Co-molten solvothermal method for synthesizing chalcopyrite CuFe1−xCrxS2 (x ≤ 0.4): high photocatalytic activity for the reduction of nitrate ions

Abstract

In literature, it is very difficult to obtain sulfides with Cr³⁺ in tetrahedral coordination. Here, a thiourea–oxalic acid co-molten solvothermal method was applied to synthesize chalcopyrite CuFe_1−xCr_xS₂ (x ≤ 0.4) solid solutions. We propose that oxalic acid plays an important role in the crystallization of CuFe_1−xCr_xS₂ and can considerably restrain the formation of other undesirable impurities. The successful incorporation of Cr³⁺ was confirmed by powder XRD, SEM and EDX mapping (2D elemental distribution). The UV-Vis reflectance spectra of CuFe_1−xCr_xS₂ suggest that the bandgap energies decrease from 0.80 to 0.61 eV along with an increase in the Cr³⁺ concentration. All the CuFe_1−xCr_xS₂ (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 Fe³⁺–Cr³⁺ sulfides, but can also be helpful to incorporate Cr³⁺ to other sulfide systems with MS₄ tetrahedra.