Noncentrosymmetric Helical Se3 and Second Polymorph β-Nb2Se3: Synthesis, Crystal Structure, and Their Impact on Photocatalytic Methylene Blue Degradation, Adsorptive Removal of Heavy Metals, and Antioxidant Activity.

Acentric helical Se3 and the β-polymorph of Nb2Se3 were grown using the hydrothermal method. The existence of chirality in Se3 was demonstrated in both P3121 and P3221 enantiomorphs with lattice parameters a = 4.3654(2) Å, c = 4.9603(2) Å, V = 81.863(6) Å3, and Z = 1. The structure consists of infinite helical chains running along the c axis. Conversely, β-Nb2Se3 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) Å3, and Z = 4. The crystal structure contains infinite [Nb2Se3]∞ chains parallel to the b axis. A needle-shaped morphology was observed in both materials. Nb2Se3 and Se3 attain 94% and 73% degradation of methylene blue dye in the presence of 6% H2O2, indicating H2O2-induced dark catalysis. Both Se3 and Nb2Se3 efficiently adsorb Cr(VI) ions with qmax 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 Nb2Se3 and Se3 respectively, and EC50 values are 0.12 and 0.85 mg, indicating their excellent potential as antioxidants.