Conductive rutile TiO2 has received considerable attention recently due to multiple applications. However, the permittivity in conductive, reduced or doped TiO2 appears to cause controversy with reported values in the range 100–10,000. In this work, we propose a method for measurements of the permittivity in conductive, n-type TiO2 that involves: (i) hydrogen ion-implantation to form a donor concentration peak at a known depth, and (ii) capacitance–voltage measurements for donor profiling. We cannot confirm the claims stating an extremely high permittivity of single crystalline TiO2. On the contrary, the permittivity of conductive, reduced single crystalline TiO2 is similar to that of insulating TiO2 established previously, with a Curie–Weiss type temperature dependence and the values in the range 160–240 along with the c-axis.

Electronic states in the upper part of the bandgap of reduced and/or hydrogenated n-type rutile TiO2 single crystals have been studied by means of thermal admittance and deep-level transient spectroscopy measurements. The studies were performed at sample temperatures between 28 and 300 K. The results reveal limited charge carrier freeze-out even at 28 K and evidence the existence of dominant shallow donors with ionization energies below 25 meV. Interstitial atomic hydrogen is considered to be a major contributor to these shallow donors, substantiated by infrared absorption measurements. Three defect energy levels with positions of about 70 meV, 95 meV, and 120 meV below the conduction band edge occur in all the studied samples, irrespective of the sample production batch and the post-growth heat treatment used. The origin of these levels is discussed in terms of electron polarons, intrinsic point defects, and/or common residual impurities, where especially interstitial titanium atoms, oxygen vacancies, and complexes involving Al atoms appear as likely candidates. In contrast, no common deep-level defect, exhibiting a charge state transition in the 200–700 meV range below the conduction band edge, is found in different samples. This may possibly indicate a strong influence on deep-level defects by the post-growth heat treatments employed.

Schottky barrier diodes (SBDs) were fabricated by depositing Pd, Pt or Ni on single crystal, conductive n-type rutile TiO2 using e-beam evaporation. As-grown and nominally undoped rutile TiO2 single crystals are semi-insulating, and were heat-treated in forming gas flow, N2 flow or H2 gas to obtain conductive n-type crystals displaying electrical conductivities in the range of ( 0.5 − 8 ) × 10 − 2 Ω − 1 cm − 1 . Additionally, SBDs were deposited on Nb-doped conductive n-type rutile TiO2 with a conductivity of around 0.25 Ω − 1 cm − 1 . Generally, SBDs displaying a rectification of up to eight orders of magnitude were obtained, when comparing the current under reverse and forward bias. The extracted ideality factors were in the range of 1.1 − 4.0 . From Capacitance-Voltage measurements, the built-in voltage was derived to be around 1.2 V – 1.9 V , depending on the doping concentration of the specific TiO2 single crystal. Series resistances as low as 19 Ω were achieved. A considerable variation in the electrical characteristics of different SBDs deposited on the same crystal was found, regardless of the metal or doping strategy used. Moreover, the SBD characteristics change over time, particularly seen as a degradation in rectification, mainly related to an increase in the current under reverse bias. Additional surface treatments such as boiling in H2O2 and etching in HF do not have a significant effect on the quality of the SBDs. Clear indications for poor adhesion between TiO2 and Pd are shown. In conclusion, we demonstrate the fabrication of SBDs which are suitable for studying the fundamental properties of metal/TiO2 junctions and the characteristics of electrically-active defects in TiO2 using space-charge spectroscopy.

We have used a combination of optical absorption and electrical conductivity measurements to study the effect of the main donor on small polarons in rutile TiO 2 single crystals rendered n-type conductive by hydrogenation or doping with Nb. The electrical conductivity measured at 295 K for hydrogenated samples shows a clear correlation with the interstitial hydrogen (H i ) concentration, which is consistent with reports that H i is the main shallow donor in rutile TiO 2 . Conductive samples exhibit two distinct optical absorption bands in the IR spectral region, at ω 1 = 6500 cm −1 (∼0.8 eV) and ω 2 = 3100 cm −1 (∼0.4 eV), which are present in both hydrogen-rich and Nb-doped samples. The intensities of the absorption bands are proportional to the electrical conductivity, and they exhibit an Arrhenius-like temperature dependence for temperatures between 25-50 K and 50-100 K for H-doped and Nb-doped samples, respectively. The thermal activation energies (E A s) for the absorption bands depend strongly on the main donor: ω 2 exhibits E A (H) and E A (Nb) of ∼4 and ∼10 meV, respectively, whereas ω 1 shows E A (H) and E A (Nb) of ∼1 and ∼2 meV, respectively. The combination of temperature-dependent data for the optical absorption bands and interstitial deuterium (D i )-small polaron vibrational lines support a model where the thermal activation is associated with the reconfiguration of small polarons involving Ti sites far away from the donor. The thermal activation of the optical absorption bands gives us insight into the dynamics of donor-dependent small polaron reconfiguration in rutile TiO 2 .

TiO2 nanowires growth was investigated varying the synthesis parameters. Nanowires demonstrated improved photocatalytic activity, especially when treated in forming gas.