Electrical conductivity and charge compensation in Nb doped TiO2 rutile

Baumard, Jean‐François; Tani, Eiji

doi:10.1063/1.434952

Cited by 143 publications

(98 citation statements)

References 18 publications

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“…34,35 The variation in defect formation energies with sample composition also explains the reported absence of an EPR signal for photogenerated holes in reduced TiO 2 .…”

Section: 11mentioning

confidence: 81%

Polaronic trapping of electrons and holes by native defects in anataseTiO2

2009

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We have investigated the formation of native defects in anatase TiO 2 using density functional theory ͑DFT͒ modified with on-site Coulomb terms ͑DFT+ U͒ applied to both Ti d and O p states. Oxygen vacancies and titanium interstitials are found to be deep donors that trap two and four electrons, with transition levels that explain the two features seen in deep level transient spectroscopy experiments. Titanium vacancies are deep acceptors accommodating four holes. Self-trapping of both electrons and holes is also predicted. In all cases both donor and acceptor trap states correspond to strongly localized small polarons, in agreement with experimental EPR data. Variation in defect formation energies with stoichiometry explains the poor hole-trapping of reduced TiO 2 . DOI: 10.1103/PhysRevB.80.233102 PACS number͑s͒: 71.38.Ht, 71.55.Ϫi, 82.50.Ϫm In semiconductors used in photocatalysis and photovoltaics, the primary process is the light-induced production of charge carriers.1 Photocatalysis proceeds by photoexcitation of electrons to unoccupied bands, producing free electronhole pairs. These charge carriers then typically undergo trapping, followed by diffusion to the surface where they can initiate reactions of adsorbed molecules. Electron-hole recombination is often a major competing process, and it is highly desirable to promote charge separation and inhibit subsequent charge-carrier-recombination in order to improve the quantum efficiency of photocatalytic processes.TiO 2 has been widely studied as a photocatalyst for the degradation of environmental pollutants, 1 and for water splitting. 2 Nanocrystalline anatase TiO 2 is often used, since it is more photocatalytically active than the ground-state polymorph, rutile, and the diffusion pathways of photogenerated charge carriers to the surface are shortened, resulting in increased quantum efficiencies.Following photoexcitation, trapping of charge carriers occurs on a nanosecond time scale.3 Defects are thought to play a critical role in the trapping process by acting as recombination centers. For example, n-type doping of anatase reduces the photocatalytic activity of experimental samples, 4 and reduced anatase is less efficient than stoichiometric TiO 2 in the trapping of photogenerated holes.5 Understanding the interaction of defects with charge carriers is essential for the optimization of TiO 2 samples for photocatalysis. Electronand hole traps have both been observed in EPR spectra. [5][6][7][8] These data have been interpreted as charge localization at single atoms to give small polarons: hole trapping has been associated with O − sites; O O • , and electron trapping with Ti 3+ species; Ti Ti Ј . Additional evidence for charge localization at Ti 3+ states in n-type TiO 2 comes from core-level XPS shifts, and characteristic gap-state features observable in UPS spectra, with samples annealed to produce oxygen vacancies giving identical EPR signals to those seen following uv irradiation. Deep traps 0.9 and 0.5 eV below the conduction band edge have also bee...

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“…34,35 The variation in defect formation energies with sample composition also explains the reported absence of an EPR signal for photogenerated holes in reduced TiO 2 .…”

Section: 11mentioning

confidence: 81%

Polaronic trapping of electrons and holes by native defects in anataseTiO2

2009

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“…22 Outside this x range, the major defects that arise are titanium interstitials (Ti i ) and oxygen vacancies (V O ). [23][24][25][26] Both defect types can explain oxygen-deficient non-stoichiometric TiO 2 x . The calculated activation energy of 0.65 eV matches that of the first ionization energy of oxygen vacancies (V Á O ), [27][28][29] and the ALD TiO 2 relaxation process is mainly attributed to first ionized V Á O .…”

Section: (C)mentioning

confidence: 99%

Anomalous C-V response correlated to relaxation processes in TiO2 thin film based-metal-insulator-metal capacitor: Effect of titanium and oxygen defects

Marichy

Sylvestre

Pinna

2015

Journal of Applied Physics

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Capacitance-voltage (C V) and capacitance-frequency (C f) measurements are performed on atomic layer deposited TiO 2 thin films with top and bottom Au and Pt electrodes, respectively, over a large temperature and frequency range. A sharp capacitance peak/discontinuity (C V anomalous) is observed in the C V characteristics at various temperatures and voltages. It is demonstrated that this phenomenon is directly associated with oxygen vacancies. The C V peak irreversibility and dissymmetry at the reversal dc voltage are attributed to difference between the Schottky contacts at the metal/TiO 2 interfaces. Dielectric analyses reveal two relaxation processes with degeneration of the activation energy. The low trap level of 0.60 0.65 eV is associated with the first ionized oxygen vacancy at low temperature, while the deep trap level of 1.05 eV is associated to the second ionized oxygen vacancy at high temperature. The DC conductivity of the films exhibits a transition temperature at 200 C, suggesting a transition from a conduction regime governed by ionized oxygen vacancies to one governed by interstitial Ti 3þ ions. Both the C V anomalous and relaxation processes in TiO 2 arise from oxygen vacancies, while the conduction mechanism at high temperature is governed by interstitial titanium ions. V C 2015 AIP Publishing LLC.

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“…Nb ions, substitutionally soluted to Ti, are therefore expected to increase the electrical conductivity, acting as donordopant. 8 …”

Section: Film Preparationmentioning

confidence: 97%

“…This characteristic limited the low-temperature operation of TiO 2 up to now. 8,9 We have therefore undertaken a research plan to investigate Au-titania thin-films for low-temperature sensing of CO. The microstructural features of the composite system was investigated by electron microscopy, while the electronic structure of single Au clusters was characterized by electron holography.…”

Section: Introductionmentioning

confidence: 99%