Electronic Structure and Conductivity Modifications of Rutile TiO₂(100) by Hydrogen Ion Irradiation

抄録

TiO₂ has promising applications in solar conversion and hydrogen storage^1,2. Black titania formation due to hydrogen doping is reported to reduce the band gap from 3.1 eV to 1.54 eV, which significantly improves solar absorption efficiency. TiO₂ is also capable of storing hydrogen up to 1.4 w.t. % which is facilitated by hydrogen diffusion at 7 MPa and 450^oC. In order to investigate the hydrogen effect at higher concentration, hydrogen ion irradiation at ultrahigh vacuum environment is employed. This technique eliminates the necessity of using high pressure, high temperature, and Pd overlayer which facilitates hydrogen dissociation. Therefore, we investigate the electrical conductivity and electronic properties changes of TiO₂(100) by hydrogen ion irradiation.

Hydrogen ion irradiation (2 keV) on Nb-doped rutile TiO₂(100) at 295 K causes its measured electrical resistance to decrease, which is due to electron donations from the irradiated hydrogen to TiO₂. The donated electron reduces the Ti⁴⁺ to Ti³⁺ which is observed by measuring the chemical state change of Ti 2p core level in x-ray photoelectron spectroscopy (XPS). In ultraviolet photoelectron spectroscopy (UPS), the donated electron in the Ti 3d orbital forms an in-gap state (IGS) at 1 eV below the Fermi level. Moreover, the UP spectra also shows that the valence band bends downward which suggests electron donations to the TiO₂. These results indicate that Ti³⁺ state behaves as a small polaronic center which contributes to the conductivity.

In contrast to the results at 295 K, the hydrogen ion irradiation at 125 K causes the electrical resistance to increase. The XP spectra shows that the Ti³⁺ chemical state increases after the irradiation. However, it is observed that the IGS forms at a deeper energy level (1.3 eV below the Fermi level) compared to the former IGS by irradiation at 295 K. This suggests that there are at least two Ti³⁺-related IGS whose transport properties are different. Upon annealing to 295 K and cooling again to 125 K, the IGS composition changes permanently. Therefore, the deeper IGS is likely to be a metastable state.

References:

1. Wang et. al., Advanced Functional Materials 2013, 23, 5444-5450

2. Sun et al., The Journal of Physical Chemistry C 2011, 115, 25590-25594