“…Metal ions of noble metals (Pt, Pd, Ag, and Au) [166] and transition metals (Cr, Cu, Mn, Zn, Co, Fe, and Ni) [167] are used as dopants [168]. Even non-metals such as C, N, S, and P are used for this purpose [160,161,163,169]. Transition metals are used as an alternative to noble metals to reduce the cost.…”