Effect of NH4F treatment on the chromaticity of the Ba2+-doped γ-Ce2S3 red pigment

The Ba2+-doped cerium precursor powder, (Ce, Ba)CO3, was successfully prepared via a co-precipitation reaction method using Ce(NO3)3·6H2O, Ba(NO3)3, and (NH4)2CO3 as raw materials. The above powder was placed in a muffle furnace and calcined at 1000°C for 120 min to obtain the corresponding oxide, (Ce, Ba)O2, subsequently, and the oxide was vulcanized at 850°C for 150 min with CS2 as a sulfur source to obtain the Ba2+-doped γ-Ce2S3 red pigment. The effects of different NH4F concentrations (1.0, 2.5, and 4.0 M) and treatment times (30, 60, 90, and 120 min) on the phase composition and chromaticity of the pigment were studied by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffractometry (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and CIELAB colorimetry. The SEM and EDS analyses revealed the presence of irregular flaky particles on the pigment surface after the NH4F treatment; the amounts of particles gradually increased with the NH4F concentration. The XRD and Raman spectra confirmed that these particles were CeF3 and CeFS heterophases. Furthermore, the Raman spectra denoted the presence of amorphous carbon. The XPS analysis demonstrated that the NH4F treatment did not change the valence of Ce (Ce3+ for γ-Ce2S3). The chromaticity of a sample reached its maximum (L* = 34.24, a* = 38.19, and b* = 39.95) after it was treated using 2.5 M NH4F for 60 min, indicating that the elimination of amorphous carbon can effectively improve the pigment chromaticity.