Growth and Characterization of Arsenic-Based Polymorphs of Co2AsO4F: Visible-Light-Induced Photocatalytic Dye Degradation, Electrocatalytic Hydrogen Evolution, and Catalytic Solvent-Free Sustainable Knoevenagel Condensation.

Single crystals of two polymorphs, i.e., α- and β-Co2AsO4F, were grown by pH-mediated hydrothermal synthesis technique. The α-Co2AsO4F crystallizes in monoclinic symmetry with a = 13.066(5) Å, b = 6.536(2) Å, c = 9.866(4) Å, β = 116.380°, and V = 754.7(5) Å3 (SG: C2/c), whereas β-Co2AsO4F crystallizes in tetragonal symmetry with a = b = 5.500(3) Å, c = 13.006(7) Å, and V = 393.4(4) Å3 (SG: I41/amd). The structural analysis reveals that the α-phase comprises two different distorted Co(II) units, namely [CoO4F3]/[Co(1)O4F(1)0.5(F(2)0.5)2] and [Co(2)O4(F(1)0.5)2F(2)0.5], and one [As(V)O4] tetrahedral unit, whereas the β-phase comprises one [Co(II)O4F2] octahedral unit and [As(V)O4] tetrahedral unit. Both phases showed their efficiency toward methylene blue (MB) degradation, and the rate was enhanced up to 4-fold with the addition of 0.1 mL 6% H2O2. Both materials were established as excellent electrocatalysts for hydrogen production through water splitting, with low overpotentials of 101 mV and 91 mV at a current density of 10 mA cm-2, Tafel slopes of 155 and 151 mV dec-1, respectively, and prolonged reusability, with only a 12 mV increase in overpotential after 1000 CV cycles at 10 mA cm-2. Moreover, they were also explored as catalysts in the solvent-free Knoevenagel condensation reaction.