Noncentrosymmetric Helical Se 3 and Second Polymorph β - Nb 2 Se 3 : Synthesis, Crystal Structure, and Their Impact on Photocatalytic Methylene Blue Degradation, Adsorptive Removal of Heavy Metals, and Antioxidant Activity

Acentric helical Se 3 and the β - polymorph of Nb 2 Se 3 were grown using the hydrothermal method. The existence of chirality in Se 3 was demonstrated in both P 3 1 21 and P 3 2 21 enantiomorphs with lattice parameters a = 4.3654(2) Å, c = 4.9603(2) Å, V = 81.863(6) Å 3, and Z = 1. The structure consists of infinite helical chains running along the c axis. Conversely, β - Nb 2 Se 3 exhibits orthorhombic symmetry (SG: Pnma ) with lattice parameters a = 11.7748(2) Å, b = 3.9719(5) Å, c = 11.6323(5) Å, V = 544.02(7) Å 3, and Z = 4. The crystal structure contains infinite [Nb 2 Se 3 ] ∞ chains parallel to the b axis. A needle-shaped morphology was observed in both materials. Nb 2 Se 3 and Se 3 attain 94% and 73% degradation of methylene blue dye in the presence of 6% H 2 O 2, indicating H 2 O 2 -induced dark catalysis. Both Se 3 and Nb 2 Se 3 efficiently adsorb Cr(VI) ions with q max of 150 and 241 mg/g, respectively, and Fe(III) ions with 248 and 266 mg/g capacity, respectively. BET and ζ-potential analyses confirm that electrostatic interactions predominantly govern the adsorption processes. The antioxidant activity against DPPH exhibits 94% and 83% efficacy in 5 and 30 min with only 1 mg of Nb 2 Se 3 and Se 3 respectively, and EC 50 values are 0.12 and 0.85 mg, indicating their excellent potential as antioxidants.