Transition-metal (TM)-ion-doped TiO2 materials are of great importance for photocatalysis- and photoelectrochemical (PEC)-related applications. We introduced a facile, low-cost, and scalable doping method of TM ions (Cr, Co, Cu, Fe, Mn, etc.) into TiO2 nanotubes (NTs), while maintaining their high-ordered tubular structures (with similar to 120 nm outside diameter). Both crystallization and doping processes were simultaneously accomplished in aqueous solution at a temperature as low as similar to 90 degrees C, and the fastest doping process could be accomplished in 30 min for Fe doping. Besides, the surface areas of the doped TiO2 NTs were increased to 129.0 m(2)/g, and their absorption ranges could be expanded from 380 to >500 nm. This study shed light on a facile method for doping TM ions that is extendable to other semiconductors in the field of PEC water splitting and could improve their efficiencies as well.