JOURNAL OF RARE EARTHS, Vol. 29, No. 4, Apr. 2011, P. 317

Foundation item: Project supported by the National High Technology Research and Development Program of China (863 Program) (2009AA03Z431)

Corresponding author: JIE Wanqi (E-mail: jwq@nwpu.edu.cn; Tel.: +86-29-88486065)

DOI: 10.1016/S1002-0721(10)60451-6

Preparation of lanthanum sulfide nanoparticles by thermal decomposition of lanthanum complex

LI Peisen (李培森), LI Huanyong (李焕勇), JIE Wanqi (介万奇) (School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China)

Received 31 August 2010; revised 19 February 2011

Abstract: γ-La2S3 nanoparticles were successfully prepared by thermal decomposition of lanthanum complex La(Et2S2CN)3·phen at low temperature. The obtained sample was characterized by the X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and ele-ment analysis. The decomposition mechanism of lanthanum complex was studied by thermogravimetric analyses (TGA). The results showed that the obtained samples were cubic phase particles with uniform sizes among 10–30 nm and γ-La2S3 was prepared by decomposition of La(Et2S2CN)3·phen via La4(Et2S2CN)3 as an intermediate product. The band gap of γ-La2S3 was 2.97 eV, which was bigger than bulk crystal because of pronounced quantum confinement effect.

Keywords: lanthanum sesquisulfide; nano-particle; lanthanide complex; thermal decomposition; rare earths

Lanthanum sesquisulfide, having the wide transmission range (0.5–14 mm), possesses a superior combination of low thermal expansion coefficient, high melting point and suffi-cient mechanical hardness, making it an attractive alternative for future IR window and dome materials[1–3]. Lanthanum sulfide has three different allotropic forms (α, β, γ-La2S3) in the crystalline state. The high-temperature cubic phase γ- La2S3 is metastable at room temperature, while it shows po-tential application as a far IR (8–14 mm) window material. So γ-La2S3 has been the focus of many research groups in last several decades. But due to the strong oxygen affinity of metallic lanthanum, pure γ-La2S3 powders are difficult to obtain below the transformation temperature of 1300 ºC from γ (cubic) phase to β (tetrahedron) phase.

Generally, the synthesis of γ-La2S3 is performed by the reaction of lanthanum oxides, chlorides, nitrates with H2S or CS2 at above 1000 ºC for several hours in the absence of oxygen and moisture[4–6]. Oxygen atoms from air and mois-ture easily lead to the formation of β-La2S3

[7,8]. Kumta and Risbud[7] synthesized oxysulfide precursors from metal alkoxides in a wet chemical route. The product crystallites prepared via high-temperature reaction were usually in the size of microns[9]. Lanthanum chalcogenolate complexes also attract considerable attention due to their potential ap-plications in the preparation of new advanced materials[10]. While most researchers focus on syntheses, structures and applications of lanthanum chalcogenolate complexes.

In this contribution, we presented the results about the prepa-ration of γ-La2S3 nanoparticles by the thermal decomposition of lanthanum complex La(Et2S2CN)3·phen at low temperature. And the formation mechanism of γ-La2S3 was also studied.

1 Experimental

1.1 Synthesis of lanthanum complex

All the chemical reagents used in our experiments except hydrochloric acid were of analytical grade. Hydrous lantha-num chlorides were prepared by reaction of lanthanum oxide (99.99%) with hydrochloric acid[10]. The as-prepared LaCl3·7H2O was placed in vacuum desiccators until its masses kept constant in several days.

Lanthanum complex La(Et2S2CN)3·phen was synthesized by the following chemical equation: LaCl3·nH2O+3NaEt2S2CN·3H2O+phen·H2O= La(Et2S2CN)3·phen+3NaCl+(n+10)H2O (1)

Hydrous lanthanum chlorides, sodium diethyldithiocar-bamate (NaEt2S2CN·3H2O) and 1,10-phenanthroline (phen·H2O) in the mole ratio of 1:3:1 were dissolved in a minimal amount of anhydrous ethanol respectively, then al-coholic solution of phen and NaEt2dtc were mixed together. The lanthanum chlorides alcoholic solution was dropped slowly into the above mixture under stirring. The final yel-low complex La(Et2S2CN)3·phen was obtained after it was washed by anhydrous ethanol for many times.

1.2 Sample preparation

Some La(Et2S2CN)3·phen was weighed and placed in a small high pure quartz crucible. Then it was transferred into a quartz tube in a horizontal furnace, one of the quartz tube’s ends was sealed and the other was connected with a vacuum system. The crucible’s temperature was maintained at 650 ºC, and the quartz tube was under dynamic vacuum of 1.3 Pa.

318 JOURNAL OF RARE EARTHS, Vol. 29, No. 4, Apr. 2011

1.3 Measurement method

The IR transmission spectra of the complex and the prod-ucts were given through Nicolet Nexus FTIRS in the wave number range from 500 to 4000 cm−1. X-Ray powder dif-fraction patterns of the products were obtained using a Ri-gaku D/max-3C X-ray diffractometer equipped with Cu Ka radiation (λ=0.1548 nm). The accelerating voltage and cur-rent were 40 kV and 35 mA respectively. The morphologies were analyzed with Supera 55 instrument. The UV-Vis diffuse reflectance spectrum of the product was obtained from UV-3150. Thermogravimetric analyses (TGA) of the complex were performed on a HI-Res TGA2950 Thermo-gravimetric Analyzer at 5 K/min in a 60 ml/min high pure N2 atmosphere.

2 Results and discussion

2.1 IR spectrum analysis

The IR spectra of complex La(Et2S2CN)3·phen and its products are shown Fig. 1. The complex had some absorbing bands between 1650 and 500 cm–1, seen in Fig. 1(a) curve. The band at 1617 cm–1 was assigned to the vibration of νC=C in the complex. Bands of 848 and 726 cm–1 were assigned to the νC–H bend vibration of benzene ring in the complex. Band of 1480 cm–1 was assigned to νCN and the band of 998 cm–1 was assigned to νCSS in the complex[10]. The absorption of hydroxyl group did not present in the complex, showing that the complex did not contain water. Not any absorbing bands observed in Fig. 1(a) curve were seen in the IR spectra curve of the products in Fig. 1(b). It proved that the complex La(Et2S2CN)3·phen decomposed completely to the products at 650 ºC.

2.2 Phase analysis

Typical X-ray powder diffraction patterns of the nanopar-ticles are shown in Fig. 2, which demonstrates the high crys-talline of the sample. All the diffraction peaks 2θ=24.99°, 32.42°, 38.53°, 46.57°, 48.98°, 53.46°, 57.83°, 66.97° and

Fig. 1 IR transmission spectra of complex La(Et2S2CN)3·phen (1)

and the decomposed products (2)

Fig. 2 XRD pattern of as-prepared γ-La2S3

Table 1 Calculated sizes of as-prepared γ-La2S3 nanoparticles

Serial numbers Pos.[2Th] FWHM[2Th] Size/nm 1 24.9911 0.6494 12.4508 2 3

32.4274 38.5366

0.3247 0.4546

25.3173 18.3938

69.82° were assigned to the (211), (310), (321), (420), (332), (510), (521), (620) and (541) planes of cubic or Th3P4 cell. The lattice parameter of the sample was calculated to be 8.7896, which was consistent with the reported value from the Ref. [11]. It could be seen in Fig. 2 that no impurities, such as La2O3, La2O2S2 and La2O2S, could be observed in the XRD pattern. The XRD result of the sample indicated that the nanoparticles were pure γ-La2S3.

The average grain size L was estimated according to the Debby-Scherer’s equation, L=Kλ/B·cosθ (2) where L is the average size of crystal particle, K is a constant taking a value of 0.89, λ is the wavelength of X-ray, B is the half width of the diffraction peak, and θ is the corresponding incidence angle. Half widths of the three strongest peaks in Fig. 2 and the corresponding incidence angles were substi-tuted into the Eq. (2) and the calculated results are listed in Table 1. It could be seen that the particle size was among 10–30 nm.

2.3 Morphology and composition analysis

Fig. 3 shows the morphology of the as-prepared sample. It is obvious that the as-prepared sample mainly consisted of a lot of nanoparticles with the diameters among 10–30 nm, which was consistent with the results of XRD analyses. It is also seen that the particles were dispersive. Three different particles were used (Fig. 3) to measure quantitatively the elemental proportion of the as-prepared γ-La2S3 nanoparticles by EDX. The results of EDX are shown in Table 2. It can be seen that the as-prepared sample have an approximate stoichiometric composition close to the ideal stoichiometry of γ-La2S3. Sulfur-deficient might be an inevitably problem since the vapor pressure of sulfur is much larger than lanthanum. Carbon was also detected by EDX.

LI Peisen et al., Preparation of lanthanum sulfide nanoparticles by thermal decomposition of lanthanum complex 319

Fig. 3 Morphology of as-prepared γ-La2S3

Table 2 Composition of as-prepare γ-La2S3

Spectrum La/at.% S/at.% La/S

1 40.52 59.48 1:1.4679

2 40.84 59.16 1:1.4486

3 40.70 59.30 1:1.4570

The presence of carbon might be ascribed to the decomposi-tion of organic groups in the lanthanum complex and carbon was not excluded completely from the products when the products were washed for many times by carbon disulfides liquid and anhydrous ethanol.

2.4 Band gap analysis

The band gap of the sample could be measured by diffuse reflectance spectra. Fig. 4 displays the UV-Vis diffuse re-flectance spectra of the as-prepared sample at room tem-perature. According to Kubelka-Munk relationship:

(3)

Where R∞ is the absolute reflectance of the sample, s is a scattering coefficient, and α is the absorption coefficient. The value between absorption coefficient and scattering coeffi-cient could be calculated by use of diffuse reflectance spec-tra. In the meanwhile, its band gap could be expressed as the following equation for direct gap materials:

Fig. 4 Diffuse reflectance spectrum of the as-prepared sample (inset:

its transformed sprectrum)

(4)

Where Eg the energy of the forbidden gap, h is Planks con-stant, ν is optical frequency. So the band gap Eg could be calculated from the inset spectrum in Fig. 4, which was plot-ted by Eqs. (3) and (4). The band gap of the as-prepared sample could be estimated at 2.97 eV, which was blue shift compared to the bulk crystal, such as 2.91 eV by optical gap[12]. Such a phenomenon might be due to the pronounced quantum confinement effect on the nano-particles.

2.5 Thermal analysis

Thermal analyses were carried out for lanthanum complex La(Et2S2CN)3·phen to study the possible decompose mecha-nism to form γ-La2S3. The TG/DTA showed that the com-plex La(Et2S2CN)3·phen decomposed in the temperature range of 200–445 ºC to La2S3, seen in the Fig. 5. The weight loss below 100 ºC was ascribed to the evaporation of the ab-sorbed water. There were two main phases for the weight loss between 200–445 ºC in the TG and they could be cor-responding to the endothermic peaks in the DSC curve. So the decomposition of the complex La(Et2S2CN)3·phen also might go though two processes to γ-La2S3. The possible de-composition processes are seen in Table 3. Firstly, the com-plex La(Et2S2CN)3·phen restructured and gradually expulsed parts of the diethyldithiocarbamate groups after it lost the phen groups in the temperature range of 200–279.31 ºC. The residual weights decreased to 49.13%. Further, the interme-diate product continued to decompose to the γ-La2S3 in the temperature range of 280–445 ºC and the percentage of the final product is 40.4%. Carbon was deposited on the γ-La2S3 because of the decomposition of the organic groups.

Fig. 5 TG-DTA (a) and DSC (b) curves of the La(Et2S2CN)3·phen

320 JOURNAL OF RARE EARTHS, Vol. 29, No. 4, Apr. 2011

Table 3 Decomposition mechanism of the La(Et2S2CN)3·phen

Steps Possible residues Temperature/ºC Percentage/% (calculated)

1 0.25La4(Et2S2CN)3+10.5C 200–279.31 49.13% (49.5%)

2 0.5La2S3+10.5C 279.31–445 40.4% (40.97%)

3 Conclusions

We successfully prepared γ-La2S3 nanoparticles with the size of 10–30 nm in diameter via thermal decomposition of the lanthanum complex at lower temperature. The results of XRD, SEM, and EDS showed that this routine would pro-vide a favorable low temperature way to prepare pure and symmetrical γ-La2S3 nano-particles. The band gap of γ-La2S3 was bigger than bulk crystal because of pronounced quantum confinement effect. TG/DTA results showed that the lan-thanum complex La(Et2S2CN)3·phen decomposes to γ-La2S3 by La4(Et2S2CN)3 as an intermediate product.

References:

[1] Kukli K, Heikkinen H, Nykanen E, Niinisto L. Deposition of lanthanum sulfide thin films by atomic layer epitaxy. Journal of Alloys and Compounds, 1998, 275-277: 10.

[2] Kumta PN, Risbud SH. Review rare-earth chalcogenide-an emerging class of optical materials. Journal of Materials Sci-ence, 1994, 29: 1135.

[3] Tian L, Ouyang T, Loh K P, Vittal J J. La2S3 thin films from metal organic chemical vapor deposition of single-source pre-

cursor. Journal of Materials Chemistry, 2006, 16: 272. [4] Sato N, Shinohara G, Kirishima A, Tochiyama O. Sulfuriza-

tion of rare-earth oxides with CS2. Journal of Alloys and Compounds, 2008, 451(1-2): 669.

[5] Ohta M, Yuan H, Hirai S, Uemura Y, Shimakage K. Prepara-tion of R2S3 (R: La, Pr, Nd, Sm) powders by sulfurization of oxide powders using CS2 gas. Journal of Alloys and Com-pounds, 2004, 374: 112.

[6] Yuan H, Zhang J, Yu R, Su Q. Synthesis of rare earth sulfides and their UV-vis absorption spectra. Journal of Rare Earths, 2009, 27(2): 308.

[7] Kumta P N, Risbud S H. Low temperature synthesis of cubic lanthanum sulfide (La2S3) powders. Materials Science and Engineering: B, 1989, 2(4): 281.

[8] Roméro S, Mosset A, Trombe J-C, Macaudière P. Low-tem-perature process of the cubic lanthanide sesquisulfides: re-markable stabilization of the -Ce2S3 phasei. Journal of Materi-als Chemistry, 1997, 7: 1541.

[9] Tang K, An C, Xie P, Shen G, Qian Y. Low-temperature synthesis and characterization of β-La2S3 nanorods. Journal of Crystal Growth, 2002, 245(3-4): 304.

[10] Fan X Z, Chen S P, Xie G, Gao S L, Shi Q Z. Regularity of thermochemical properties of ternary complexes RE[(pdtc)3

(phen)]. Chinese Journal of Chemistry, 2006, 24(8): 1013. [11] Toide T, Utsunomiya T, Sato M, Hoshino Y, Hatano T, Aki-

moto Y. Preparation of lanthanum sulfides using carbon disul-fide as a sulfurizing agent and the change of these sulfides on heating in air. Bull. Tokyo. Inst. Tech., 1973, 117: 41.

[12] Biswas K, Varadaraju U V. Stabilization of γ-La2S3 by alkali metal ion doping. Materials Research Bulletin, 2007, 42: 385.