The nitrogen–boron paramagnetic center in visible light sensitized N–B co-doped TiO2. Experimental and theoretical characterization

Nitrogen boron co-doped TiO2 prepared via sol–gel synthesis and active under visible light, contains two types of paramagnetic extrinsic defects, both exhibiting a well resolved EPR spectrum. The first center is the well characterized [NiO]˙ species (i = interstitial) also present in N-doped TiO2, while the second one involves both N and B. This latter center (labeled [NOB]˙) exhibits well resolved EPR spectra obtained using either 14N or 15N which show a high spin density in a N 2p orbital. The structure of the [NOB]˙ species is different from that previously proposed in the literature and is actually based on the presence of interstitial N and B atoms both bound to the same lattice oxygen ion. The interstitial B is also linked to two other lattice oxygen ions reproducing the trigonal planar structure typical of boron compounds. The energy level of the [NOB]˙ center lies near the edge of the valence band of TiO2 and, as such, does not contribute to the visible light absorption. However, [NOB]˙ can easily trap one electron generating the [NOB]− diamagnetic center which introduces a gap state at about 0.4 eV above the top of the valence band. This latter species can contribute to the visible light activity.