b)-plane mobility in the exfoliation sample increased from 5.9 to 9.8 cm2 V-1 s-1 with an almost unaffected carrier concentration, in spite of the increased scattering due to grain boundaries. As a result, a tremendously high power factor of up to 16 μW cm-1 K-2 at 673 K was achieved, which is 60% higher than that of the pristine TiS2 and is the highest for bulk TiS2 at high temperatures.

Correlation between Crystal Structure and Thermoelectric Properties of Sr1−xTi0.9Nb0.1O3−δ Ceramics

Polycrystalline Sr1−xTi0.9Nb0.1O3−δ (x = 0, 0.1, 0.2) ceramics have been prepared by the solid state method and their structural and thermoelectric properties have been studied by neutron powder diffraction (NPD), thermal, and transport measurements. The structural analysis of Sr1-xTi0.9Nb0.1O3−δ (x = 0.1, 0.2) confirms the presence of a significant amount of oxygen vacancies, associated with the Sr-deficiency of the materials. The analysis of the anisotropic displacement parameters (ADPs) indicates a strong softening of the overall phonon modes for these samples, which is confirmed by the extremely low thermal conductivity value (κ ≈ 1.6 W m-1 K−1 at 823 K) found for Sr1−xTi0.9Nb0.1O3−δ (x = 0.1, 0.2). This approach of introducing A-site cation vacancies for decreasing the thermal conductivity seems more effective than the classical substitution of strontium by rare-earth elements in SrTiO3 and opens a new optimization scheme for the thermoelectric properties of oxides.

Electron Density Optimization and the Anisotropic Thermoelectric Properties of Ti Self-Intercalated Ti1+ xS2 Compounds

Polycrystalline Ti_{1+ x}S₂ (0.111 ≤ x ≤ 0.161) with high density and controllable composition were successfully prepared using solid-state reaction combined with plasma-activated sintering. Ti_{1+ x}S₂ showed strong (00 l) preferred orientation with Lotgering factor of 0.32-0.60 perpendicular to the pressing direction (⊥), whereas the preferred orientation was not obvious along the pressing direction (∥). This structural anisotropy resulted in distinct anisotropic thermoelectric transport properties in Ti_{1+ x}S₂. At 300 K, while the Seebeck coefficient was weak anisotropic, the power factor and lattice thermal conductivity of Ti_{1+ x}S₂ was much larger in the perpendicular direction as compared to that of the parallel direction, with an anisotropic ratio of 1.8-2.7 and 1.3-1.7, respectively. Theoretical calculations of formation energy of defects suggested that the excess Ti was most probably intercalated into the van der Waals gaps in metal-rich Ti_{1+ x}S₂, consistent with X-ray diffraction, high-resolution transmission electron microscopy characterization and transport measurements. With increasing x, the carrier concentration and power factor of Ti_{1+ x}S₂ dramatically increased because of the donor behavior of Ti interstitials, which was accompanied by a significant decrease in the lattice thermal conductivity owing to the strengthened phonon scattering from structural disorder. Because of its strongest (00 l) preferred orientation and largest carrier mobility among all samples, Ti_1.112S₂ had the highest power factor of 22 μW cm^-1 K^-2 at 350 K perpendicular to the pressing direction, close to the value (37.1 μW cm^-1 K^-2) achieved in single-crystal TiS₂. We found out that the maximum power factor and dimensionless figure of merit ZT could be achieved at an optimum carrier concentration of about 5.0 × 10²⁰ cm^-3. Finally, Ti_1.142S₂ acquired the highest ZT value of 0.40 at 725 K perpendicular to the pressing direction because of the beneficial preferred orientation, improved power factor, and reduced lattice thermal conductivity.

Metallic 1T-TiS2 nanodots anchored on a 2D graphitic C3N4 nanosheet nanostructure with high electron transfer capability for enhanced photocatalytic performance

The introduction of metallic TiS₂ nanodots in 2D-C₃N₄ nanosheets improved the photocatalytic activity due to the suppression of e–h recombination.