Unique photoelectrochemical behavior of TiO2 nanorods wrapped with novel titanium Oxy-Nitride (TiOxNy) nanoparticles

In this work, we developed novel titanium oxynitride (TiOxNy) nanoparticles with diameter of 25 ± 2 nm and crystalline size of ∼15 nm on hydrothermally grown one-dimensional (1D) TiO2 nanorod (TNR) arrays. Herein, the TiOxNy nanoparticles were synthesized by facile nitridation using TiO2 powder at 100% NH3 gas atmosphere. Titanium oxynitride composed of potentially energetic metal-nitrogen bonds (TiN), compared to the weaker TiO bond, becomes chemically stable in the alkaline environment, and is considered as a suitable material for photoelectrochemical (PEC) system. The PEC performance of TiOxNy decorated TNR (abbreviated as TiOxNy @TNR) films was evaluated in 0.1 M KOH solution under solar illumination condition, and achieved the potentially high photocurrent density (J) of 2.1 mA/cm2 at 1.23 V versus reversible hydrogen electrode (RHE) (abbreviated as VRHE) in the TiOxNy@TNR arrays, in comparison with the poor photoresponse (0.7 mA/cm2 at 1.23 VRHE) of the pristine TNR arrays. A nearly three-fold enhancement was attained in the TiOxNy decorated TNR arrays, attributed to the high visible light absorption and fast carrier separation, due to the hybridization with the visible active TiOxNy nanoparticles in the cascading band alignment between the TiOxNy and TNR materials. Furthermore, the introduction of TiOxNy layer on the TNR surface quite reduces the interfacial resistance in the solid-liquid interface region, and further, the TiOxNy layer contributes to the passivation of the surface states (e.g., defect, trap sites etc.) where the charge recombination reaction frequently happens, leading to the improvement of PEC performance.