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

Acentric helical Se₃ and the β<i>-</i>polymorph of Nb₂Se₃ were grown using the hydrothermal method. The existence of chirality in Se₃ was demonstrated in both <i>P</i>3₁21 and <i>P</i>3₂21 enantiomorphs with lattice parameters <i>a</i> = 4.3654(2) Å, <i>c</i> = 4.9603(2) Å, <i>V</i> = 81.863(6) ų, and <i>Z</i> = 1. The structure consists of infinite helical chains running along the <i>c</i> axis. Conversely, β<i>-</i>Nb₂Se₃ exhibits orthorhombic symmetry (SG: <i>Pnma</i>) with lattice parameters <i>a</i> = 11.7748(2) Å, <i>b</i> = 3.9719(5) Å, <i>c</i> = 11.6323(5) Å, <i>V</i> = 544.02(7) ų, and <i>Z</i> = 4. The crystal structure contains infinite [Nb₂Se₃]_∞ chains parallel to the <i>b</i> 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 <i>q</i>_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 Nb₂Se₃ and Se₃ respectively, and EC₅₀ values are 0.12 and 0.85 mg, indicating their excellent potential as antioxidants.