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Engineering the crystal structure of γ-[Li]-Ce2S3 red pigments for enhanced thermal stability
Journal Pre-proof Engineering the crystal structure of γ-[Li]-Ce2S3 red pigments for enhanced thermal stability Fusheng Song, Yueming Li, Yongzhi Yu, Zongyang Shen, Zhumei Wang, Xin Li PII:
S0022-4596(19)30615-2
DOI:
https://doi.org/10.1016/j.jssc.2019.121110
Reference:
YJSSC 121110
To appear in:
Journal of Solid State Chemistry
Received Date: 30 September 2019 Revised Date:
28 November 2019
Accepted Date: 2 December 2019
Please cite this article as: F. Song, Y. Li, Y. Yu, Z. Shen, Z. Wang, X. Li, Engineering the crystal structure of γ-[Li]-Ce2S3 red pigments for enhanced thermal stability, Journal of Solid State Chemistry (2020), doi: https://doi.org/10.1016/j.jssc.2019.121110. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Engineering the crystal structure of γ-[Li]-Ce2S3 red pigments for enhanced thermal stability Fusheng Song a,b,c, Yueming Li a,b,c*, Yongzhi Yu b, Zongyang Shen a,b,c, Zhumei Wang a,b,c, Xin Li a,b,c a
b
c
School of Materials Science and Engineering, Jingdezhen Ceramic Institute; Jingdezhen 333403, China
National Engineering Research Center for Domestic and Building Ceramics; Jingdezhen 333403, China
Key Laboratory of Functional Ceramic Materials of China Light Industry, Jingdezhen 333403, China
Abstract: The search for high thermal stability, environmentally-friendly, and low-cost red pigments for glazes remains a significant challenge in ceramic research. Here, we report a simple gas-solid reaction method to synthesis red inorganic pigments, namely γ-[Li]-Ce2S3. The effects of different molar ratio of lithium to cerium nLi/Ce (nLi/Ce =0.05, 0.10, 0.15, 0.20, 0.25, 0.30) on the phase composition, crystal structure, chromatic properties and thermal stability of pigments were systematically studied. The XRD and XPS characterization combined with Rietveled structural refinement showed that Li+ entered into the γ-Ce2S3 lattice structure to form γ-[Li]-Ce2S3 solid solutions. The two different nearest-neighbors Ce-S bond lengths in the γ-[Li]-Ce2S3 and the lattice parameters decreased with the content of nLi/Ce, while these values displayed a constant beyond nLi/Ce=0.20, indicating that the vacancies in the lattice were fully filled and Li+ no longer entered the lattice. The substitution of Li+ for Ce3+ and the elimination of the cationic vacancies in γ-[Li]-Ce2S3 led to an increase in the band gap from 1.95 to 2.00 eV, corresponding to a slight change in the red hue. Furthermore, from Raman spectra, a trace of cerium polysulfide as an impurity phase was found in the pigments, *Corresponding author. E-mail address: [email protected]
which was responsible for the observed dark red color of as-prepared γ-[Li]-Ce2S3. In addition, the thermal stability was evaluated by TG-DSC and the thermal stable temperature of γ-[Li]-Ce2S3 can be raised to 450°C when nLi/Ce was equal to 0.20, showing that the stability of the pigments can be improved by adjusting the structural parameters. Keywords: γ-[Li]-Ce2S3; red pigments; bond length; thermal stability;
1. Introduction Inorganic pigments have been widely used in various fields covering ceramic inks, paints and others because of their strong tinting power, high weather resistance and thermal stability [1-5]. Among numerous inorganic pigments, the red pigment has been especially favored over time owing to its noble and luxuriant characteristics. Although there have been numerous reports of red pigments, few can show a pure and bright red color. During the past several decades, CdSxSe1-x red pigments have been widely utilized in the decoration industry due to its bright red color [6, 7]. Unfortunately, traditional cadmium red pigments contain highly toxic elements (Cd and Se), which are dangerous for human health and the environment. Therefore, concerns over safety, availability and cost have motivated researchers to develop new red pigments based on environmentally-friendly and cost-effective materials [8, 9]. Recently, γ-Ce2S3 has attracted tremendous attention owing to its bright red color and non-toxicity. This emerging pigment is expected to replace highly toxic cadmium red pigments [3, 5, 10]. However, γ-Ce2S3 is prone to oxidation and discoloration at more than 350 °C, which greatly limits its application, especially in high-temperature
fields such as ceramics [11]. To improve its thermal stability, there have, recently, been extensive efforts to introduce a layer of a colorless transparent material (such as SiO2) coating onto γ-Ce2S3, which leads to a stable temperature of 550 °C has been identified [12-14]. However, due to the difference in refractive index, composition and structure between the coating material and pigment, the coating material weakens the color of the pigment. Theoretically, three phases of cerium sesquisulfides exist, including α, β and γ structures [15, 16]. Among these allotropes, γ-Ce2S3 is a high-temperature phase and isomorphic to Ce3S4, which is analogous to the cubic structure of the Th3P4-type. In fact, the structure of γ-Ce2S3 can be regarded as being formed by a partial removal of cerium ions of Ce3S4 [17-19]. Therefore, the widely claimed “γ-Ce2S3” in the literature is actually Ce2.67V0.33S4 (here, we use “γ-Ce2S3” to describe this phase to maintain agreement with general usage), where cationic vacancies are randomly distributed on the site of the cerium atom in the perfect lattice [10, 17-19]. It is found that there are two different Ce-S bonds in the Ce3S4 structure, and the two different nearest-neighbor Ce-S bond lengths are 2.896 and 3.087 Å, respectively [18]. However, the bond lengths for these two kinds of Ce-S bonds in the γ-Ce2S3 structure change to 2.901 and 3.092 Å, respectively [19]. It is not difficult to find that the existence of cationic vacancies leads to a lengthening of the Ce-S bond length in the structure of γ-Ce2S3. It is well known that the vacancy defect concentration and bond length are both closely related to the stability of materials. On the other hand, doping, which involves the intentional incorporation of suitable ions into host lattices, is considered to be an effective method
to regulate the properties of materials. Taking Na+ doped γ-Ce2S3 for example, it has been identified that the vacancies in the γ-Ce2S3 lattice can be partially or wholly filled with Na+, leading to changes in chromatic properties, energy level structure and thermal stability [19-21]. However, for several decades, great efforts have been devoted to investigating the impact of the synthesis temperature on dopant behavior because γ-Ce2S3 is a high-temperature phase, which makes it difficult to synthesize under normal conditions [16, 22-23]. On the contrary, few works have studied the ion doping-induced change in Ce-S bond lengths and the stability of γ-Ce2S3. Herein, this work focused on the control of the substitution of Li+ for Ce3+ in γ-Ce2S3 with the aim of intrinsically enhancing its thermal stability. The effects of Li-substitution on the composition, crystal structure, chromatic properties and thermal stability of the pigments were systematically studied. The obtained γ-[Li]-Ce2S3 exhibited a high thermal stability, and the thermal stable temperature was raised to 450°C in air when the molar ratio of lithium to cerium was equal to 0.20. This excellent performance exceeds that of commercially available products, confirming promising potential for application in the area of ceramics. 2. Experimental 2.1 Synthesis of Li-substituted γ-Ce2S3 All of the reagents were used without further treatment. A series of Li-substituted γ-Ce2S3 powders were prepared via a gas-solid reaction method. The raw materials of lithium carbonate (Li2CO3), cerium oxide (CeO2) and carbon disulfide (CS2), used in the experiments, were all of analytical grade and purchased from Shanghai Sinopharm Chemical reagent Co., Ltd., China. First, Li2CO3 and CeO2 were mixed in an
appropriate molar ratio of lithium to cerium of 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30. The raw precursor mixture was ground and then placed in a pottery boat crucible. Subsequently, the crucible was transferred to a tubular atmosphere furnace. Prior to heating, the furnace was evacuated to -0.1 MPa, and then, argon gas was injected several times to remove the air. The entire following process for both heating and cooling was protected by an inert argon atmosphere with a flow rate of 200 ml/min. Afterwards, the mixture was fired to 900 °C for 4 h with a ramp rate of 5 °C/min in an airstream of CS2/Ar, which was introduced into a quartz tube by bubbling liquid CS2 with flowing argon as the mixture was heated to 200 °C. Finally, the samples were cooled inside a furnace to room temperature in an Ar atmosphere to avoid oxidation. The schematic diagram of the reaction system is shown in Fig. 1. 2.2 Characterization The XRD data for the samples was collected by a D8 Advance X-ray diffractometer (XRD) (Bruker Corporation, Germany), which was operated with voltage and current settings of 40 kV and 40 mA, respectively, using Cu Kα radiation (λ = 1.5418 Å), with a 2θ range of 20~80°, step size of 0.02°and acquisition time per step of 0.2 s. Phase composition was analyzed from these data. For the calculations of the cell parameters and bond length, Rietveld structure refinement was performed using the Total Pattern Solution (TOPAS) program. X-ray photoemission spectroscopy (XPS) was measured using a Thermo escalab 250Xi (Thermo Electron Corporation, USA) to explore the elements on the surface and determine the valence state of the cerium ion in the γ-[Li]-Ce2S3. The Raman spectrum in the range of 100~500 cm-1 was investigated by
an inVia (Renishaw, England) using a 780 nm laser as the excitation source. The diffuse reflectance from the samples was measured with a UV-Vis spectrophotometer (Lambda-850, PerkinElmer Corporation, USA) using BaSO4 as a reference. The optical band gap Eg can be obtained from the Kubelka-Munk (K-M) function. The formula F(R) is shown as follows:
F (R ) =
(1−R )2 2R
(1)
F(R) is the Kubelka-Munk function, where R represents the reflectance of the sample at the corresponding wavelength. According to Eq. (1), F(R) can be calculated by substituting for the R value. Then, the curve of [F(R)hν]2 vs hν (h- Planck constant, ν-the frequency of light) can be drawn. By extrapolating the linear part to the intersection point of the abscissa, the band gap value Eg of the sample can be obtained [24]. The colorimetric values for the samples were measured by a color analyzer (ACM402, Yante, China) using the Commission International de l’Eclairage (CIE) 1976 L*a*b*colorimetric method, where L* represents lightness (from 0 to 100, 0 for black and 100 for white), a* denotes the red value (from -a* (green) to +a*(red)) and b* denotes the yellowness (-b* for blue and + b* for yellow). The thermal stability of the pigment samples was evaluated by simultaneous thermogravimetric-differential scanning calorimetry(TG-DSC, STA 449C, NETZSCH, Germany) from room temperature to 1000 °C at a rate of 10 °C/min in air atmosphere. 3. Results and discussion 3.1 X-ray diffraction analysis Fig. 2 shows the XRD patterns for γ-[Li]-Ce2S3 pigments with the nLi/Ce ranging
from 0.05 to 0.30 sulfurized at 900 °C for 4 h. As seen from Fig. 2, all the observed diffraction peaks are in accordance with the standard cubic structure of γ-Ce2S3 (PDF 01-089-2260), indicating the formation of a single phase for all samples with varying Li-substitution amounts. In addition, no other Li-containing phases are found from the XRD patterns, which clearly demonstrates that Li+ enters the γ-Ce2S3 lattice to form solid solution. To further explore the effect of Li+ substitution on the structure of γ-[Li]-Ce2S3, the cell parameters and bond length of Ce-S in the γ-[Li]-Ce2S3 lattice are calculated by Rietveld structural refinement with the TOPAS program. Fig. 3 shows the Rietveld refinement results for X-ray diffraction pattern for the γ-[Li]-Ce2S3 sample with nLi/Ce= 0.05 and nLi/Ce= 0.20. The cell parameters and bond length of Ce-S for each composition are listed in Table 1 and Table 2. The plot of the cell volume as a function of nLi/Ce is shown in Fig.4. It can be seen that the cell parameters and the two different nearest bond lengths of Ce-S1 and Ce-S2 in the lattice gradually decrease with increasing nLi/Ce molar ratio from 0.05 to 0.20. This change is attributed to the formation of γ-[Li]-Ce2S3 solid solution and the smaller ionic radius of Li+ (0.92 Å, CN = 8) compared to that of Ce3+ (1.14 Å, CN=8) [25, 26]. However, when the molar ratio of nLi/Ce further increases to 0.30, these structural parameters display no change, indicating that Li+ no longer enters the lattice beyond nLi/Ce= 0.20. Meanwhile, it can be seen from Fig. 4 that the cell volume also shows no change when the nLi/Ce above 0.20. This result is mainly ascribed to the fact that the cationic vacancies in the γ-[Li]-Ce2S3 lattice are completely filled up when nLi/Ce= 0.20[12, 15]. Therefore, the above observations imply that the excess
lithium element is likely to exist in the form of small amounts of other compounds, such as LiCeS2, in addition to forming the γ-[Li]-Ce2S3 solid solution, when the nLi/Ce molar ratio exceeds 0.20. Mauricot et al. reported that a NaCeS2 impurity phase appeared for Na-doped γ-Ce2S3 when the nNa/Ce molar ratio was larger than 0.20. However, in our case, further Li+ addition into the system has no obvious change on the phase composition according to the XRD results (Fig. 2), possibly because a LiCeS2 phase is indeed formed when the nLi/Ce molar ratio exceeds 0.20, while its content is too small to be detected by XRD [13]. 3.2 XPS analysis The chemical composition and structure of γ-[Li]-Ce2S3 was further investigated by XPS measurements. Fig. 5(a) shows the survey spectra for γ-[Li]-Ce2S3 with nLi/Ce=0.20. It can be found that the surface of the γ-[Li]-Ce2S3 consists of Ce, S, C, O and Li. The C1s peak located at approximately 284.8 eV is typically attributed to the calibration peak of the instrument itself. The O1s peak in the survey spectra may be mainly ascribed to hydroxyl oxygen and dissociated oxygen adsorbed onto the surface of the sample due to the strong oxygen affinity of Ce. High-resolution spectra for Ce3d, S2p and Li1s are shown in Fig. 5(b)-(d), respectively. The binding energy peaks located at 884.9 and 903.4 eV are attributed to Ce3d5/2 and Ce3d3/2 for Ce3+ in γ-[Li]-Ce2S3, respectively [27]. Additionally, the intense characteristic peaks observed at 160.2 eV and 161.4eV in the XPS spectra for S2p are assigned to S2p2/3 and S2p1/3, respectively [28]. The corresponding binding energy of 55.3 eV in Figure 2(d) is the characteristic peak of Li1s [29]. 3.3 UV-Vis diffuse reflectance spectra and analysis of the chromatic properties
The UV-Vis diffuse reflectance spectra for γ-[Li]-Ce2S3 for varying nLi/Ce are presented in Fig. 6. It is clearly seen that the reflectance spectra for different components display a similar shape with an almost identical slope. Interestingly, the reflectance gradually decreases and the reflection edge shifted slightly towards the shortwave direction with increasing Li+ content. A strong absorption was observed in the region 500~560 nm, which is assigned to the electronic transition Ce4f→Ce5d, thus leading to a visible red color as a complementary. The calculated band gap energies are shown in Fig. 7 and listed in Table 3 along with the corresponding CIE L*a*b* color coordinates. Accordingly, the band gap energies (Eg) slightly increase from 1.95 to 2.00 eV when the nLi/Ce increases from 0.05 to 0.20. The increase in Eg can be explained by the unique effect of ion doping on the lattice structure of the γ-Ce2S3 samples. As low-valent Li+ is introduced into the γ-Ce2S3 with cationic vacancies randomly distributed in the lattice to form γ-[Li]-Ce2S3 solid solution, Li+ will fill vacancies accompanied with the replacement of Ce3+ to maintain electrical neutrality. As a result, the substitution of Li+ for Ce3+ and the elimination of the cationic vacancies in γ-[Li]-Ce2S3 changes the crystal field environment, thus causing an increase in Eg when nLi/Ce is in the range 0.05 to 0.20. The corresponding change in the red color can be observed in Table. 3. It is worth noting that the effect of the addition amount on band gaps above a nLi/Ce of 0.20 is negligible, which is mainly attributed to the fact that all of the cationic vacancies in the γ-[Li]-Ce2S3 lattice are filled up at nLi/Ce = 0.20. The chromatic properties of the γ-[Li]-Ce2S3 pigments are evaluated from the CIE1976 L*a*b* coordinate values. As shown in Table 3, the value of L* increases
from 25.95 to 27.25 and decreases to 24.78 with increasing nLi/Ce, corresponding to a pigment color change from dark to bright and then back again to dark. In addition, the values of a* and b* display a variation in a narrow range for further Li+ addition. The results for the L*a*b* coordinate are consistent with the color change of the γ-[Li]-Ce2S3 pigments. Interestingly, it can also be observed from Table 3 that the as-prepared γ-[Li]-Ce2S3 exhibits a dark red hue as a whole. According to Perrin and Vasilyeva [18,21], although the color of γ-[Li]-Ce2S3 pigments is mainly determined by the intrinsic nature of the Ce4f→Ce5d electron transitions, one should keep in mind the influence of external factors, such as impurities with even small quantities, showing an impact. Therefore, to explore whether small amounts of impurities exist, Raman spectroscopy was used to characterize the vibrational spectra of components with nLi/Ce=0.05, 0.20 and 0.30 under ambient conditions. As illustrated in Fig. 8, the Raman spectra for all the samples display six characteristic peaks at 113, 194, 241, 298, 401, and 413 cm-1. It can be noted that the Raman bands located at 113, 194, 241, and 298 cm-1 are four broadened peaks that are very similar to the peaks found for the rare-earth sesquisulfide γ-Ln2S3 with a Th3P4-type structure [30]. Because Li+, Ce3+ and vacancies occupy the same cation sites, there is a disordered structure in the γ-[Li]-Ce2S3 lattice, which leads to a broadening of the Raman bands [31]. Two narrow and sharp peaks located at 401 and 413 cm-1 are assigned to the characteristic frequency of CeS2-x [21]. Combined with XRD analysis, CeS2-x should be an impurity exists with a small concentration in all samples. Since the detected impurity CeS2-x is brown, it is likely to alter the color of the γ-[Li]-Ce2S3 pigments. As a result, the color shade of the
as-obtained pigments presents a dark red hue. 3.4 Thermal stability The thermal stability of the pigments can be evaluated by simultaneous thermogravimetric-differential scanning calorimetry (TG-DSC). If a negligible weight loss is observed, the pigment will be thermally stable [32]. Fig. 9 presents the TG-DSC curves for the γ-[Li]-Ce2S3 pigments with nLi/Ce=0.05 and nLi/Ce=0.20, respectively. A similar broad peak is observed in the TG curves shown in Fig. 9 (a) and (b), exhibiting a single mass gain and single mass loss process. The process of weight gain is mainly due to the formation of intermediate oxidation products such as oxysulfides and sulfates [33], and the process of the weight loss is attributed to sulfur overflow in the form of SO2 or other gases, with CeO2 being the final oxidation product. In addition, the weight change of the nLi/Ce=0.05 component sample can be neglected before 410 °C, with the temperature of the weight gain of the nLi/Ce=0.20 component begining to appear at 450 °C. The corresponding exothermic peaks appear at 459.3 and 503.8 °C respectively, as shown in the DSC curves in Fig. 9 (a) and (b), suggesting that the stability of the nLi/Ce=0.20 component sample is higher than that of the nLi/Ce=0.05 component sample. mainly because with the increasing Li+ addition content, the bond lengths of the two different Ce-S nearest-neighbors and the cationic vacancy concentration gradually decrease, as described in the XRD refinement results. The exothermic peaks located at 580.9 and 619.7 °C in Fig. 9 (a) and (b), respectively, correspond to the further oxidation of γ-[Li]-Ce2S3 and the oxysulfieds. To further reveal the thermal stability of γ-[Li]-Ce2S3 with varying Li+ addition content, the nLi/Ce=0.05 and nLi/Ce=0.20
component samples were calcined at different temperature in air. Fig. 10 shows the XRD patterns of nLi/Ce=0.05 and nLi/Ce=0.20 samples calcined at 450 °C for 30 min. The color coordinates and photos of of nLi/Ce=0.05 and nLi/Ce=0.20 samples calcined at different temperatures are listed in Table 4. It can be clearly seen that the phase composition of the nLi/Ce=0.05 component sample shows an obvious change after heating at 450 °C. The XRD patterns identified the formation of CeS2 due to the oxidation of Ce2S3. In contrast, for the nLi/Ce=0.20 component sample, the phase composition remains only γ-Ce2S3 after heating at 450 °C, suggesting better thermal stability due to a shortened Ce-S bond length and reduced cationic vacancy concentration. Meanwhile, it can be seen from Table 3 that for the sample of nLi/Ce=0.20, it still maintains an excellent red color (L*=25.67, a*=30.45, b*=12.36) when it is heated to 450 °C, while for the sample of nLi/Ce=0.05, the color coordinates (L*=20.57, a*=17.89, b*=12.53) decreases significantly when it is heated to 450 °C. Obviously, the thermal stable temperature of the nLi/Ce=0.20 component sample can reach 450 °C through adjustment of the structural parameters, while that of pure γ-Ce2S3 is only 350 °C [5]. Thus, the γ-[Li]-Ce2S3 pigment with nLi/Ce=0.20 can be applied at higher temperatures and in more application fields.
4. Conclusion A series of Li-substituted γ-Ce2S3 powders were prepared via a gas-solid reaction method. The results revealed that the substitution of Li+ for Ce3+ changed the Ce-S bond length and the γ-[Li]-Ce2S3 lattice parameters with increasing nLi/Ce. In addition, these
values displayed no change beyond nLi/Ce=0.20, indicating that the vacancies in the lattice were fully filled and that Li+ no longer entered the lattice. The color shade of the as-prepared γ-[Li]-Ce2S3 pigments exhibited a dark red hue as a whole due to a small amount of brown CeS2-x impurity existing in all the samples. Furthermore, the stability of the pigments can be improved by adjusting the structural parameters. Especially, the thermal stability temperature of γ-[Li]-Ce2S3 with nLi/Ce =0.20 can reach 450°C, which is promising for application at high temperature and in more fields. Acknowledgements This work is supported by the Natural Science Foundation of Jiangxi Province (No. 20161BAB206132, No.20171ACB20022), Science and Technology Research Project of Jiangxi Education Department (No.GJJ180715) and Jingdezhen City Science and Technology project (2017GYZD019-012).
References [1] S.W. Kim, T. Hasegawa, M. Watanabe, et al., Environmentally friendly Rb3V5O14 fluorescent red pigment, Dyes Pigments. 136 (2017) 219-223. [2] T. Masui, T. Honda, Wendusu, et al., Novel and environmentally friendly (Bi,Ca,Zn)VO4 yellow pigments, Dyes Pigments. 99 (2013) 636-641. [3] W.X. Mao, W. Zhang, Z.X. Chi, et al., Core–shell structured Ce2S3@ZnO and its potential as a pigment, J. Mater. Chem. A. 3 (2015) 2176-2180 [4] L. Yuan, A. Han, M. Ye, et al., Synthesis and characterization of environmentally benign inorganic pigments with high NIR reflectance: Lanthanum-doped BiFeO3, Dyes Pigments.148
(2018) 137-146 [5] S. Zhang, M. Ye, S. Chen, et al., Synthesis and characterization of mica/γ-Ce2-xYxS3 composite red pigments with UV absorption and high NIR reflectance, Ceram. Int. 42 (2016) 16023-16030. [6] H. F. Liu, W. B. Dai, H. Wang, et al., Study on the preparation of the CdSxS1−x@ZrSiO4 red ceramic pigments and its properties, J. Sol-Gel. Sci. Technol. 75 (2015) 198-205. [7] B. E. Yekta, M. Tamizifar, N. Rahimi, Synthesis of a zircon-cadmium sulfo selenide pigment by a sol-gel technique, J. Ceram. Soc. Jpn. 115 (2007) 757-760. [8] S. Hirai, K. Shimakage, Y. Saitou, et al., Synthesis and Sintering of Cerium(III) Sulfide Powders, J. Am. Ceram. Soc. 81 (1998) 145-151 [9] M. Jansen, H. P. Letschert, Inorganic yellow-red pigments without toxic metals, Nature.404 (2000) 980. [10] S. Roméro, A. Mosset, J. C. Trombe, et al., Low-temperature process of the cubic lanthanide sesquisulfides: remarkable stabilization of the γ-Ce2S3 phase, J. Mater. Chem. 7 (1997) 1541-1547. [11] G. Chen, Z. Zhu, H Liu, et al. Preparation of SiO2 coated Ce2S3 red pigment with improved thermal stability, J. Rare. Earth. 31 (2013) 891-896. [12] S. Yu, D. Wang, Y. Liu, et al., Preparations and characterizations of γ-Ce2S3@SiO2 pigments from precoated CeO2 with improved thermal and acid stabilities, RSC. Adv. 4 (2014) 23653-23657. [13] S. G. Liu, Y. M. Li, Z. M. Wang, et al., Enhanced high temperature oxidization resistance of silica coated γ-Ce2S3 red pigments, Appl. Surf. Sci. 387 (2016) 1147-1153. [14] Y. M. Li, S. G. Liu, F. S. Song, et al., Preparation and thermal stability of silica layer
multicoated γ-Ce2S3 red pigment microparticles, Surf. Coat. Tech. 345 (2018) 70-75. [15] F. Marrot, A. Mosset, J.C. Trombe, et al., The stabilization of γ-Ce2S3 at low temperature by heavy rare earths, J. Alloy. Comp. 259 (1997) 145-152. [16] S. Roméro, A. Mosset, P. Macaudière, et al., Effect of some dopant elements on the low temperature formation of γ-Ce2S3, J. Alloy. Comp. 302 (2000) 118-127. [17] C. Witz, D. Huguenin, J. Lafait, et al., Comparative optical studies of Ce2S3 and Gd2S3 compounds, J. Appl. Phys. 79 (1996) 2038-2042. [18] M. A. Perrin, E. Wimmer., Color of pure and alkali-doped cerium sulfide: A local-density-functional study. Phys. Rev. B. 54 (1996) 2428–2435 [19] R. Mauricot, P. Gressier, M. Evain, et al., Comparative study of some rare earth sulfides: doped γ-[A]M2S3 (M = La, Ce and Nd, A = Na, K and Ca) and undoped γ-M2S3 (M = La, Ce and Nd), J. Alloy. Compd. 223 (1995) 130-138. [20] T. Chopin, H. Guichon, O. Touret, Rare earth sesquisulfide compositions comprising alkali/alkaline earth metal values, US Pat No 5348581,1994. [21] I. G. Vasilyeva, B. M. Ayupov, A. A. Vlasov, et al., Color and chemical heterogeneities of γ-[Na]-Ce2S3 solid solutions, J. Alloy. Comp. 268 (1998) 72-77. [22] J. C. Trombe, M. Verelst, Stabilization of the γ-Ce2S3 Phase, J. Alloy. Comp. 323 (2001) 66-69. [23] S. Romero, A. Mosset, J. C. Trombe, et al., Study of some ternary and quaternary systems based on γ-Ce2S3 using oxalate complexes: stabilization and coloration, J. Alloy. Comp. 269 (1998) 98-106. [24] L. Liu, A. Han, M. Ye, et al., Synthesis and characterization of Al3+ doped LaFeO3 compounds: A novel inorganic pigments with high near-infrared reflectance, Sol. Energ. Mat. Sol. C. 132 (2015) 377-384. [25] R. D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in
halides and chalcogenides, Acta. Cryst. 32(1976):751-767. [26] Y. Shi, M. Zhong, Z. Zhang, et al., Crystal structure and near-infrared reflective properties of Fe3+ doped AlPO4 pigments, Ceram. Int. 43(2017) 5979-5983. [27] O. Ryohei, S. Yusuke, A. Fumiya, et al., Synthesis and characterization of SrY2-xCexO4 as environmentally friendly reddish-brown pigments, Rsc. Adv. 87 (2017):55081-55087 [28] K. S. Ranjith, A. Senthamizhan, B. Balusamy, et al., Nanograined surface shell wall controlled ZnO–ZnS core–shell nanofibers and their shell wall thickness dependent visible photocatalytic properties, Catal. Sci. Technol. 7 (2017) 1167-1180. [29] T. Eriksson, A. M. Andersson, A.G. Bishop, et al., Surface Analysis of LiMn2O4 Electrodes in Carbonate-Based Electrolytes, J. Electrochem. Soc. 149 (2002) A69-A78. [30] D. S. Knight, W. B. White, Raman spectroscopic study of the rare earth sesquisulfides, Spectrochim. Acta. A. 46(1990) 381-387. [31] P. L. Provenzano, S. I. Boldish, W. B. White, Vibrational spectra of ternary sulfides with the Th3P4 structure, Mater. Res. Bull., 12 (1977) 939-946. [32] S. Jose, D. Joshy, S. B, et al., Recent advances in infrared reflective inorganic pigments, Sol. Energ. Mat. Sol. C. 194 (2019) 7-27 [33] M. Leskelä, Thermal stability of Ce2O2S. DOES Ce2O2SO4 exist?[J]. Thermochim. Acta. 92 (1985):739-742.
Fig. 1 The schematic diagram of the experimental process Fig. 2 XRD patterns for γ-[Li]-Ce2S3 sulfurized at 900 °C for 4h Fig. 3 Observed and calculated X-ray diffraction patterns obtained by Rietveld refinement for γ-[Li]-Ce2S3 with nLi/Ce= 0.05(a) and nLi/Ce= 0.20(b) Fig. 4 The plot of the cell volume as a function of nLi/Ce
Fig. 5 XPS spectra for γ-[Li]-Ce2S3 with nLi/Ce=0.20
(a)Survey XPS spectra of
γ-[Li]-Ce2S3 with nLi/Ce=0.20; (b) Ce3d, (c) S 2p and (d) Li1s XPS of γ-[Li]-Ce2S3 with nLi/Ce=0.20 Fig. 6 Diffuse reflectance spectrum of γ-[Li]-Ce2S3 Fig. 7 Kubelka-Munk transformed reflectance spectrum of γ-[Li]-Ce2S3 with varying nLi/Ce Fig. 8 Raman spectrum of γ-[Li]-Ce2S3 Fig. 9 TG-DSC curves for γ-[Li]-Ce2S3 pigments (a): nLi/Ce=0.05; (b): nLi/Ce=0.20 Fig. 10 XRD patterns for γ-[Li]-Ce2S3 pigments after calcination at 450 °C for 30 min Table 1 The cell parameters for γ-[Li]-Ce2S3 with varying nLi/Ce Table 2 Selected bond lengths for γ-[Li]-Ce2S3 with varying nLi/Ce Table 3 Color coordinates and band gaps for different nLi/Ce samples Table 4 Color coordinates and photos at different calcination temperature for nLi/Ce= 0.05 and nLi/Ce= 0.20
Fig. 1 The schematic diagram of the experimental process
Fig. 2 XRD patterns for γ-[Li]-Ce2S3 sulfurized at 900 °C for 4h
Fig. 3 Observed and calculated X-ray diffraction patterns obtained by Rietveld refinement for γ-[Li]-Ce2S3 with nLi/Ce= 0.05(a) and nLi/Ce= 0.20(b)
Fig. 4 The plot of the cell volume as a function of nLi/Ce
Fig. 5 XPS spectra for γ-[Li]-Ce2S3 with nLi/Ce=0.20 (a) Survey XPS spectra for γ-[Li]-Ce2S3 with nLi/Ce=0.20; (b) Ce3d, (c) S 2p and (d) Li1s XPS of γ-[Li]-Ce2S3 with nLi/Ce=0.20
Fig. 6 Diffuse reflectance spectrum of γ-[Li]-Ce2S3
Fig. 7 Kubelka-Munk transformed reflectance spectrum of γ-[Li]-Ce2S3 for varying nLi/Ce
Fig. 8 Raman spectrum of γ-[Li]-Ce2S3
Fig. 9 TG-DSC curves for γ-[Li]-Ce2S3 pigments (a): nLi/Ce=0.05; (b): nLi/Ce=0.20
Fig. 10 XRD patterns for the γ-[Li]-Ce2S3 pigments after calcination at 450 °C for 30 min
Table 1 The cell parameters for γ-[Li]-Ce2S3 with varying nLi/Ce Sample
a (Å)
Vcell (Å3)
Rwp%
nLi/Ce=0.05
a=8.6268(4)
642.0248(9)
7.63
nLi/Ce=0.10
a=8.6209(3)
640.7092(7)
6.25
nLi/Ce =0.15
a=8.6153(4)
639.4500(8)
6.19
nLi/Ce =0.20
a=8.6107(4)
638.4262(8)
5.91
nLi/Ce =0.25
a=8.6106(4)
638.4060(8)
5.77
nLi/Ce =0.30
a=8.6108(4)
638.4305(10)
5.96
Table 2 Selected bond lengths for γ-[Li]-Ce2S3 with varying nLi/Ce Sample
Ce1-S1(×4) (Å)
Ce1-S1(×4) (Å)
nLi/Ce=0.05
2.8926(2)
3.0838(2)
nLi/Ce=0.10
2.8907(2)
3.0817(2)
nLi/Ce =0.15
2.8888(2)
3.0797(2)
nLi/Ce =0.20
2.8872(2)
3.0780(2)
nLi/Ce =0.25
2.8872(2)
3.0781(2)
nLi/Ce =0.30
2.8872(2)
3.0780(2)
Table 3 Color coordinates and band gaps for different nLi/Ce samples Sample
L*
a*
b*
photo
Eg/eV
nLi/Ce=0.05
25.95
29.44
12.22
1.95
nLi/Ce =0.10
26.99
31.14
13.41
1.97
nLi/Ce =0.15
26.84
31.99
14.65
1.98
nLi/Ce =0.20
27.25
32.83
13.23
2.00
nLi/Ce =0.25
26.86
31.25
13.54
2.00
nLi/Ce =0.30
24.78
30.45
12.98
2.00
Table 4 Color coordinates and photos at different calcination temperature for nLi/Ce= 0.05 and nLi/Ce= 0.20 Sample
L*
a*
b*
350°C (nLi/Ce=0.05)
25.58
29.23
13.27
400°C (nLi/Ce=0.05)
25.16
28.89
14.25
450°C (nLi/Ce=0.05)
20.57
17.89
12.53
350°C (nLi/Ce=0.20)
27.23
32.56
14.01
400°C (nLi/Ce=0.20)
28.16
31.55
14.03
450°C (nLi/Ce=0.20)
25.67
30.45
12.36
460°C (nLi/Ce=0.20)
24.25
16.59
12.25
photo
This manuscript reports a facile and effective method to improve the thermal stability of γ-Ce2S3 pigments. By introducing Li+ into the γ-Ce2S3 framework, the concentration of cationic vacancy defects and the Ce-S bond length can be regulated, thus adjusting the properties of γ-Ce2S3 pigments. The thermal stability temperature of γ-[Li]-Ce2S3 was raised to 450 °C when nLi/Ce was equal to 0.20.
Highlights
(1) The Li+ doped γ-Ce2S3 red pigments were prepared via gas-solid reaction.
(2) The structural parameters were calculated by Rietveld structural refinement with TOPAS program.
(3) The band gap and structure parameters of γ-[Li]-Ce2S3 show a similar trend with the doping amount.
(4) The thermal stability of the pigments can be improved by adjusting the structural parameters.
Fusheng Song: Conceived and designed the study. Performed the experiments. Writing- Original draft preparation. Yueming Li: Conceived and designed the study. Reviewed and edited the manuscript. Yongzhi Yu: Reviewed and edited the manuscript. Zongyang Shen: Provided the characterization. Zhumei Wang: Provided the characterization. Xin Li: Performed the experiments. All authors read and approved the manuscript.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0022-4596(19)30615-2
DOI:
https://doi.org/10.1016/j.jssc.2019.121110
Reference:
YJSSC 121110
To appear in:
Journal of Solid State Chemistry
Received Date: 30 September 2019 Revised Date:
28 November 2019
Accepted Date: 2 December 2019
Please cite this article as: F. Song, Y. Li, Y. Yu, Z. Shen, Z. Wang, X. Li, Engineering the crystal structure of γ-[Li]-Ce2S3 red pigments for enhanced thermal stability, Journal of Solid State Chemistry (2020), doi: https://doi.org/10.1016/j.jssc.2019.121110. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Engineering the crystal structure of γ-[Li]-Ce2S3 red pigments for enhanced thermal stability Fusheng Song a,b,c, Yueming Li a,b,c*, Yongzhi Yu b, Zongyang Shen a,b,c, Zhumei Wang a,b,c, Xin Li a,b,c a
b
c
School of Materials Science and Engineering, Jingdezhen Ceramic Institute; Jingdezhen 333403, China
National Engineering Research Center for Domestic and Building Ceramics; Jingdezhen 333403, China
Key Laboratory of Functional Ceramic Materials of China Light Industry, Jingdezhen 333403, China
Abstract: The search for high thermal stability, environmentally-friendly, and low-cost red pigments for glazes remains a significant challenge in ceramic research. Here, we report a simple gas-solid reaction method to synthesis red inorganic pigments, namely γ-[Li]-Ce2S3. The effects of different molar ratio of lithium to cerium nLi/Ce (nLi/Ce =0.05, 0.10, 0.15, 0.20, 0.25, 0.30) on the phase composition, crystal structure, chromatic properties and thermal stability of pigments were systematically studied. The XRD and XPS characterization combined with Rietveled structural refinement showed that Li+ entered into the γ-Ce2S3 lattice structure to form γ-[Li]-Ce2S3 solid solutions. The two different nearest-neighbors Ce-S bond lengths in the γ-[Li]-Ce2S3 and the lattice parameters decreased with the content of nLi/Ce, while these values displayed a constant beyond nLi/Ce=0.20, indicating that the vacancies in the lattice were fully filled and Li+ no longer entered the lattice. The substitution of Li+ for Ce3+ and the elimination of the cationic vacancies in γ-[Li]-Ce2S3 led to an increase in the band gap from 1.95 to 2.00 eV, corresponding to a slight change in the red hue. Furthermore, from Raman spectra, a trace of cerium polysulfide as an impurity phase was found in the pigments, *Corresponding author. E-mail address: [email protected]
which was responsible for the observed dark red color of as-prepared γ-[Li]-Ce2S3. In addition, the thermal stability was evaluated by TG-DSC and the thermal stable temperature of γ-[Li]-Ce2S3 can be raised to 450°C when nLi/Ce was equal to 0.20, showing that the stability of the pigments can be improved by adjusting the structural parameters. Keywords: γ-[Li]-Ce2S3; red pigments; bond length; thermal stability;
1. Introduction Inorganic pigments have been widely used in various fields covering ceramic inks, paints and others because of their strong tinting power, high weather resistance and thermal stability [1-5]. Among numerous inorganic pigments, the red pigment has been especially favored over time owing to its noble and luxuriant characteristics. Although there have been numerous reports of red pigments, few can show a pure and bright red color. During the past several decades, CdSxSe1-x red pigments have been widely utilized in the decoration industry due to its bright red color [6, 7]. Unfortunately, traditional cadmium red pigments contain highly toxic elements (Cd and Se), which are dangerous for human health and the environment. Therefore, concerns over safety, availability and cost have motivated researchers to develop new red pigments based on environmentally-friendly and cost-effective materials [8, 9]. Recently, γ-Ce2S3 has attracted tremendous attention owing to its bright red color and non-toxicity. This emerging pigment is expected to replace highly toxic cadmium red pigments [3, 5, 10]. However, γ-Ce2S3 is prone to oxidation and discoloration at more than 350 °C, which greatly limits its application, especially in high-temperature
fields such as ceramics [11]. To improve its thermal stability, there have, recently, been extensive efforts to introduce a layer of a colorless transparent material (such as SiO2) coating onto γ-Ce2S3, which leads to a stable temperature of 550 °C has been identified [12-14]. However, due to the difference in refractive index, composition and structure between the coating material and pigment, the coating material weakens the color of the pigment. Theoretically, three phases of cerium sesquisulfides exist, including α, β and γ structures [15, 16]. Among these allotropes, γ-Ce2S3 is a high-temperature phase and isomorphic to Ce3S4, which is analogous to the cubic structure of the Th3P4-type. In fact, the structure of γ-Ce2S3 can be regarded as being formed by a partial removal of cerium ions of Ce3S4 [17-19]. Therefore, the widely claimed “γ-Ce2S3” in the literature is actually Ce2.67V0.33S4 (here, we use “γ-Ce2S3” to describe this phase to maintain agreement with general usage), where cationic vacancies are randomly distributed on the site of the cerium atom in the perfect lattice [10, 17-19]. It is found that there are two different Ce-S bonds in the Ce3S4 structure, and the two different nearest-neighbor Ce-S bond lengths are 2.896 and 3.087 Å, respectively [18]. However, the bond lengths for these two kinds of Ce-S bonds in the γ-Ce2S3 structure change to 2.901 and 3.092 Å, respectively [19]. It is not difficult to find that the existence of cationic vacancies leads to a lengthening of the Ce-S bond length in the structure of γ-Ce2S3. It is well known that the vacancy defect concentration and bond length are both closely related to the stability of materials. On the other hand, doping, which involves the intentional incorporation of suitable ions into host lattices, is considered to be an effective method
to regulate the properties of materials. Taking Na+ doped γ-Ce2S3 for example, it has been identified that the vacancies in the γ-Ce2S3 lattice can be partially or wholly filled with Na+, leading to changes in chromatic properties, energy level structure and thermal stability [19-21]. However, for several decades, great efforts have been devoted to investigating the impact of the synthesis temperature on dopant behavior because γ-Ce2S3 is a high-temperature phase, which makes it difficult to synthesize under normal conditions [16, 22-23]. On the contrary, few works have studied the ion doping-induced change in Ce-S bond lengths and the stability of γ-Ce2S3. Herein, this work focused on the control of the substitution of Li+ for Ce3+ in γ-Ce2S3 with the aim of intrinsically enhancing its thermal stability. The effects of Li-substitution on the composition, crystal structure, chromatic properties and thermal stability of the pigments were systematically studied. The obtained γ-[Li]-Ce2S3 exhibited a high thermal stability, and the thermal stable temperature was raised to 450°C in air when the molar ratio of lithium to cerium was equal to 0.20. This excellent performance exceeds that of commercially available products, confirming promising potential for application in the area of ceramics. 2. Experimental 2.1 Synthesis of Li-substituted γ-Ce2S3 All of the reagents were used without further treatment. A series of Li-substituted γ-Ce2S3 powders were prepared via a gas-solid reaction method. The raw materials of lithium carbonate (Li2CO3), cerium oxide (CeO2) and carbon disulfide (CS2), used in the experiments, were all of analytical grade and purchased from Shanghai Sinopharm Chemical reagent Co., Ltd., China. First, Li2CO3 and CeO2 were mixed in an
appropriate molar ratio of lithium to cerium of 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30. The raw precursor mixture was ground and then placed in a pottery boat crucible. Subsequently, the crucible was transferred to a tubular atmosphere furnace. Prior to heating, the furnace was evacuated to -0.1 MPa, and then, argon gas was injected several times to remove the air. The entire following process for both heating and cooling was protected by an inert argon atmosphere with a flow rate of 200 ml/min. Afterwards, the mixture was fired to 900 °C for 4 h with a ramp rate of 5 °C/min in an airstream of CS2/Ar, which was introduced into a quartz tube by bubbling liquid CS2 with flowing argon as the mixture was heated to 200 °C. Finally, the samples were cooled inside a furnace to room temperature in an Ar atmosphere to avoid oxidation. The schematic diagram of the reaction system is shown in Fig. 1. 2.2 Characterization The XRD data for the samples was collected by a D8 Advance X-ray diffractometer (XRD) (Bruker Corporation, Germany), which was operated with voltage and current settings of 40 kV and 40 mA, respectively, using Cu Kα radiation (λ = 1.5418 Å), with a 2θ range of 20~80°, step size of 0.02°and acquisition time per step of 0.2 s. Phase composition was analyzed from these data. For the calculations of the cell parameters and bond length, Rietveld structure refinement was performed using the Total Pattern Solution (TOPAS) program. X-ray photoemission spectroscopy (XPS) was measured using a Thermo escalab 250Xi (Thermo Electron Corporation, USA) to explore the elements on the surface and determine the valence state of the cerium ion in the γ-[Li]-Ce2S3. The Raman spectrum in the range of 100~500 cm-1 was investigated by
an inVia (Renishaw, England) using a 780 nm laser as the excitation source. The diffuse reflectance from the samples was measured with a UV-Vis spectrophotometer (Lambda-850, PerkinElmer Corporation, USA) using BaSO4 as a reference. The optical band gap Eg can be obtained from the Kubelka-Munk (K-M) function. The formula F(R) is shown as follows:
F (R ) =
(1−R )2 2R
(1)
F(R) is the Kubelka-Munk function, where R represents the reflectance of the sample at the corresponding wavelength. According to Eq. (1), F(R) can be calculated by substituting for the R value. Then, the curve of [F(R)hν]2 vs hν (h- Planck constant, ν-the frequency of light) can be drawn. By extrapolating the linear part to the intersection point of the abscissa, the band gap value Eg of the sample can be obtained [24]. The colorimetric values for the samples were measured by a color analyzer (ACM402, Yante, China) using the Commission International de l’Eclairage (CIE) 1976 L*a*b*colorimetric method, where L* represents lightness (from 0 to 100, 0 for black and 100 for white), a* denotes the red value (from -a* (green) to +a*(red)) and b* denotes the yellowness (-b* for blue and + b* for yellow). The thermal stability of the pigment samples was evaluated by simultaneous thermogravimetric-differential scanning calorimetry(TG-DSC, STA 449C, NETZSCH, Germany) from room temperature to 1000 °C at a rate of 10 °C/min in air atmosphere. 3. Results and discussion 3.1 X-ray diffraction analysis Fig. 2 shows the XRD patterns for γ-[Li]-Ce2S3 pigments with the nLi/Ce ranging
from 0.05 to 0.30 sulfurized at 900 °C for 4 h. As seen from Fig. 2, all the observed diffraction peaks are in accordance with the standard cubic structure of γ-Ce2S3 (PDF 01-089-2260), indicating the formation of a single phase for all samples with varying Li-substitution amounts. In addition, no other Li-containing phases are found from the XRD patterns, which clearly demonstrates that Li+ enters the γ-Ce2S3 lattice to form solid solution. To further explore the effect of Li+ substitution on the structure of γ-[Li]-Ce2S3, the cell parameters and bond length of Ce-S in the γ-[Li]-Ce2S3 lattice are calculated by Rietveld structural refinement with the TOPAS program. Fig. 3 shows the Rietveld refinement results for X-ray diffraction pattern for the γ-[Li]-Ce2S3 sample with nLi/Ce= 0.05 and nLi/Ce= 0.20. The cell parameters and bond length of Ce-S for each composition are listed in Table 1 and Table 2. The plot of the cell volume as a function of nLi/Ce is shown in Fig.4. It can be seen that the cell parameters and the two different nearest bond lengths of Ce-S1 and Ce-S2 in the lattice gradually decrease with increasing nLi/Ce molar ratio from 0.05 to 0.20. This change is attributed to the formation of γ-[Li]-Ce2S3 solid solution and the smaller ionic radius of Li+ (0.92 Å, CN = 8) compared to that of Ce3+ (1.14 Å, CN=8) [25, 26]. However, when the molar ratio of nLi/Ce further increases to 0.30, these structural parameters display no change, indicating that Li+ no longer enters the lattice beyond nLi/Ce= 0.20. Meanwhile, it can be seen from Fig. 4 that the cell volume also shows no change when the nLi/Ce above 0.20. This result is mainly ascribed to the fact that the cationic vacancies in the γ-[Li]-Ce2S3 lattice are completely filled up when nLi/Ce= 0.20[12, 15]. Therefore, the above observations imply that the excess
lithium element is likely to exist in the form of small amounts of other compounds, such as LiCeS2, in addition to forming the γ-[Li]-Ce2S3 solid solution, when the nLi/Ce molar ratio exceeds 0.20. Mauricot et al. reported that a NaCeS2 impurity phase appeared for Na-doped γ-Ce2S3 when the nNa/Ce molar ratio was larger than 0.20. However, in our case, further Li+ addition into the system has no obvious change on the phase composition according to the XRD results (Fig. 2), possibly because a LiCeS2 phase is indeed formed when the nLi/Ce molar ratio exceeds 0.20, while its content is too small to be detected by XRD [13]. 3.2 XPS analysis The chemical composition and structure of γ-[Li]-Ce2S3 was further investigated by XPS measurements. Fig. 5(a) shows the survey spectra for γ-[Li]-Ce2S3 with nLi/Ce=0.20. It can be found that the surface of the γ-[Li]-Ce2S3 consists of Ce, S, C, O and Li. The C1s peak located at approximately 284.8 eV is typically attributed to the calibration peak of the instrument itself. The O1s peak in the survey spectra may be mainly ascribed to hydroxyl oxygen and dissociated oxygen adsorbed onto the surface of the sample due to the strong oxygen affinity of Ce. High-resolution spectra for Ce3d, S2p and Li1s are shown in Fig. 5(b)-(d), respectively. The binding energy peaks located at 884.9 and 903.4 eV are attributed to Ce3d5/2 and Ce3d3/2 for Ce3+ in γ-[Li]-Ce2S3, respectively [27]. Additionally, the intense characteristic peaks observed at 160.2 eV and 161.4eV in the XPS spectra for S2p are assigned to S2p2/3 and S2p1/3, respectively [28]. The corresponding binding energy of 55.3 eV in Figure 2(d) is the characteristic peak of Li1s [29]. 3.3 UV-Vis diffuse reflectance spectra and analysis of the chromatic properties
The UV-Vis diffuse reflectance spectra for γ-[Li]-Ce2S3 for varying nLi/Ce are presented in Fig. 6. It is clearly seen that the reflectance spectra for different components display a similar shape with an almost identical slope. Interestingly, the reflectance gradually decreases and the reflection edge shifted slightly towards the shortwave direction with increasing Li+ content. A strong absorption was observed in the region 500~560 nm, which is assigned to the electronic transition Ce4f→Ce5d, thus leading to a visible red color as a complementary. The calculated band gap energies are shown in Fig. 7 and listed in Table 3 along with the corresponding CIE L*a*b* color coordinates. Accordingly, the band gap energies (Eg) slightly increase from 1.95 to 2.00 eV when the nLi/Ce increases from 0.05 to 0.20. The increase in Eg can be explained by the unique effect of ion doping on the lattice structure of the γ-Ce2S3 samples. As low-valent Li+ is introduced into the γ-Ce2S3 with cationic vacancies randomly distributed in the lattice to form γ-[Li]-Ce2S3 solid solution, Li+ will fill vacancies accompanied with the replacement of Ce3+ to maintain electrical neutrality. As a result, the substitution of Li+ for Ce3+ and the elimination of the cationic vacancies in γ-[Li]-Ce2S3 changes the crystal field environment, thus causing an increase in Eg when nLi/Ce is in the range 0.05 to 0.20. The corresponding change in the red color can be observed in Table. 3. It is worth noting that the effect of the addition amount on band gaps above a nLi/Ce of 0.20 is negligible, which is mainly attributed to the fact that all of the cationic vacancies in the γ-[Li]-Ce2S3 lattice are filled up at nLi/Ce = 0.20. The chromatic properties of the γ-[Li]-Ce2S3 pigments are evaluated from the CIE1976 L*a*b* coordinate values. As shown in Table 3, the value of L* increases
from 25.95 to 27.25 and decreases to 24.78 with increasing nLi/Ce, corresponding to a pigment color change from dark to bright and then back again to dark. In addition, the values of a* and b* display a variation in a narrow range for further Li+ addition. The results for the L*a*b* coordinate are consistent with the color change of the γ-[Li]-Ce2S3 pigments. Interestingly, it can also be observed from Table 3 that the as-prepared γ-[Li]-Ce2S3 exhibits a dark red hue as a whole. According to Perrin and Vasilyeva [18,21], although the color of γ-[Li]-Ce2S3 pigments is mainly determined by the intrinsic nature of the Ce4f→Ce5d electron transitions, one should keep in mind the influence of external factors, such as impurities with even small quantities, showing an impact. Therefore, to explore whether small amounts of impurities exist, Raman spectroscopy was used to characterize the vibrational spectra of components with nLi/Ce=0.05, 0.20 and 0.30 under ambient conditions. As illustrated in Fig. 8, the Raman spectra for all the samples display six characteristic peaks at 113, 194, 241, 298, 401, and 413 cm-1. It can be noted that the Raman bands located at 113, 194, 241, and 298 cm-1 are four broadened peaks that are very similar to the peaks found for the rare-earth sesquisulfide γ-Ln2S3 with a Th3P4-type structure [30]. Because Li+, Ce3+ and vacancies occupy the same cation sites, there is a disordered structure in the γ-[Li]-Ce2S3 lattice, which leads to a broadening of the Raman bands [31]. Two narrow and sharp peaks located at 401 and 413 cm-1 are assigned to the characteristic frequency of CeS2-x [21]. Combined with XRD analysis, CeS2-x should be an impurity exists with a small concentration in all samples. Since the detected impurity CeS2-x is brown, it is likely to alter the color of the γ-[Li]-Ce2S3 pigments. As a result, the color shade of the
as-obtained pigments presents a dark red hue. 3.4 Thermal stability The thermal stability of the pigments can be evaluated by simultaneous thermogravimetric-differential scanning calorimetry (TG-DSC). If a negligible weight loss is observed, the pigment will be thermally stable [32]. Fig. 9 presents the TG-DSC curves for the γ-[Li]-Ce2S3 pigments with nLi/Ce=0.05 and nLi/Ce=0.20, respectively. A similar broad peak is observed in the TG curves shown in Fig. 9 (a) and (b), exhibiting a single mass gain and single mass loss process. The process of weight gain is mainly due to the formation of intermediate oxidation products such as oxysulfides and sulfates [33], and the process of the weight loss is attributed to sulfur overflow in the form of SO2 or other gases, with CeO2 being the final oxidation product. In addition, the weight change of the nLi/Ce=0.05 component sample can be neglected before 410 °C, with the temperature of the weight gain of the nLi/Ce=0.20 component begining to appear at 450 °C. The corresponding exothermic peaks appear at 459.3 and 503.8 °C respectively, as shown in the DSC curves in Fig. 9 (a) and (b), suggesting that the stability of the nLi/Ce=0.20 component sample is higher than that of the nLi/Ce=0.05 component sample. mainly because with the increasing Li+ addition content, the bond lengths of the two different Ce-S nearest-neighbors and the cationic vacancy concentration gradually decrease, as described in the XRD refinement results. The exothermic peaks located at 580.9 and 619.7 °C in Fig. 9 (a) and (b), respectively, correspond to the further oxidation of γ-[Li]-Ce2S3 and the oxysulfieds. To further reveal the thermal stability of γ-[Li]-Ce2S3 with varying Li+ addition content, the nLi/Ce=0.05 and nLi/Ce=0.20
component samples were calcined at different temperature in air. Fig. 10 shows the XRD patterns of nLi/Ce=0.05 and nLi/Ce=0.20 samples calcined at 450 °C for 30 min. The color coordinates and photos of of nLi/Ce=0.05 and nLi/Ce=0.20 samples calcined at different temperatures are listed in Table 4. It can be clearly seen that the phase composition of the nLi/Ce=0.05 component sample shows an obvious change after heating at 450 °C. The XRD patterns identified the formation of CeS2 due to the oxidation of Ce2S3. In contrast, for the nLi/Ce=0.20 component sample, the phase composition remains only γ-Ce2S3 after heating at 450 °C, suggesting better thermal stability due to a shortened Ce-S bond length and reduced cationic vacancy concentration. Meanwhile, it can be seen from Table 3 that for the sample of nLi/Ce=0.20, it still maintains an excellent red color (L*=25.67, a*=30.45, b*=12.36) when it is heated to 450 °C, while for the sample of nLi/Ce=0.05, the color coordinates (L*=20.57, a*=17.89, b*=12.53) decreases significantly when it is heated to 450 °C. Obviously, the thermal stable temperature of the nLi/Ce=0.20 component sample can reach 450 °C through adjustment of the structural parameters, while that of pure γ-Ce2S3 is only 350 °C [5]. Thus, the γ-[Li]-Ce2S3 pigment with nLi/Ce=0.20 can be applied at higher temperatures and in more application fields.
4. Conclusion A series of Li-substituted γ-Ce2S3 powders were prepared via a gas-solid reaction method. The results revealed that the substitution of Li+ for Ce3+ changed the Ce-S bond length and the γ-[Li]-Ce2S3 lattice parameters with increasing nLi/Ce. In addition, these
values displayed no change beyond nLi/Ce=0.20, indicating that the vacancies in the lattice were fully filled and that Li+ no longer entered the lattice. The color shade of the as-prepared γ-[Li]-Ce2S3 pigments exhibited a dark red hue as a whole due to a small amount of brown CeS2-x impurity existing in all the samples. Furthermore, the stability of the pigments can be improved by adjusting the structural parameters. Especially, the thermal stability temperature of γ-[Li]-Ce2S3 with nLi/Ce =0.20 can reach 450°C, which is promising for application at high temperature and in more fields. Acknowledgements This work is supported by the Natural Science Foundation of Jiangxi Province (No. 20161BAB206132, No.20171ACB20022), Science and Technology Research Project of Jiangxi Education Department (No.GJJ180715) and Jingdezhen City Science and Technology project (2017GYZD019-012).
References [1] S.W. Kim, T. Hasegawa, M. Watanabe, et al., Environmentally friendly Rb3V5O14 fluorescent red pigment, Dyes Pigments. 136 (2017) 219-223. [2] T. Masui, T. Honda, Wendusu, et al., Novel and environmentally friendly (Bi,Ca,Zn)VO4 yellow pigments, Dyes Pigments. 99 (2013) 636-641. [3] W.X. Mao, W. Zhang, Z.X. Chi, et al., Core–shell structured Ce2S3@ZnO and its potential as a pigment, J. Mater. Chem. A. 3 (2015) 2176-2180 [4] L. Yuan, A. Han, M. Ye, et al., Synthesis and characterization of environmentally benign inorganic pigments with high NIR reflectance: Lanthanum-doped BiFeO3, Dyes Pigments.148
(2018) 137-146 [5] S. Zhang, M. Ye, S. Chen, et al., Synthesis and characterization of mica/γ-Ce2-xYxS3 composite red pigments with UV absorption and high NIR reflectance, Ceram. Int. 42 (2016) 16023-16030. [6] H. F. Liu, W. B. Dai, H. Wang, et al., Study on the preparation of the CdSxS1−x@ZrSiO4 red ceramic pigments and its properties, J. Sol-Gel. Sci. Technol. 75 (2015) 198-205. [7] B. E. Yekta, M. Tamizifar, N. Rahimi, Synthesis of a zircon-cadmium sulfo selenide pigment by a sol-gel technique, J. Ceram. Soc. Jpn. 115 (2007) 757-760. [8] S. Hirai, K. Shimakage, Y. Saitou, et al., Synthesis and Sintering of Cerium(III) Sulfide Powders, J. Am. Ceram. Soc. 81 (1998) 145-151 [9] M. Jansen, H. P. Letschert, Inorganic yellow-red pigments without toxic metals, Nature.404 (2000) 980. [10] S. Roméro, A. Mosset, J. C. Trombe, et al., Low-temperature process of the cubic lanthanide sesquisulfides: remarkable stabilization of the γ-Ce2S3 phase, J. Mater. Chem. 7 (1997) 1541-1547. [11] G. Chen, Z. Zhu, H Liu, et al. Preparation of SiO2 coated Ce2S3 red pigment with improved thermal stability, J. Rare. Earth. 31 (2013) 891-896. [12] S. Yu, D. Wang, Y. Liu, et al., Preparations and characterizations of γ-Ce2S3@SiO2 pigments from precoated CeO2 with improved thermal and acid stabilities, RSC. Adv. 4 (2014) 23653-23657. [13] S. G. Liu, Y. M. Li, Z. M. Wang, et al., Enhanced high temperature oxidization resistance of silica coated γ-Ce2S3 red pigments, Appl. Surf. Sci. 387 (2016) 1147-1153. [14] Y. M. Li, S. G. Liu, F. S. Song, et al., Preparation and thermal stability of silica layer
multicoated γ-Ce2S3 red pigment microparticles, Surf. Coat. Tech. 345 (2018) 70-75. [15] F. Marrot, A. Mosset, J.C. Trombe, et al., The stabilization of γ-Ce2S3 at low temperature by heavy rare earths, J. Alloy. Comp. 259 (1997) 145-152. [16] S. Roméro, A. Mosset, P. Macaudière, et al., Effect of some dopant elements on the low temperature formation of γ-Ce2S3, J. Alloy. Comp. 302 (2000) 118-127. [17] C. Witz, D. Huguenin, J. Lafait, et al., Comparative optical studies of Ce2S3 and Gd2S3 compounds, J. Appl. Phys. 79 (1996) 2038-2042. [18] M. A. Perrin, E. Wimmer., Color of pure and alkali-doped cerium sulfide: A local-density-functional study. Phys. Rev. B. 54 (1996) 2428–2435 [19] R. Mauricot, P. Gressier, M. Evain, et al., Comparative study of some rare earth sulfides: doped γ-[A]M2S3 (M = La, Ce and Nd, A = Na, K and Ca) and undoped γ-M2S3 (M = La, Ce and Nd), J. Alloy. Compd. 223 (1995) 130-138. [20] T. Chopin, H. Guichon, O. Touret, Rare earth sesquisulfide compositions comprising alkali/alkaline earth metal values, US Pat No 5348581,1994. [21] I. G. Vasilyeva, B. M. Ayupov, A. A. Vlasov, et al., Color and chemical heterogeneities of γ-[Na]-Ce2S3 solid solutions, J. Alloy. Comp. 268 (1998) 72-77. [22] J. C. Trombe, M. Verelst, Stabilization of the γ-Ce2S3 Phase, J. Alloy. Comp. 323 (2001) 66-69. [23] S. Romero, A. Mosset, J. C. Trombe, et al., Study of some ternary and quaternary systems based on γ-Ce2S3 using oxalate complexes: stabilization and coloration, J. Alloy. Comp. 269 (1998) 98-106. [24] L. Liu, A. Han, M. Ye, et al., Synthesis and characterization of Al3+ doped LaFeO3 compounds: A novel inorganic pigments with high near-infrared reflectance, Sol. Energ. Mat. Sol. C. 132 (2015) 377-384. [25] R. D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in
halides and chalcogenides, Acta. Cryst. 32(1976):751-767. [26] Y. Shi, M. Zhong, Z. Zhang, et al., Crystal structure and near-infrared reflective properties of Fe3+ doped AlPO4 pigments, Ceram. Int. 43(2017) 5979-5983. [27] O. Ryohei, S. Yusuke, A. Fumiya, et al., Synthesis and characterization of SrY2-xCexO4 as environmentally friendly reddish-brown pigments, Rsc. Adv. 87 (2017):55081-55087 [28] K. S. Ranjith, A. Senthamizhan, B. Balusamy, et al., Nanograined surface shell wall controlled ZnO–ZnS core–shell nanofibers and their shell wall thickness dependent visible photocatalytic properties, Catal. Sci. Technol. 7 (2017) 1167-1180. [29] T. Eriksson, A. M. Andersson, A.G. Bishop, et al., Surface Analysis of LiMn2O4 Electrodes in Carbonate-Based Electrolytes, J. Electrochem. Soc. 149 (2002) A69-A78. [30] D. S. Knight, W. B. White, Raman spectroscopic study of the rare earth sesquisulfides, Spectrochim. Acta. A. 46(1990) 381-387. [31] P. L. Provenzano, S. I. Boldish, W. B. White, Vibrational spectra of ternary sulfides with the Th3P4 structure, Mater. Res. Bull., 12 (1977) 939-946. [32] S. Jose, D. Joshy, S. B, et al., Recent advances in infrared reflective inorganic pigments, Sol. Energ. Mat. Sol. C. 194 (2019) 7-27 [33] M. Leskelä, Thermal stability of Ce2O2S. DOES Ce2O2SO4 exist?[J]. Thermochim. Acta. 92 (1985):739-742.
Fig. 1 The schematic diagram of the experimental process Fig. 2 XRD patterns for γ-[Li]-Ce2S3 sulfurized at 900 °C for 4h Fig. 3 Observed and calculated X-ray diffraction patterns obtained by Rietveld refinement for γ-[Li]-Ce2S3 with nLi/Ce= 0.05(a) and nLi/Ce= 0.20(b) Fig. 4 The plot of the cell volume as a function of nLi/Ce
Fig. 5 XPS spectra for γ-[Li]-Ce2S3 with nLi/Ce=0.20
(a)Survey XPS spectra of
γ-[Li]-Ce2S3 with nLi/Ce=0.20; (b) Ce3d, (c) S 2p and (d) Li1s XPS of γ-[Li]-Ce2S3 with nLi/Ce=0.20 Fig. 6 Diffuse reflectance spectrum of γ-[Li]-Ce2S3 Fig. 7 Kubelka-Munk transformed reflectance spectrum of γ-[Li]-Ce2S3 with varying nLi/Ce Fig. 8 Raman spectrum of γ-[Li]-Ce2S3 Fig. 9 TG-DSC curves for γ-[Li]-Ce2S3 pigments (a): nLi/Ce=0.05; (b): nLi/Ce=0.20 Fig. 10 XRD patterns for γ-[Li]-Ce2S3 pigments after calcination at 450 °C for 30 min Table 1 The cell parameters for γ-[Li]-Ce2S3 with varying nLi/Ce Table 2 Selected bond lengths for γ-[Li]-Ce2S3 with varying nLi/Ce Table 3 Color coordinates and band gaps for different nLi/Ce samples Table 4 Color coordinates and photos at different calcination temperature for nLi/Ce= 0.05 and nLi/Ce= 0.20
Fig. 1 The schematic diagram of the experimental process
Fig. 2 XRD patterns for γ-[Li]-Ce2S3 sulfurized at 900 °C for 4h
Fig. 3 Observed and calculated X-ray diffraction patterns obtained by Rietveld refinement for γ-[Li]-Ce2S3 with nLi/Ce= 0.05(a) and nLi/Ce= 0.20(b)
Fig. 4 The plot of the cell volume as a function of nLi/Ce
Fig. 5 XPS spectra for γ-[Li]-Ce2S3 with nLi/Ce=0.20 (a) Survey XPS spectra for γ-[Li]-Ce2S3 with nLi/Ce=0.20; (b) Ce3d, (c) S 2p and (d) Li1s XPS of γ-[Li]-Ce2S3 with nLi/Ce=0.20
Fig. 6 Diffuse reflectance spectrum of γ-[Li]-Ce2S3
Fig. 7 Kubelka-Munk transformed reflectance spectrum of γ-[Li]-Ce2S3 for varying nLi/Ce
Fig. 8 Raman spectrum of γ-[Li]-Ce2S3
Fig. 9 TG-DSC curves for γ-[Li]-Ce2S3 pigments (a): nLi/Ce=0.05; (b): nLi/Ce=0.20
Fig. 10 XRD patterns for the γ-[Li]-Ce2S3 pigments after calcination at 450 °C for 30 min
Table 1 The cell parameters for γ-[Li]-Ce2S3 with varying nLi/Ce Sample
a (Å)
Vcell (Å3)
Rwp%
nLi/Ce=0.05
a=8.6268(4)
642.0248(9)
7.63
nLi/Ce=0.10
a=8.6209(3)
640.7092(7)
6.25
nLi/Ce =0.15
a=8.6153(4)
639.4500(8)
6.19
nLi/Ce =0.20
a=8.6107(4)
638.4262(8)
5.91
nLi/Ce =0.25
a=8.6106(4)
638.4060(8)
5.77
nLi/Ce =0.30
a=8.6108(4)
638.4305(10)
5.96
Table 2 Selected bond lengths for γ-[Li]-Ce2S3 with varying nLi/Ce Sample
Ce1-S1(×4) (Å)
Ce1-S1(×4) (Å)
nLi/Ce=0.05
2.8926(2)
3.0838(2)
nLi/Ce=0.10
2.8907(2)
3.0817(2)
nLi/Ce =0.15
2.8888(2)
3.0797(2)
nLi/Ce =0.20
2.8872(2)
3.0780(2)
nLi/Ce =0.25
2.8872(2)
3.0781(2)
nLi/Ce =0.30
2.8872(2)
3.0780(2)
Table 3 Color coordinates and band gaps for different nLi/Ce samples Sample
L*
a*
b*
photo
Eg/eV
nLi/Ce=0.05
25.95
29.44
12.22
1.95
nLi/Ce =0.10
26.99
31.14
13.41
1.97
nLi/Ce =0.15
26.84
31.99
14.65
1.98
nLi/Ce =0.20
27.25
32.83
13.23
2.00
nLi/Ce =0.25
26.86
31.25
13.54
2.00
nLi/Ce =0.30
24.78
30.45
12.98
2.00
Table 4 Color coordinates and photos at different calcination temperature for nLi/Ce= 0.05 and nLi/Ce= 0.20 Sample
L*
a*
b*
350°C (nLi/Ce=0.05)
25.58
29.23
13.27
400°C (nLi/Ce=0.05)
25.16
28.89
14.25
450°C (nLi/Ce=0.05)
20.57
17.89
12.53
350°C (nLi/Ce=0.20)
27.23
32.56
14.01
400°C (nLi/Ce=0.20)
28.16
31.55
14.03
450°C (nLi/Ce=0.20)
25.67
30.45
12.36
460°C (nLi/Ce=0.20)
24.25
16.59
12.25
photo
This manuscript reports a facile and effective method to improve the thermal stability of γ-Ce2S3 pigments. By introducing Li+ into the γ-Ce2S3 framework, the concentration of cationic vacancy defects and the Ce-S bond length can be regulated, thus adjusting the properties of γ-Ce2S3 pigments. The thermal stability temperature of γ-[Li]-Ce2S3 was raised to 450 °C when nLi/Ce was equal to 0.20.
Highlights
(1) The Li+ doped γ-Ce2S3 red pigments were prepared via gas-solid reaction.
(2) The structural parameters were calculated by Rietveld structural refinement with TOPAS program.
(3) The band gap and structure parameters of γ-[Li]-Ce2S3 show a similar trend with the doping amount.
(4) The thermal stability of the pigments can be improved by adjusting the structural parameters.
Fusheng Song: Conceived and designed the study. Performed the experiments. Writing- Original draft preparation. Yueming Li: Conceived and designed the study. Reviewed and edited the manuscript. Yongzhi Yu: Reviewed and edited the manuscript. Zongyang Shen: Provided the characterization. Zhumei Wang: Provided the characterization. Xin Li: Performed the experiments. All authors read and approved the manuscript.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: