Preparations and characterizations of γ-Ce₂S₃@SiO₂ pigments from precoated CeO₂ with improved thermal and acid stabilities

Abstract

A series of γ-Ce₂S₃@SiO₂ pigments are prepared successfully by sulfurizing their corresponding precoated CeO₂ with different thicknesses of SiO₂ shell. Properties of the samples are investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric (TG) analysis and a spectrophotometer. The results show that when a well controlled thickness of SiO₂ coated CeO₂ is used as a precursor, γ-Ce₂S₃ nanoparticles with excellent properties are obtained. With the shell thickness increasing, the colours of the samples are expanded from red to orange to yellow. At the same time, the thermal and acid stabilities are also improved gradually.

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Introduction

Generally, cadmium sulfoselenide, lead molybdate and iron(III) oxide are known as pigments, particularly in the red colour range. However, the impact of legislation on limiting the use of some pigments has given an impetus to the search for new inorganic non-toxic pigments with red colour because pyrolysis of cadmium-containing materials results in the emission of cadmium oxide, which is dangerous for public health and the environment. The other two phases are orange and dark red, respectively. The rare earth sesquisulfide-based pigment, exhibits some specific physical thermal, mechanical, electronic and optical stability. The light rare earth sesquisulfides exist in three crystal structures of α, β and γ phase. The γ phase is the high temperature phase and presents a cubic structure of Th₃P₄. The chemical formula is expressed as RE_3−x□_xS₄ (where RE is the rare earth metal, □ is a rare earth metal vacancy, and 0 ≤ x ≤ 1/3).

The usual method to synthesis the γ-RE₂S₃ is treated with corresponding oxides with H₂S at about 1200 °C for 2 h, but so obtained γ-RE₂S₃ contains oxysulfides, which well-known as β-Ln₂S₃.^1,2 It demonstrates that the oxysulfides are the preferred phase and the oxygen is difficult to removing. Over the past years, Researchers focused their study on synthesis of γ-RE₂S₃ at low temperature. Firstly, as mostly studied dopant, alkali metal can be filled in the vacancies of γ-RE₂S₃, leading to the limit formula RE_2.5A_0.5S₄ (A = Na, K and Ca).³ Besides, the lanthanide elements, such Pr, Nd,⁴ Eu,^5,6 Dy, Ho, Er, Tb⁷ are also studied. Secondly, as kinds of promising procedures, with amorphous phase, under very low temperature, lanthanide molecular complex⁸ providing a better reactivity. Finally, studies shows that CS₂ is a powerful sulfurizing reagent, although elemental sulphur,⁹ H₂S,¹⁰ CS₂¹¹ are all investigated.

In recent years, nanometre rare earth sulphides are reported due to its specific properties. Li et al.^8,12 pointed out that γ-La₂S₃ nanoparticles can be prepared from La(Et₂S₂CN)₃·phen though pyrolysis method. The band gap of so obtained γ-La₂S₃ is boarder than the bulk one, due to the pronounced quantum confinement effect. Besides, research¹³ has reported that, with bright colour, series NaRES₂-based nano-pigments have prepared successfully in solution. But, synthesis of nanometre γ-RE₂S₃ by traditional solid-phase sintering reaction is still a challenging research.

Among above-mentioned rare earth sesquisulfide, the high temperature γ-Ce₂S₃ phase is widely studied due to it can be used as nontoxic red pigment with heavy metal free.¹⁴ Although with many advantages, the application of γ-Ce₂S₃ is still limited owe its thermal and chemical stability. Thierry et al.¹⁵ reported that the thermal and chemical stability of the rare earth sesquisulfide can be improved by coating a transparent layer. Chen et al.¹⁶ have investigated the properties of SiO₂ coated Ce₂S₃ red pigment and its thermal stability temperature is improved to 450 °C. Besides, the colour of Ce₂S₃ pigment is not affected significantly.

CeO₂–SiO₂ core–shell nanoparticles has attracted much attention due to their outstanding properties.^17,18 In this article, in order to enhance thermal and chemical stability of γ-Ce₂S₃, SiO₂ shell was coated on the γ-Ce₂S₃ nanoparticles to protect them and improve the wettability and biocompatibility of pigments for further plastics industry application. Series γ-Ce₂S₃@SiO₂ nanoparticles with different thickness SiO₂ shell are synthesized by sulfurizing CeO₂@SiO₂, and improved properties are also measured and analysed. The whole procedure is depicted in Scheme 1.

Scheme 1 Illustration of the formation of γ-Ce₂S₃@SiO₂ nanoparticles (route A) and badly sintered γ-Ce₂S₃ (route B).

Experimental

The formation of CeO₂ nanoparticles with spherical shape were derived from previous work.¹⁹ Similarly, 2.0 g Ce(NO₃)₃·6H₂O was dissolved in 2 ml deionized water by ultrasonic methods. Then, 2 ml CH₃COOH and 60 ml glycol were added with stirring for 30 min to form a uniform solution. The mixed solution was sealed in a Teflon-lined stainless steel autoclave and treated for 200 min after the temperature reaches 180 °C. Then the products were separated by centrifugation, washed with water and ethanol repeatedly, and dried at 60 °C in air.

SiO₂ coated CeO₂ was synthesized successfully by sol–gel method.²⁰ Typically, 0.6 g CeO₂ was well dispersed in a mixture solution of ethanol by using ultrasonic method, deionized water and 25 wt% concentrated ammonia aqueous stirred for 0.5 h at room temperature. Then, a certain volume of tetraethyl orthosilicate (TEOS) was added into the mixture. After stirring for 6 h at room temperature, the product was filtered and washed with water and ethanol, and then dried at 60 °C for 12 h. The detailed experimental conditions of samples are listed in Table 1.

Table 1 Preparing conditions of CeO₂@SiO₂

sample	CeO2/g	H2O/ml	C2H5OH/ml	NH3·H2O/ml	TEOS/ml
a0	0.6	0	0	0	0
b0	0.6	40	160	2.5	0.5
c0	0.6	40	160	5.0	1.0
d0	0.6	40	160	7.5	1.5
e0	0.6	40	160	10	2.0
f0	0.6	40	160	15	3.0

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Series of sulfurized products were obtained by following steps: Firstly, a fine mixture of CeO₂@SiO₂ and anhydrous Na₂CO₃ were obtained with a starting mole ratio of Na/Ce = 0.2. Then, the mixture, contained in a mullite boat, was introduced into a high temperature furnace and kept at 800 °C for 2 h. The tube was purged with dried N₂ for 20 min before heated. Afterwards, the N₂ was bubbled thought a CS₂ liquid when the temperature reach to 500 °C.

The powder X-ray diffraction (XRD) measurements were performed with PANalytical/X'Pert PRO diffractometer by using Cu Kα (40 kV, 40 mA) radiation. The morphologies of products were obtained by a transmission electron microscope (TEM, FEI/tecnai F20). Before the TEM test, the samples were dispersed in ethanol by ultrasonication for 5 min in an ultrasonic cleaner. The pigmentary performances of the samples were carried out with a spectrophotometer JENA SPECORD 50/Analytik Jena AG, and D65 illuminant. The thermogravimetric (TG) analysis was recorded on a NETZSCH/STA 409 PC/PG analyzer at a heating rate of 10 °C min⁻¹ from room temperature to 1000 °C.

Results and discussion

Fig. 1 shows the XRD patterns for the series precursors prepared by sol–gel method. All peaks of uncoated (a₀) and precoated samples (b₀–f₀) can be exactly indexed to the cubic CeO₂ (JCPDS 41-0013) and no traces of additional peaks belonging to SiO₂ were observed. This situation is mainly attributed to the in contrasting environments of crystallinity between amorphous SiO₂ and cubic CeO₂.

Fig. 1 XRD patterns of CeO₂@SiO₂ samples, the preparation conditions of the samples are listed in Table 1.

XRD patterns for the series sulfurized samples are shown in Fig. 2. It is clearly that the series stabilized γ-Ce₂S₃ (JCPDS 50-0851) was synthesized successfully by treating corresponding precursors with CS₂. Interestingly, when TEOS increasing to 1.0 ml, additional peaks that are essentially identical to the standard data for sulphur (23–0562) were observed (Fig. 2, b₁), it reveals that SiO₂ was coated successfully by means of sol–gel technology and the SiO₂ shell act as an obstacle that prevents the contact and reaction of CeO₂ and sulphur. Additionally, with the volume of TEOS increasing, peaks belonging to sulphur were becoming stronger and stronger (Fig. 2, b₁–f₁), it indicates that the SiO₂ shell was getting thicker.

Fig. 2 XRD patterns of γ-Ce₂S₃@SiO₂ samples. a₁, b₁, c₁, d₁, e₁ and f₁ are prepared by sulfurizing a₀, b₀, c₀, d₀, e₀ and f₀, respectively.

The sizes and morphologies of nanoparticle in samples were examined by TEM (see in Fig. 3). As shown in Fig. 3a, it is clearly that the CeO₂ before sulfurizing are nanoparticles with spherical morphology and about 100 nm in size. The SiO₂ shells were precoated on the CeO₂ nanoparticles with different thickness were synthesized successfully with sol–gel technology.

Fig. 3 TEM images of CeO₂@SiO₂ samples. From a to f: sample a₀, b₀, c₀, d₀, e₀ and f₀.

TEM images for series sulfurized samples are given in Fig. 4. As can be seen in Fig. 4a, badly sintered product was obtained when uncoated CeO₂ nanoparticles were applied, the formation process belongs to route B is illustrated in Scheme 1. Interestingly, when SiO₂ shell is introduced, γ-Ce₂S₃ nanoparticles with their original morphology and size basically were obtained. It reveals that SiO₂ shell can play as a protective layer to prevent sintering process between the nanoparticles. At the same time, unfortunately, under the treatment of high temperature, the SiO₂ shell is destroy and fused together (Fig. 4b). But, these results can be improved gradually with the increase of the thickness of SiO₂ (Fig. 4c–f). SiO₂ coated γ-Ce₂S₃ nanoparticle with fine core–shell structure was obtained when the SiO₂ is thick enough (Fig. 3f, 110 nm). Formation processes of γ-Ce₂S₃ nanoparticles belongs to route A is illustrated in Scheme 1, when SiO₂ layers are introduced.

Fig. 4 TEM images of γ-Ce₂S₃@SiO₂ samples. From a to f: sample a₁, b₁, c₁, d₁, e₁ and f₁.

The chromatic coordinates compare to the 1931 CIE Standard Source C and photograph of the samples are shown in Fig. 5. Fig. 5a shows coordinates for a₁, b₁, c₁ and d₁ are more closer to the red area than e₁ and f₁, it is in accord with the fact that the colour of samples from a₁ to f₁ are red, orange and yellow, respectively, see Fig. 5b. The colorimetric parameters are summarized in Table 2. With the thickness of SiO₂ shell increasing, the lightness (L*) of the sulphide powder is increases gradually from 40.38 to 66.73 while the red-green index a* and dominant wavelength are decrease from 34.62 to 9.16 and 602 to 580 nm, respectively. Additionally, products present a stronger value of b* between 22.66 and 43.34, which correspond to yellow-blue index. It is reveals that the introduction of SiO₂ can improve the lightness and broaden the colour of the pigments.

Fig. 5 (a) Chromatic coordinates and (b) photograph of γ-Ce₂S₃@SiO₂ samples.

Table 2 Chromaticity coordinates, L*, a*, b* and wavelength for γ-Ce₂S₃@SiO₂ samples

Sample number	CIE (x, y)	L*	a*	b*	Dominant wavelength/nm
a1	(0.5001, 0.3447)	40.38	34.62	26.33	602
b1	(0.4866, 0.3561)	47.60	32.87	30.22	596
c1	(0.4793, 0.3728)	55.70	30.10	37.21	590
d1	(0.4913, 0.3823)	57.47	30.82	43.34	589
e1	(0.3926, 0.3648)	63.63	11.74	23.29	582
f1	(0.3830, 0.3648)	66.73	9.16	22.66	580

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Thermal stability is studied by heat treatment of the samples in air. TG curves for all sulfurized samples are given in Fig. 6. A similar broad peak band clearly exhibited one mass gain and one mass loss processes, it can attribute to the oxidation of the γ-Ce₂S₃ in air: oxysulfides are the possible intermediates and CeO₂ is the final product. The value for corresponding temperature on the curve which follows it is the mass lose process can express thermal stability for sample. Obviously, these values are increasing gradually from sample a₁ to f₁. Therefore, it seems reasonable to conclude that the thickness of SiO₂ can be directly attributed to thermal stability. In addition, The values for corresponding weight (%) are declining basically from sample a₁ to f₁, except for e₁ shows a boarder peak make its value lower than f₁. It reveals the content of γ-Ce₂S₃ in a₁ to f₁ reducing gradually, this is in good agreement with the XRD and TEM results.

Fig. 6 TG curves for γ-Ce₂S₃@SiO₂ samples.

Acid stability was studied by treatment of samples (0.01 g) with hydrochloric acid (10 ml, pH = 0.5). The times which samples lose their original colour (t_loc) completely are recorded in Table 3. When coated sample b₁ is selected, weak acid stability is observed (t_loc = 70 s), the value even smaller than uncoated sample a₁ (t_loc = 120 s). The reasonable explanation is that b₁ has a smaller size and incompletely coated shell. At the same time, with the thickness of SiO₂ shell increasing, the acid stability increase dramatically, and the maximum value is 4 h for d₁. As an abnormal result, the acid stabilities of e₁ and f₁ are weaker than d₁. It is because that they have a relatively lower content of γ-Ce₂S₃. All results illustrate that the acid stability of γ-Ce₂S₃ is improved gradually, when SiO₂ layer is introduced.

Table 3 Results of acid stability test

Sample	a1	b1	c1	d1	e1	f1
tloc	120 s	70 s	40 min	≈4 h	≈3 h	≈3 h

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Conclusions

In this paper, series of coated γ-Ce₂S₃ nanoparticles are synthesized successfully by use traditional sulfurization method, when colourless and nontoxic SiO₂ layer is introduced. On the one hand, SiO₂ can plays as a protective layer to prevent the agglomeration of γ-Ce₂S₃ nanoparticles. On the other hand, field of application for γ-Ce₂S₃ pigment may be expended, owing to their unique colour, improved thermal and acid stability.

Acknowledgements

The authors are grateful to the financial aid from the National Natural Science Foundation of China (grant no. 20961006), the Inner Mongolia Natural Science Foundation (grant no. 20080404MS0201), and the Inner Mongolia Technology Innovation and Guidance Funds.

Notes and references

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