Co-molten solvothermal method for synthesizing chalcopyrite CuFe(1-x)Cr(x)S₂ (x ≤ 0.4): high photocatalytic activity for the reduction of nitrate ions.

In literature, it is very difficult to obtain sulfides with Cr(3+) in tetrahedral coordination. Here, a thiourea-oxalic acid co-molten solvothermal method was applied to synthesize chalcopyrite CuFe(1-x)Cr(x)S2 (x ≤ 0.4) solid solutions. We propose that oxalic acid plays an important role in the crystallization of CuFe(1-x)Cr(x)S2 and can considerably restrain the formation of other undesirable impurities. The successful incorporation of Cr(3+) was confirmed by powder XRD, SEM and EDX mapping (2D elemental distribution). The UV-Vis reflectance spectra of CuFe(1-x)Cr(x)S2 suggest that the bandgap energies decrease from 0.80 to 0.61 eV along with an increase in the Cr(3+) concentration. All the CuFe(1-x)Cr(x)S2 (0 ≤ x ≤ 0.4) samples show considerably higher photocatalytic activities than P25 toward the reduction of nitrate ions in aqueous solution. We speculate that the thiourea-oxalic acid co-molten method may not only be effective to synthesize Fe(3+)-Cr(3+) sulfides, but can also be helpful to incorporate Cr(3+) to other sulfide systems with MS4 tetrahedra.