J. Cent. South Univ. Technol. (2008) 15: 748-752

DOI: 10.1007/s11771-008-0138-0

Synthesis and coloring properties of Cd(S_1-xSe_x) pigments by precipitate-hydrothermal method

SONG Xiao­lan(宋晓岚), PENG Lin(彭 林), DING Yi(丁 意), QIU Guan­zhou(邱冠周)

 (School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China)

Abstract: Cd(S_1-xSe_x) pigments (red to yellow) were synthesized by precipitate-hydrothermal method. The structure, morphology and hue of the powder were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDAX) and CIE chromaticity. The optimum synthesis conditions were obtained and reaction mechanism was further analyzed as well. The results show that molar ratio of S to Se, pH value and hydrothermal reaction conditions have great effects on the hues of the pigments. Pigments with vivid hues are obtained under the conditions that pH value is about 13.0, hydrothermal reaction condition is at 140 ℃ for 4 h or at 160 ℃ for 6 h. The reaction mechanism is that Se^2- of Cd(S_1-xSe_x) substitutes S^2- of CdS and then forms a continuous solid solution.

Key words: Cd(S_1-xSe_x) pigments; precipitate-hydrothermal reaction; reaction mechanism

1 Introduction

Red to yellow is particularly important color in the ceramic pigments field. The consumption of yellow pigments exceeds that of any other colored pigment^[1]. The three important yellow pigments are tin vanadium yellow (11-22-4 DCMA, dry color manufacturers association), praseodymium zircon (14-43-4 DCMA) and zirconium vanadium yellow (1-01-4 DCMA)^[2]. However, these pigments are sensitive to reducing conditions and are incompatible with chromium-containing pigments. The praseodymium zircon yellow pigment has some drawbacks in the bulk coloration of porcelain stoneware^[3] and this pigment is expensive because of high cost of Pr³⁺ and high temperature synthesis^[4]. Cd(S_1-xSe_x) is pigment with colorful hues. In the case of Cd(S_1-xSe_x), increasing S content gives red to yellow hues^[5-6]. Furthermore, they are the only pigments that can give bright red hues^[7-8]. Recently, the main synthesis techniques are solid phase sintering method^[9], co-precipitation^[10] and sol-precipitation method^[6]. All of the methods need the process of sintering as CdS is easily oxidized and volatilized when temperature is above 600 ℃^[7]. Thus these methods have effects on the coloring of Cd(S_1-xSe_x). Hydrothermal synthesis can obtain powder by precursor at a relatively low temperature^[11-12], therefore, it is particular importance to easily prepare volatile pigment.

In this work, Cd(S_1-xSe_x) pigments (red to yellow) were synthesized by precipitate-hydrothermal method with cadmium nitrate (Cd(NO₃)₂?4H₂O), selenium powder (Se) and sodium sulfide (Na₂S?9H₂O) as raw materials. The effects of molar ratio of S to Se, pH value and hydrothermal reaction conditions on the structure, morphology and hues of the Cd(S_1?xSe_x) pigments were studied by means of XRD, SEM, EDAX and CIE chromaticity. Furthermore, the optimum synthesis conditions were obtained and reaction mechanism was analyzed.

2 Experimental

Raw materials for the preparation of Cd(S_1-xSe_x) pigments were cadmium nitrate (Cd(NO₃)₂?4H₂O, AR), selenium powder (Se, 99.9%) and sodium sulfide (Na₂S?9H₂O, AR). Na₂S was dissolved in solution and pH was controlled by the volume of the solution. Then Se powders with the required molar ratio (molar ratio of S to Se=0.82/0.18, 0.85/0.15, 0.88/0.12, 0.91/0.09) were homogenized in solution and pH values were adjusted slightly by ammonia. pH value was controlled seriously and Cd(NO₃)₂ solution was added with stoichiometric ratio to take place precipitate reaction at room temperature. Sample was obtained after the deposition was washed with deionized water, filtered and dried. Then the sample was put into the autoclave with polytetrafluorethylene liner. Deionized water was used as hydrothermal reaction medium. Hydrothermal synthesis was carried out by strengthening reaction conditions gradually (i.e. increasing reaction temperature and time). Later, the powder was washed with deionized water and dried again.

Crystalline phase constitution and structure of samples were characterized by Dmax/35 X-ray diffractometer (Rigaku, Japan). The X-ray diffraction (XRD) used Pr filtered Cu K_α radiation and the patterns were collected in a 2θ range of 10?-90?, at a scanning rate of 0.075 (?)/s. Powder morphology was investigated by scanning electron microscope (SEM, JSM-6360LV, Japan) coupled with an energy dispersive X-ray spectrometric microanalyzer (EDAX, GENESIS 60S, USA). The hues of the samples were characterized by ADCI-60-C CIE chromaticity (Beijing) and evaluated according to the Commission International de l’Eclairage (CIE) through L*a*b* parameters. In this system, L* is lightness; a* represents red hues, from green (-) to red (+); b* represents yellow hues, from blue (-) to yellow (+).

3 Results and discussion

3.1 X-ray diffraction (XRD) analysis

The crystalline phase constitutions and structures of the samples obtained at molar ratio of S to Se of 0.88/0.12, different pH values and different hydrothermal reaction conditions were characterized by XRD. The results are shown in Fig.1 and Fig.2. XRD patterns show that whether with hydrothermal treatment or not, CdS crystalline phase with α-ZnS (wurtzite) structure is obtained, while Se compound phase is not detected both for samples obtained at pH values of 13.0 and 13.3. This indicates that Se has entered CdS lattice and formed Cd(S_0.88Se_0.12) solid solution. The difference is that at pH value of 13.0, only CdS crystalline phase is detected, however, at pH value of 13.3, not only CdS but also Cd(OH)₂ are detected. This indicates that pH value has great effects on the synthesis of Cd(S_1-xSe_x) pigments. As S and Se can substitute mutually and form solid solution, only CdS structure appears in the Cd(S_0.88Se_0.12). After hydrothermal treatment, diffraction peak becomes narrow and intensity becomes strong, which shows that crystallinity of the Cd(S_0.88Se_0.12) increases and structure becomes better.

Fig.1 XRD patterns for samples obtained at pH=13.0:(a) Without hydrothermal treatment; (b) Hydrothermal treatment at 160 ℃ for 6 h

Fig.2 XRD patterns for samples obtained at pH=13.3: (a) Without hydrothermal treatment; (b) Hydrothermal treatment at 180 ℃ for 8 h

3.2 Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDAX) analysis

Fig.3 shows the morphologies of samples obtained at molar ratio of S to Se of 0.88/0.12 and pH value of 13.0. It can be seen that, particle size of sample without hydrothermal treatment is big and inhomogeneous (Fig.3(a)). However, after hydrothermal treatment, samples are spherical particles and well distributed (Fig.3(b)). This effect is beneficial to enhancing the stability of pigments’ hues.

Fig.3 Scanning electron microscope (SEM) images of samples obtained at molar ratio of S to Se of 0.88/0.12 and pH=13.0: (a) Without hydrothermal treatment; (b) Hydrothermal treatment at 160 ℃ for 6 h

Fig.4 shows the EDAX pattern of sample in Fig.3(b), and Table 1 lists the chemical elements and their contents corresponding to Fig.4. It can be seen that, three elements of Cd, S and Se all appear and molar ratio is 1.0/0.79/0.21, near to 1.0/0.88/0.12. Combing with XRD analysis, it shows that Se^2- appears in product, which further indicates that Se^2- has entered CdS lattice by substituting S^2- of CdS and formed Cd(S_1-xSe_x) continuous solid solution. The ratio is slightly smaller than 1.0/0.88/0.12. The reason may be hydrolysis of S^2- and volatilization of H₂S in the synthesis process.

Fig.4 EDAX pattern of product in Fig.3(b)

Table 1 Chemical elements and their contents in sample corresponding to Fig.4

3.3 Effect of molar ratio of S to Se on hues of samples

Fig.5 shows the hues of the samples at different molar ratios of S to Se. It shows that at the same pH value, increasing S content leads to an increase in the yellow hues. This can be assigned to that CdS crystal is yellow and CdSe is black. When the concentration of S^2- is

higher, the formation of saturated solution of CdS is easier. Consequently, forming rate of crystal nucleus is quick and number is large. Then, solute deposits on them and particles grow bigger and bigger. Therefore, with increasing concentration of S^2-, sample obviously presents the hues of CdS, indicating that yellow hues(b*) become vivid increasingly.

Fig.5 Effect of molar ratio of S to Se on hues of sample

3.4 Effect of pH value on hues of samples

Fig.6 shows the hues of samples at different pH values. It shows that at the same molar ratio of S to Se, samples present high yellow hues (b*) at pH value of 13.0, and when pH value is too low or too high, yellow hues (b*) are lower. This can be assigned to hydrolysis of S^2- in the solution. The reactions can be described as follows:

S^2-+H₂OHS^-+OH^-                        (1)

HS^-+H₂OH₂S+OH^-                       (2)

Fig.6 Effect of pH value on hues of samples

When the concentration of OH^- is low, the reaction of hydrolysis is obvious. The result is that the concentration of S^2- is reduced and the deposition of CdS is also reduced. Thus yellow hues (b*) decrease at this situation. However, when the concentration of OH^- is too high, Cd²⁺ reacts with OH^-. This reaction can be described as follows:

Cd²⁺+2OH^-=Cd(OH)₂↓                      (3)

Furthermore, Cd²⁺ can form various ligands with OH^-. The coordination number can be from 1 to 4 and stability constants lg β_n are 4.17, 8.33, 9.02 and 8.62, respectively^[13]. Therefore, deposition of CdS is reduced and yellow hues (b*) are also decreased. Fig.1 and Fig.2 further confirm the interpretation. When pH value is 13.3, a lot of Cd(OH)₂ is observed; when pH value is 13.0, the main crystalline phase is Cd(S_1-xSe_x).

3.5 Effect of hydrothermal reaction on hues of samples

The effect of hydrothermal reaction on red hues (a*) and yellow hues (b*) of samples (obtained at pH=13.0) is shown in Table 2. It can be seen that when hydrothermal condition is at 140 ℃ for 4 h or at 160 ℃ for 6 h, red hues (a*) and yellow hues (b*) of the samples are high. This can be assigned to that at low temperature, hydrothermal reaction is unobvious, most of the samples are amorphous and these formed crystals are defective. At proper temperature, crystals grow well with small particle size (Fig.3(b)); furthermore, in the process of growth, crystals have better ability to repel impurities, so the hues are vivid^[14]. When temperature continually increases, the growth of crystals is quicker and larger crystals are formed. As particle size grows, the hues become dark gradually. To the pigments with different molar ratios of S to Se, the higher the ratio, the lower the change of yellow hues (b*) after hydrothermal treatment. This can be assigned to that in the process of hydrothermal reaction, both CdS and CdSe have effects on the formation of Cd(S_1-xSe_x) crystal nucleus. When the ratio is high, the quantity of CdS is dominative. Although a small part of CdS is dissolved in the hydro- thermal reaction ((CdS)=8.0×10^-27＞(CdSe)= 6.31×10^-36,  is solubility of product), it changes slightly to the whole CdS quantity. Thus, the changes of hues are unobvious. Table 3 shows the hues of samples with or without hydrothermal treatment. It can be seen that, with hydrothermal treatment, yellow hues (b*) of the pigments are decreased while red hues (a*) are increased. This is because hydrothermal reaction benefits the formation of Cd(S_1-xSe_x).

Table 2 Effects of hydrothermal reaction conditions on hues of samples

Table 3 Hues comparison of samples with or without hydrothermal treatment

3.6 Reaction mechanism

Both CdS and CdSe are hexagonal crystal (space group is P63mC) and α-ZnS (wurtzite) structure. S^2- and Se^2- are arranged as the hexagonal close-packed structure, and Cd²⁺ is packed in half of the tetrahedron cavity^[15]. The radii of S^2- and Se^2- are 0.184 and 0.198 nm, respectively. |?r/r|=|(0.198-0.184)/0.198|=7.07%＜15%. According to the solid solution theory^[16], with precipitate reaction, CdS and CdSe devitrify from solution and form continuous substitute solid solution. The reaction can be described as follows:

xCdSe+(1-x)CdS→Cd(S_1-xSe_x)                 (4)

Table 3 confirms that it is helpful Eqn.(4) to take place at high temperature and high pressure. From Fig.1, it can be seen that at pH=13.0, crystalline phases of samples (hydrothermal reaction temperature is 160 ℃ and time is 6 h) are CdS phases. The variation is the diffraction peak intensity and width; EDAX analysis shows that three elements of Cd, S and Se all appear in the product. The results confirm that S^2- of CdS is substituted by Se^2- to form Cd(S_1-xSe_x) continuous solid solution and the structure is also CdS structure. The formation of the defect reaction can be described as follows:

CdSeCd_Cd+Se_S                        (5)

4 Conclusions

1) With precipitate reaction, Cd(S_1-xSe_x) continuous substitute solid solution with low crystallinity is synthesized. The mechanism is that Se^2- substitutes S^2- of CdS. But after hydrothermal reaction, the Cd(S_1-xSe_x) with higher crystallinity is obtained and structure is better. In addition, yellow hues decrease and red hues increase.

2) The molar ratio of S to Se has an important effect on the pigments’ hues. In the case of Cd(S_1-xSe_x), when x decreases (i.e. S content increases), the yellow hues of the pigments become more and more bright.

3) pH value and hydrothermal reaction conditions also have a very important influence. When pH value is about 13.0 and hydrothermal reaction condition is at 140 ℃ for 4 h or at 160 ℃ for 6 h, Cd(S_1-xSe_x) pigments (red to yellow) are obtained with vivid color.

References

[1] EPPLER R A, EPPLER D R. Which colors can and cannot be produced in ceramic glazes [J]. Ceramic Engineering and Science Proceedings, 1994, 15(1): 281-288.

[2] SORLI S, TENA M A，BADENES J A, CALBO J, LLUSAR M, MONROS G. Structure and color of Ni_xA_1-3xB_2xO₂ (A=Ti, Sn; B=Sb, Nb) solid solutions [J]. Journal of the European Ceramic Society, 2004, 24(8): 2425-2432.

[3] BADENES J A, LLUSAR M, TENA M A, CALBO J, MONROS G. Praseodymium-doped cubic Ca-ZrO₄ ceramic stain [J]. Journal of the European Ceramic Society, 2002, 22(12): 1981-1990.

[4] EKAMBARAM S. Combustion synthesis and characterization of new class of ZnO-based ceramic pigments [J]. Journal of Alloys and Compounds, 2005, 390(2): 4-6.

[5] BONDIOLI F, FERRARI A M, LEONELLI C, MANFREDINI T. Synthesis of Fe₂O₃/silica red inorganic inclusion pigments for ceramic applications [J]. Materials Research Bulletin, 1998, 33(5): 723-729.

[6] ZHANG Yang, DENG Jian-cheng, GU Xiao-dan. New method synthesizes scarlet pigment at low temperature [J]. American Ceramic Society Bulletin, 2006, 85(3): 9301-9305.

[7] MARINOVA Y, HOHEMBERGER J M, CORDONCILLO E, ESCRIBANO P, CARDA J B. Study of solid solutions, with perovskite structure, for application in the field of the ceramic pigments [J]. Journal of the European Ceramic Society, 2003, 23(2): 213-220.

[8] COSTA A L, MATTEUCCI F, DONDI M, ZAMA I, ALBONETTI S, BALDI G. Heterocoagulation-spray drying process for the inclusion of ceramic pigments [J]. Journal of the European Ceramic Society, 2008, 28(1): 169-176.

[9] VOELKER W. New range of colored glazes [J]. Ceramic Magazine, 1987, 39(8): 514-515.

[10] SUN Zai-qing, LIU Shu-xing. Fabrication and application of ceramic pigments [M]. Beijing: Chemical Industry Press, 2007. (in Chinese)

[11] SU B, BUTTON T W, PONTON C B. Control of the particle size and morphology of hydrothermally synthesized lead zirconate titanate powders [J]. Journal of Materials Science, 2004, 39(21): 6439-6447.

[12] LIU Su-qin, HUANG Ke-long. Influences of reaction conditions on the structure and properties of analcimes [J]. Journal of Central South University of Technology, 2000, 31(3): 232-234. (in Chinese)

[13] NI Jing-an, SHANG Shao-ming, ZHAI Bing. Inorganic and analytic chemistry [M]. Beijing: Chemical Industry Press, 2004. (in Chinese)

[14] ZHANG Ping, CI Li-jie, ZHANG Xing-chen, DUAN Shu-de, XU Bao-en, ZHAO Jian-lu. Preparation of bismuth vanadate yellow pigment and its influence factors [J]. Journal of Shijiazhuang University, 2006, 8(6): 9-11. (in Chinese)

[15] YU Kang-tai, TIAN Gao, HU Ya-ping, XU Wang-hui. Study on the colour development mechanism and composition of the glase of occlusion pigment Cd(S_xSe_1-x) [J]. Ceramics, 1999, 6(4): 25-28. (in Chinese)

[16] SONG Xiao-lan, HUANG Xue-hui. Foundation of inorganic material science [M]. Beijing: Chemical Industry Press, 2006. (in Chinese)

Foundation item: Project(2005DFBA028) supported by the International Cooperation of Science and Technology of Ministry of China; Project(LA06030) supported by the Undergraduate Innovation Education of Central South University, China

Received date: 2008-03-19; Accepted date: 2008-05-20

Corresponding author: SONG Xiao-lan, Associate professor; Tel: +86-731-8830346; E-mail: xlsong@hnu.cn, xlsong365@126.com

(Edited by YANG Hua)