Preparation, characterization, and properties of a Ba²⁺–Sm³⁺ co-doped γ-Ce₂S₃ red pigment

Highlights

• γ-Ce₂S₃ was prepared by codoping with Ba²⁺ and Sm³⁺ at 850 °C for 150 min.

• The chromaticity value of the pigments increased from L∗ = 31.84, a∗ = 30.95, b∗ = 23.63, and C∗ = 38.94 (S.Sm_0.00) to L∗ = 34.63, a∗ = 35.36, b∗ = 38.88, C∗ = 52.55 (S.Sm_0.01).

• The S.Sm_0.01 sample still exhibited a pure γ-phase and showed excellent red color (L∗ = 33.18, a∗ = 33.74, b∗ = 36.69, and C∗ = 49.84) after the heat treatment at 440 °C for 10 min in air.

Abstract

In this study, Ba²⁺–Sm³⁺ co-doped γ-Ce₂S₃ (abbreviated as γ-[Ba,Sm]-Ce₂S₃) red pigments were synthesized by the coprecipitation method with a composition of n(Ba)/n(Ce_1−xSm_x) = 0.1(molar ratio, x = 0, 0.01, 0.03, 0.05, and 0.10 mol). The corresponding vulcanized products, γ-[Ba,Sm]-Ce₂S₃ red pigments (abbreviated as S.Sm_x), were prepared using CS₂ as a sulfur source at 850 °C for 150 min. The effect of the Sm³⁺ doping content on the phase composition, chromaticity, and thermal stability of Ba²⁺–Sm³⁺ co-doped γ-Ce₂S₃ was systematically investigated by FE-SEM, EDS, XRD, Raman spectroscopy, HR-TEM, XPS, CIELAB colorimetry, and TG-DTA. The results show that a pure γ phase can be obtained for S.Sm_x, when x is varied from 0 to 0.10 mol at 850 °C. With an increase in the Sm³⁺ content, the band gap of γ-[Ba,Sm]-Ce₂S₃ increased from 2.12 to 2.14 eV, which resulted in a color change from red to red-orange. The chromaticity value of the pigments increased from L∗ = 31.84, a∗ = 30.95, b∗ = 23.63, and C∗ = 38.94 (S.Sm_0.00) to L∗ = 34.63, a∗ = 35.36, b∗ = 38.88, C∗ = 52.55 (S.Sm_0.01), which indicates that Ba²⁺–Sm³⁺ co-doping can effectively increase the chromaticity value. The S.Sm_0.01 sample still exhibited a pure γ-phase and showed excellent red color (L∗ = 33.18, a∗ = 33.74, b∗ = 36.69, and C∗ = 49.84) after the heat treatment at 400 °C for 10 min in air, which indicated that Ba²⁺–Sm³⁺ co-doping successfully increased the thermal stability of the S.Sm_0.01 red pigment. S.Sm_0.01 has excellent chromaticity and good thermal stability, which expands the number of methods for preparing γ-Ce₂S₃ red pigment and shows a considerable market potential.

Introduction

Inorganic red pigments have been widely used in a variety of materials including glass, plastics, and ceramics, because of their bright color, and strong coverage. However, most popular red pigments, such as CdSe_1−xS_x, contain Cd, which is toxic [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]]. Their use is restricted or even banned. The use of environmentally safe red pigments is attracting considerable attention. γ-Ce₂S₃ is a rare earth sesquisulfide red pigment with excellent tinting and environmentally safe properties; in addition, Ce is abundant in the Earth's crust, and Ce is becoming a promising substitute for CdSe_1−xS_x materials [[19], [20], [21], [22], [23]].

γ-Ce₂S₃ has a defective Th₃P₄-type structure and a cubic symmetry (I-43d space group) [19,24]; this non-stoichiometric phase occurs in the composition range between Ce₂S₃ (or Ce_8/3V_1/3S₄, where “V” denotes metal vacancies) and Ce₃S₄, formulated as Ce_3–xV_xS₄, without vacancy ordering in the cation sublattice (between vacancies and metal atoms) [[25], [26], [27], [28]]. It's typical synthesis route includes a reaction between CeO₂ and H₂S at 1200 °C for 120 min, and the resulting product is not pure [29,30]. Therefore, previous studies mainly focused on decreasing ion doping by controlling the synthesis temperature of γ-Ce₂S₃. For example, Vasilieva et al. [19] prepared Na-doped γ-Ce₂S₃ at 850 °C and investigated the distribution of Na⁺ ions in the sample. Urones-Garrote et al. [31] prepared Ca-doped γ-Ce₂S₃ at 900 °C and studied the chromaticity and structural changes of γ-Ce₂S₃ with an increase in the Ca²⁺ content. Li et al. [32] studied Sr-doped γ-Ce₂S₃ at 900 °C and studied the chromaticity and thermal stability of γ-Ce₂S₃ with an increase in the Sr²⁺ content. The abovementioned studies demonstrate that a suitable single ion can effectively reduce the synthesis temperature of γ-Ce₂S₃. In a related study, Luo et al. [33] investigated γ-La₂S₃, which has the same cubic Th₃P₄-type structure as γ-Ce₂S₃ at 700 °C, and successfully stabilized γ-La₂S₃ at 1100 °C by doping Eu³⁺ ions. This fully demonstrated that the thermal stability of the sample can be effectively improved by suitable ion doping. However, Aubert et al. [34] found that the L∗, a∗, and b∗ values for KCeS₂ samples can be effectively adjusted by co-doping rare earth ions (e.g., La, Dy, and Yb) compared to K single doping. However, despite the advances achieved in this field, the stabilization of the internal lattice of γ-Ce₂S₃ by low-valence ion doping to improve the thermal stability using equivalent ion doping to improve the chroma of the sample has not been systematically reported. Therefore, in this study, on the basis of a Ba to (Ce_1−xSm_x) molar ratio [n(Ba)/n(Ce_1−xSm_x)] of 0.1, the effects of varying Sm³⁺ to Ce³⁺ content on the phase composition, chromaticity, and thermal stability of γ-[Ba,Sm]-Ce₂S₃ were systematically investigated.