Solar Light and Dopant-Induced Recombination Effects: Photoactive Nitrogen in TiO2 as a Case Study

The spectral response of newly synthesized and doped TiO2 nanostructures, which are designed for the operation under solar light, is key to their potential performance as photocatalyst materials. In the case of nitrogen-doped TiO2 particles, the presence of the photoactive defect can be verified via characteristic spectroscopic fingerprints, such as (1) an optical absorption band above lambda > 400 nm, (2) paramagnetic defect states related to substitutional and interstitial nitrogen atoms, and (3) a photoluminescence emission feature that can be induced by band gap excitation. We report these spectroscopic fingerprints for hydrothermally derived and N-doped TiO2 powder samples. On the basis of defined annealing procedures, we also found a way to eliminate the photoactive nitrogen defects and performed light-induced charge separation experiments on samples before and after dopant elimination in O-2 atmosphere. The concentrations of adsorbed O-2(-) ions, which stand for scavenged photogenerated electrons, were determined by electron paramagnetic resonance (EPR) spectroscopy. On the basis of these data, we calculated the number of photons, which (a) were additionally utilized because of the beneficial effect of photoactive nitrogen and (b) were lost because of the dopant-induced recombination of UV light generated charge carriers. As a major result, we found for the doped materials that both effects actually compensate each other. With regard to the synthesis and characterization of doped TiO2 systems, these findings underline the importance to include dopant-induced charge-carrier recombination effects in the evaluation of the photocatalyst's spectral response in order to reach a fair evaluation of its properties.