“…However, maintaining a sufficiently high concentration of Ti 3þ is challenging because it is easily oxidized. Several strategies, such as oxidizing the doped Ti 2þ (e.g., TiH 2 or TiO) in TiO 2 [14e16], combusting the mixed Ti 2 O 3 and TiO 2 in air [17], heating TiO 2 under reducing conditions (e.g., H 2 , Zn, or urea) [18,19], high energetic particle bombardment [20,21], and electrochemical reduction [22], have been explored to increase the concentration of Ti 3þ in TiO 2 . However, nearly all of these methods do not attempt to preserve the Ti 3þ concentration throughout the synthesis (i.e., from the initial preparation to the final products).…”