Detailed XPS analysis and anomalous variation of chemical state for Mn- and V-doped TiO2 coated on magnetic particles

Manganese (Mn)- and vanadium (V)-doped titania (TiO2) coated on iron oxide (γ-Fe2O3) nanoparticles (TM and TV) were reduced in H2/N2 mixture atmospheres at different reducing temperatures TR. Furthermore, TM and TV reduced at TR = 800 °C (TM8 and TV8) were subsequently nitrided in the ammonia (NH3) atmosphere at different temperatures TN in order to change the chemical state (CS) of the Mn, V, Fe, Ti, O, and N elements. Samples were characterized by X-ray diffraction and X-ray photoelectron spectroscopy. For reduced samples, with increasing TR, anomalous variation of CS is as follows: Ratios of V4+ and V5+ increased, while the ratio of V3+ decreased, and the ratio of oxygen vacancy Ov decreased. Data conflict with the effect of reduction reaction could be understood as follows: Ov was occupied by O from H2O (product of the reduction reaction), according to previous theoretical result. For nitrided samples, compared to TM8 and TV8 samples, as TN increased, ratios of Mn4+ and V5+ with the highest valence state exhibited an increase at TN = 400 °C. Moreover, the ratio of TiOx decreased and the ratio of TiO2 increased, even though NH3 exhibited good reducibility. Data could be well interpreted as follows: Ov was occupied by N along with adsorbed O, possibly in the form of neutral NO radical and NO22− radical ion. Till date, these phenomena have not been reported experimentally, which are important to understand the mechanism for photocatalytic performance of Mn-, V-, N-, and self-doped TiO2.