Synthesis and characterization of calcium lanthanum sulfide via a wet chemistry route followed by thermal decomposition

Abstract

Calcium lanthanum sulfide (CaLa₂S₄) has been extensively studied as a promising candidate for advanced infrared optical ceramics. In the present research, we report the successful synthesis of CaLa₂S₄ via a wet chemistry method followed by thermal decomposition. CaLa₂S₄ precursor material was first prepared by a facile ethanol-based wet chemical single-source precursor route. The precursor was then thermally decomposed in argon at high temperature to form CaLa₂S₄. The phase composition and morphology of the synthesized CaLa₂S₄ powder were confirmed and observed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), respectively. Surface area and pore size analyses showed that the CaLa₂S₄ powder had a high specific surface due to a combined effect of small particle size and the existence of mesopores. Optical characterization revealed that the synthesized CaLa₂S₄ powder exhibited quantum size confinement and near-band-edge photoluminescence.

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Introduction

Due to its favorable optical performance and mechanical properties, calcium lanthanum sulfide (CaLa₂S₄), crystallizing in the cubic Th₃P₄ crystal structure, has been extensively studied as a promising candidate for infrared optical ceramics.^1,2 It has been reported that this alkaline earth and rare earth ternary sulfide compound, which displays a wide infrared transmittance range of 8–14 μm, has superior hardness and better rain erosion resistance compared with conventional infrared ceramic materials such as ZnS and ZnSe, indicating that it has the potential as an advanced infrared optical ceramic.^3–5

Since the 1980's, researchers have focused on developing various synthesis routes to prepare CaLa₂S₄ powders and ceramics. It has been reported that optical-quality CaLa₂S₄ powders can be synthesized via an alkoxide method followed by the sulfurization of precursors at high temperature in an atmosphere of H₂S or CS₂.^6–9 Alternately, by adopting a carbonate precipitation route, M. Tsai et al. fabricated CaLa₂S₄ powders via CS₂ sulfurization, and subsequently investigated the influence of varying the Ca/La stoichiometric ratio on the phase composition of the CaLa₂S₄ powders.^10–13 W. White et al. developed an evaporative decomposition of solution (EDS) route for producing CaLa₂S₄, consisting of spraying an aqueous solution of Ca and La nitrates into a hot furnace and heating the resulting oxide powder in flowing H₂S.¹⁴ In addition, researchers from the Office of Naval Research developed yet another method to fabricate CaLa₂S₄ powder, with high purity and large BET surface area, through flame spray pyrolysis of Ca(NO₃)₂ and La(NO₃)₃ followed by sulfurization in H₂S.¹⁵ In summary, the previously investigated CaLa₂S₄ synthesis procedures have mainly employed different methods to prepare the calcium lanthanum precursors, followed by sulfurization at elevated temperatures in an atmosphere of CS₂ or H₂S. However, sulfurization is a very slow and time-consuming process, which requires the use of highly toxic and flammable gases such as CS₂ and H₂S, rendering it a costly and environmentally harmful route for synthesizing CaLa₂S₄.

The single-source precursor (one-pot) method has been extensively applied to synthesize various high-quality metal sulfide nanomaterials with controllable morphologies and sizes over the last two decades.^16,17 Different sulfur-containing precursors, such as organometallic compounds, dithiocarbamate salts, trithiocarbonate salts, xanthate salts, and metal dialkyldithiophosphates have been employed to chelate metal ions with sulfur ions during wet chemical processes, with the corresponding sulfides subsequently obtained by pyrolysis of the precursors.¹⁸ Among them, it has been reported that sodium diethyldithiocarbamate and 1,10-phenanthroline have been successfully used as sulfur source and chelating agent for synthesizing various nano-sized semiconductor sulfides.^19–23 However, few studies have reported the synthesis of CaLa₂S₄ through the use of a wet chemistry method followed by thermal decomposition.

In the present investigation, CaLa₂S₄ precursors were fabricated via a single-source precursor method, and CaLa₂S₄ powder was subsequently obtained through the thermal decomposition of the as-synthesized precursor material. The phase composition and morphology of the synthesized powder were determined and observed, respectively. The specific surface area and pore size distribution of the synthesized porous CaLa₂S₄ powder were measured and analyzed through Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively. Spectroscopic characterization was performed to develop an understanding of the correlation between the optical properties and the inherent physical and chemical properties of the synthesized CaLa₂S₄ powder.

Experimental

All employed chemicals were of analytical grade and were used as received without any purification. Fig. 1 shows a brief schematic of the process used to synthesize the CaLa₂S₄ powder. 20 mmol sodium diethyldithiocarbamate trihydrate (Na(Ddtc)·3H₂O, Sigma-Aldrich) and 20 mmol 1,10-phenanthroline trihydrate ((Phen)·3H₂O, ≥99%, Sigma-Aldrich) were first dissolved in separate 100 mL anhydrous ethanol (reagent alcohol, 100%, Decon), respectively. The alcoholic solutions of Na(Ddtc) and Phen were then mixed by vigorous stirring. In addition, 5 mmol lanthanum chloride heptahydrate (LaCl₃·7H₂O, 99%, Alfa Aesar) and 2.5 mmol calcium chloride dihydrate (CaCl₂·2H₂O, 99%, Alfa Aesar) were also dissolved in separate 100 mL anhydrous ethanol, respectively. Next, the lanthanum and calcium chlorides solutions were added dropwise into the mixed solution of Na(Ddtc) and Phen while being stirred, yielding a yellowish solution. 2 mL of thioglycolic acid (TGA, ≥98%, Sigma-Aldrich) was dissolved into 10 mL of anhydrous ethanol, and then added into the previously mixed yellowish alcoholic solution as a capping agent. After 1 hour of reaction, the resulting yellow precipitates were washed and centrifuged (Allegra X-12 Centrifuge, Beckman Coulter) several times with anhydrous ethanol and then dried in air at 55 °C for 24 hours. Finally, the dried precursor material was heat treated at 1000 °C for 5 hours in flowing argon to achieve thermal decomposition and reaction to form CaLa₂S₄.

Fig. 1 Schematic of the route used to synthesize CaLa₂S₄ via a wet chemistry method followed by thermal decomposition.

The phase composition of the synthesized CaLa₂S₄ powder was determined by using XRD with Cu Kα (λ = 0.154 nm) radiation (Bruker D2 PHASER) at a voltage of 30 kV and a current of 10 mA. Measurement conditions of 0.03° 2θ step size and 0.2 s count time were employed over a measurement range of 10–75° 2θ. MDI Jade 9 was adopted for phase analysis, and Topas (Bruker) was used to perform Rietveld refinement of the XRD patterns. The morphological and microstructural features of the CaLa₂S₄ powder were investigated by SEM (FEI Quanta 200) with an acceleration voltage at 20 kV and TEM (JEOL 2100F) with an acceleration voltage at 200 kV. A Tristar II 3020 system (Micromeritics) was employed to measure the surface area and adsorption–desorption curve of the synthesized CaLa₂S₄ powder using BET and BJH methods, respectively. The powder was degassed at 25 °C for 10 min and at 150 °C for 1 hour prior to the measurement. Thermogravimetry (TG) and differential thermal analysis (DTA) (SDT Q600, TA Instruments) were conducted in a nitrogen (N₂) atmosphere at a heating rate of 10 °C min⁻¹ from 20 °C to 1100 °C with α-Al₂O₃ as reference. The band gap of the materials was determined using an ultraviolet-visible (UV-Vis) spectrophotometer (Perkin-Elmer Lambda 900) to measure the UV-Vis diffuse reflectance spectrum of the CaLa₂S₄ powder at 25 °C. Photoluminescence was investigated using photoluminescence spectra measurements (Jobin Yvon Fluorolog-3, Horiba) collected at 25 °C with a xenon lamp as the light source.

Results and discussion

Fig. 2 shows the XRD pattern of the CaLa₂S₄ precursor material after thermal decomposition and reaction at 1000 °C. The material is mostly single phase, with the main phase indexed to cubic calcium lanthanum sulfide (ICDD card no. 00-29-0339, I4̄3d, a = 8.6830 Å). Through Rietveld refinement, the lattice parameter of the material is refined to be 8.6814 Å, which is very close to the standard PDF card data. In addition, it should be noted that some minor peaks exist in the XRD pattern, which correspond to residual carbon remaining in the powder after thermal decomposition. The fact that the main phase of the powder is cubic CaLa₂S₄ demonstrates that the route applied in this research is a feasible and effective method to prepare CaLa₂S₄ without the use of any long sulfurization treatments using sulfur-containing gases. It also suggests that Na(Ddtc) and Phen are effective at chelating the La and Ca ions with sulfur ions during wet chemical precipitation. The effective chelation of the Ca, La, and S ions allows for the ternary sulfide CaLa₂S₄ to be obtained by subsequent thermal decomposition and reaction of the precursor material. Due to the relatively high temperature at which the thermal decomposition and reaction is performed, the material experiences particle growth, and as such the crystallite size of the reaction products is relatively high, apparent as the narrow peaks observed in the XRD pattern in Fig. 2.

Fig. 2 XRD pattern of the synthesized CaLa₂S₄ powder after thermal decomposition at 1000 °C for 5 hours in flowing argon.

The TG/DTA curves of the CaLa₂S₄ precursor material obtained via the wet chemistry route are displayed in Fig. 3, which illustrates the various thermal reactions that the material goes through during the heat treatment in Ar. In the temperature range between room temperature and 400 °C, the broad exothermic peak accompanied by >50% weight loss corresponds to the rapid thermal decomposition of the polymers in the precursor. The two endothermic peaks in this range are likely due to the release of the waters of hydration from the hydrated nitrate raw materials used in the precursor. As temperature increases, the precursor material's rate of weight loss decreases. The weight loss of the CaLa₂S₄ precursor becomes approximately stable with a total weight loss of 79%, at the final temperature of 1000 °C, with an endothermic peak at 1000 °C corresponding to the formation of CaLa₂S₄. The TG/DTA curves shown here provide a basic reference for the thermal decomposition profile of the precursor to synthesize CaLa₂S₄.

Fig. 3 TG/DTA curves of the CaLa₂S₄ precursors synthesized via the wet chemistry method.

The SEM image displayed in Fig. 4(a) shows that the CaLa₂S₄ powder obtained by thermal decomposition of the precursor consists of inhomogeneous submicron-sized agglomerates of particles. It is further suggested by TEM (Fig. 4(b)) that the CaLa₂S₄ particles are nanoscale, with an approximate average particle size of ∼50 nm. Detailed observations of the powder reveal neckings between adjacent particles, which can be attributed to the powder beginning to sinter at the relatively high heat treatment temperature. In addition, EDS measurements of the powder (shown in Fig. 4(c)) reveal the presence of the main cations and anions expected in CaLa₂S₄. The oxygen peak is due to the oxygen attached to the pores and surface of the CaLa₂S₄ powder. The presence of carbon indicates that some carbon species survive the thermal decomposition heat treatment of the CaLa₂S₄ precursor. The powder needs to be further purified to remove carbon impurities for infrared optical ceramic applications.

Fig. 4 (a) SEM image, (b) TEM image, and (c) EDS spectrum of the CaLa₂S₄ powder synthesized via a wet chemistry method followed by thermal decomposition.

Fig. 5(a) presents the N₂ adsorption–desorption isotherm curve of the synthesized CaLa₂S₄ powder measured using the BJH method. The hysteresis loop shown in this typical irreversible type IV isotherm curve is due to pore condensation,²⁴ suggesting the presence of mesopores within the CaLa₂S₄ powder, which are believed to form primarily during the thermal decomposition of the CaLa₂S₄ precursor. It is of interest to notice that no mesopores are shown in the particles in the TEM image. However, from the agglomerates observed in the SEM micrograph, the mesopores are speculated due to interspaces within the small nano-sized particles during the process of particles' stackings and agglomerations. The BET specific surface area of the sample was measured to be 75.82 ± 0.26 m² g⁻¹, which is attributed to the submicron-scale agglomerates of the nanoparticles, in combination with the effect of the mesopores. The pore size distribution curve displayed in Fig. 5(b) indicates that the sample has a narrow pore size distribution approximately at 23 nm, which also supports the notion that extensive mesopores exist within the sample.

Fig. 5 Adsorption–desorption isotherm curve (a) and pore size distribution curve (b) of the synthesized CaLa₂S₄ powder.

The UV-Vis spectrum of the synthesized CaLa₂S₄ powder is shown in terms of absorbance versus wavelength in Fig. 6(a). The measured diffuse reflectance is converted to absorbance in this plot. The minimum wavelength required to promote an electron to the conduction band of the material is dependent on the material's band gap, and was estimated from the curve to be 429 nm. To determine the direct band gap of the CaLa₂S₄ powder, the curve can be replotted based on the equation for the Tauc relation shown below:²⁵

(αhν)^m = A(hν − E_gn) (1)

where A is a constant and the band gap of the studied material is denoted as E_gn. The exponent m depends upon the type of transition in which an electron is excited to the conduction band: m may have values of 2, 1/2, 2/3 and 1/3 corresponding to the allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, respectively. Fig. 6(b) shows the Tauc plot of the synthesized CaLa₂S₄ powder. An absorption energy can be determined by extrapolating the value of hν to α = 0, which corresponds to the band gap energy E_gn. The band gap energy of the synthesized CaLa₂S₄ powder is estimated to be 2.89 eV, higher than that measured for the bulk material (2.70 eV).²⁶ The small crystallites within the CaLa₂S₄ powder result in quantum size confinement, which contributes to the apparent band gap enhancement of the synthesized powder.

Fig. 6 (a) UV-Vis absorption spectrum and (b) Tauc plot of the synthesized CaLa₂S₄ powder, with the UV-Vis spectrum collected at 25 °C.

Fig. 7 shows the photoluminescence emission spectrum (excitation wavelength: 365 nm) of the synthesized CaLa₂S₄ powder measured with a xenon lamp as the light source. It was reported that the characteristic Mn²⁺ peak, which originated from Mn impurities in the CaS impurity phase present in CaLa₂S₄, was observed in the photoluminescence spectra of CaLa₂S₄ powder.²⁷ However, there are no such impurity peaks in the spectra measured in this study, likely due to the difference in the employed processing routes. Here, the sample shows an emission peak at approximately 454 nm, which is assumed to be due to the near-band-edge photoluminescence of the CaLa₂S₄ semiconductor. It should be noted that the photoluminescence emission wavelength has a redshift compared with than the estimated wavelength required for electron promotion through light absorption, as determined through UV-Vis spectroscopy (shown in Fig. 6(a)), which suggests that the emitted photon energy is lower than the absorbed photon energy. The energy degradation here can be ascribed to the Stokes shift in energy (Δ_Stokes), which in semiconductors is attributed to the participation of phonons in the relaxation process.²⁸ In addition, it is believed that some sulfur defects are formed during the thermal decomposition process which forms the CaLa₂S₄ powder. According to the semi-quantitative analysis based on EDS spectra measured in different areas of the powder, it is found that sulfur has an apparent deficiency compared with the value in the stoichiometry of CaLa₂S₄. Furthermore, it has also been reported that sulfur losses and deficiencies were detected in the material ​after hot pressing of CaLa₂S₄ ceramics in vacuum and argon.^29,30 Thus, the broad peaks at 474 nm and 532 nm are assumed be due to the recombinations of electrons from conduction band and sulfur vacancy with holes in elemental sulfur species on surface, respectively.³¹

Fig. 7 Photoluminescence emission spectrum of the synthesized CaLa₂S₄ powder measured under excitation at 365 nm at 25 °C.

Conclusions

In the present study, the ternary sulfide CaLa₂S₄ was successfully synthesized by using a wet chemistry method (single-source precursor method) followed by thermal decomposition at 1000 °C for 5 hours in flowing argon. Analysis of the reaction products shows that the synthesized powder consists of submicron-scale agglomerates of nanoparticles, with cubic CaLa₂S₄ as the main phase. The CaLa₂S₄ powder possesses a large specific surface area due to the combination of small particle size and the presence of mesopores, which have a narrow size distribution approximately at 23 nm. It appears that the synthesized CaLa₂S₄ powder exhibits the quantum size effect, which in turn leads to band gap enhancement. Photoluminescence characterization indicates that the CaLa₂S₄ powder exhibits a near-band-edge photoluminescence emission with a Stokes shift and two broad emission bands due to the sulfur defects.

Acknowledgements

We gratefully acknowledge the Office of Naval Research (ONR) (contract N00014-14-1-0546) for funding and supporting this research. This research used JEOL2100F TEM of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704.

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