From nanotubes to single crystals: Co doped TiO2

Abstract

Millimeter-sized cobalt doped rutile crystals with a bi-pyramidal shape are obtained by chemical vapour transport using scroll-type H2Ti3O7 nanotubes as a precursor in which Co2+ ions are introduced by a simple ion exchange method prior to the growth. Despite the low concentration of Co2+ dopants (5 × 1019 cm−3), the resistivity of the single crystal shows a metallic behaviour above 50 K and the Seebeck coefficient has the signatures of polaronic quasiparticles. The magnetic properties of the material show a weak anti-ferromagnetic interaction between the spins on Co atoms below 50 K. This synthesis method could be beneficial for the growth of a large variety of doped TiO2 single crystals

Theoretical limits of thermoelectric figure of merit inn-typeTiO2polymorphs

“…Additionally, we find that the Seebeck coefficient is isotropic to within 1% in all three polymorphs. Our calculated Seebeck coefficients are in reasonable agreement with available experimental data for rutile 4,5 and anatase, 10,11 as summarized in the insets of Figs. 2a,c.…”

“…Three naturally occurring polymorphs-rutile, anatase, and brookite-are stable in atmosphere at high temperatures (rutile melts near 2100 K 1 while anatase and brookite transform irreversibly to rutile above approximately 900 K and 1100 K, respectively 2,3 ). Furthermore, large n-type Seebeck coefficients have been measured from -360 to -700 µV/K in rutile [4][5][6] and from -240 to -500 µV/K in anatase, [7][8][9][10][11] while power factors as high as 14 µW/(K 2 cm) have been reported. 12 Despite these promising characteristics, the highest reported value of the dimensionless thermoelectric figure of merit ZT = σS 2 T /κ to date is 0.35 at 973 K in rutile, 13 which is well below that of the current best thermoelectric materials.…”

Tuning the Two-Dimensional Electron Gas at Oxide Interfaces with Ti–O Configurations: Evidence from X-ray Photoelectron Spectroscopy

“…where A is a temperature independent constant, Ep is the activation energy for hopping condution and kB is the Boltzmann constant. Through fitting, the deduced activation energy is 14 meV that is comparable with Co doped TiO2, 43 indicating that the transport proceeds through thermal activation. 44 To reveal the physical origin of the interfacial conductivity and mechanisms that lead to the different transport properties of these three heterostructures, we further performed an analysis of the oxidization state of relevant elements by XPS.…”

Tailoring thermal conduction in anatase TiO2

“…The value of 0.014 WK −1 m −1 is important and beneficial by itself because it calls for an excellent thermal insulator, but especially also by the way it was reached: by the discreet-phonon dispersion due to the one-dimensionality of the nanowires. Since the same mechanism does not limit the electrical conductivity, this foam would be an excellent material for thermoelectric applications if one doped them to metallicity 54 . Recalling that in anatase thin films doped with 6% of Nb, the high power factor (S 2 σ, where S is the Seebeck coefficient and σ the electrical conductivity) of 14 µWK −2 cm −1 gave only a figure of merit (ZT) of 0.1 because of the high thermal conductivity (8.5 WK −1 m −1 ) of the thin film, but not attractive enough for practical purposes.…”