Revealing the true impact of interstitial and substitutional nitrogen doping in TiO on photoelectrochemical applications

Application of photocatalysts that strongly absorb within the visible range is a common strategy to improve the efficiency of photoelectrochemical (PEC) systems; this may translate to high photocurrents, but it is not always the case. Here, we show that nitrogen doping enhances visible light absorption of TiO 2 ; however, it does not necessarily result in improved PEC performance. Depending on the applied external potential, N-doping can improve, or degrade, PEC performance either under water oxidation conditions or via hole scavenging (Na 2 S/Na 2 SO 3 ). In this work, we developed a holistic approach to evaluate the true impact of N doping in TiO 2 on PEC performance. Interstitial and substitutional N doping are experimentally explored for the first time through a simple and novel PEC approach which complemented X-ray photoelectron analyses. Using this approach, we show that interstitial N doping of anatase TiO 2 dominates up to 400 °C and substitutional doping up to ca. 600 °C, without rutile formation. This reveals that the bottleneck for doping higher N-concentrations in TiO 2 is the direct transformation to thermodynamically favorable N-rich phases, such as TiN/Ti 2 N at 700 °C, inhibiting the formation of rutile phase. Transmission electron microscopy revealed that N doping proceeds mainly from the inner to the outer tube walls via nitridation and follows a preferential pathway from interstitial to substitutional doping. Direct PEC experimental evidence on visible light activation of N doped TiO 2 , and the location of interband states, showed acceptor levels of 1.0 eV for substitutional and 0.7 eV for interstitial doping above the TiO 2 valence band maximum. In addition, due to O vacancies and Ti 3+ species, donor levels below the conduction band minimum were also created. These levels act as trapping/recombination centers for charge carriers and, therefore, the gain in the visible range due to N doping does not translate to an improved PEC performance by these structural defects. Ultimately, we show that whilst there is a benefit of visible light absorption through N doping in TiO 2, the PEC performance of the samples only surpasses pristine TiO 2 at relatively high biasing (>0.3 V vs. Ag/AgCl). A holistic approach to fully characterize the physical-chemical properties of N doped TiO 2 and evaluate its true impact on photoelectrochemical applications.