Applying the Extraction–Pyrolysis Method to Modify Functional Materials Based on Metal Oxides

Abstract

The modification of functional materials with rare-earth elements (REEs) for different purposes by the low-temperature extraction–pyrolysis method has been carried out. Luminophors and magnetic and biomedical materials have been synthesized, and their compositions and properties have been studied. It has been established that the introduction of modifying additives (REE ions) could significantly improve the useful properties of the fabricated functional materials.

INTRODUCTION

At present, one can see high interest in oxide materials activated by rare-earth elements (REEs), which provide the former with unique properties and predetermine their active use. In particular, the field of application of luminescent materials in established technologies (the production of displays, lamps, X-ray detectors, etc.) is expanding [1], while new areas of their application are emerging: in advanced biomedicine, luminophors are applicable for biolabeling, optical visualization, and phototherapy [2]. Lanthanide ions often act as modifying additives for luminophors characterized with unique spectral properties, including long luminescence lifetimes and numerous and well-separated bands. The radiation spectrum for luminophors or the intensity of magnetization for multiferroics depend on the composition, crystal structure, and microstructure (particle sizes and morphology) of functional materials [3, 4]. Therefore, the production of nanomaterials with well-controlled dimensions, morphology, phase purity, chemical composition, and required preset properties remains one of the most difficult problems. The traditional method for producing complex oxide materials is a solid-phase synthesis. The disadvantage of the method consists of maintaining high synthesis temperatures. Compared to the solid-phase method, the extraction–pyrolysis method of synthesis of complex oxide materials enables one to decrease the duration and temperature of the process. Its efficiency for the production of ferrites of rare-earth elements was shown by the authors earlier [5]. This approach can be also successfully applied to the introduction of modifying additives into the composite material in any amounts [6]. In the present work, the results of applying the low-temperature extraction–pyrolysis method for introducing modifying additives in functional materials based on metal oxides were demonstrated.

EXPERIMENTAL

Metal-saturated REE extracts were obtained by mixing a benzene solution of acetylacetone (AA) and aqueous chloride or nitrate REE solutions in the presence of 1,10-phenanthroline or 2,2'-dipyridyl (DP). The pH of the aqueous phase was adjusted to 7.0–7.5 with a solution of NH₄OH. Cerium was extracted using a mixed benzene solution of capronic acid and AA, manganese(II) with a benzene solution of trioctylamine (TOA) from aqueous chloride solutions; niobium(V) and tantalum(V) were extracted with a benzene solution of trialkylbenzylammonium chloride (TABACH) and REEs for the production of phosphate-based composites were extracted with a solution of tributylphosphate (TBP).

The extraction was carried out at a temperature of 20 ± 2°C for 30 min with intensive stirring of the phases using a sieving machine, the ratio of the organic and aqueous phases was 1 : 1. For the synthesis of composites, the metal-saturated organic phase was separated, the saturated metal extracts were mixed in the required ratios, and then the solvent was distilled at 40–60°C. The precursors were pyrolyzed at the optimum temperature for each composite in the range 600–900°C in a muffle furnace.

RESULTS AND DISCUSSION

To illustrate the possibilities of the applied synthesis method, the conditions for obtaining modified functional materials are presented in Table 1.

Table 1.   Conditions for the production of functional materials

Silicates, oxysulfides, tantalates, vanadates, tungstates, etc., have been increasingly used as a matrix for REE modification. Special attention is paid to tantalates and niobates [7], which are promising materials with their own luminescence. The trivalent europium ion is a classic candidate for producing red-emitting luminophors.

Europium (EuTa₃O₉, EuTa₅O₁₄, and EuTa₇O₁₉) and terbium polytantalates (TbTa₇O₁₉), europium polyiniobates (EuNb₃O₉, EuNb₅O₁₄) and mixed yttrium (YNb_xTa_{1 – x}O₄ (x = 0.1; 0.3; 0.5; 0.7)), and gadolinium polytantalate niobates (GdNb_xTa_{1 – x}O₄ (x = 0.1; 0.3; 0.5; 0.9)) were synthesized from precursors mixed at the respective molar ratios. The luminescence characteristic of the Eu³⁺ ion was recorded for all samples of europium polytantalates and polyiniobates (Fig. 1).

Fig. 1.

Luminescence spectra: EuNb₅O₁₄ λ_ex = 238 nm (1); TbTa₇O₁₉ λ_ex = 378 nm (2); GdNb_0.3Ta_0.7O₄ λ_ex = 260 nm (3); YNb_0.1Ta_0.9O₄ λ_ex = 250 nm (4); 300 K.

Since the compositions of the obtained europium polyniobates and polytantalates in the studied pyrolysis temperature range from 600 to 900°C remained unchanged (in accordance with the data of X-ray diffraction analysis) while the character of the luminescence spectra of all samples at the same excitation wavelength did not change significantly, individual polyniobates and polytantalates were formed by a temperature of 600°C.

In mixed tantalate-niobates, when the tantalum atoms were partially or completely substituted by niobium atoms, the sensitivity of the host lattice to UV excitation increased and broad bands emerged in the luminescence spectra in the blue region with maxima at 415 and 450 nm for YNb_xTa_{1 – x}O₄ and GdNb_xTa_{1 – x}O₄, respectively (see Fig. 1). The TaO₄ or NbO₄ groups initiated blue recombination luminescence related to certain charge transfer transitions involving tetrahedral MeO₄ groups [8].

Activated materials based on rare-earth oxysulfides are widely used as high-performance luminescent and scintillation materials [9]. Luminophor based on europium and yttrium oxysulfides activated by terbium and praseodymium was obtained by the pyrolysis of a mixture of extracts of the corresponding metals and a sulfur solution in turpentine at the ratio of Eu : Tb : Pr : S : Y = 1 : 0.1 : 0.1 : 4 : 10. In order to select the optimal temperature of synthesis of functional materials, the effect of the synthesis temperature on one of the main characteristics—the luminescence intensity—was studied. The introduction of the yttrium activator and the praseodymium and terbium coactivators into europium sulfoxide resulted in an increase in the luminescence intensity of the Eu³⁺ ion by approximately five times, while the temperature of formation of this composite in comparison with europium oxysulfide increased insignificantly from 550 to 600°C (Fig. 2, curve 3). The luminophor obtained at this temperature was characterized by a maximum luminescence intensity, whereas a subsequent increase in the pyrolysis temperature, as in the case of europium sulfoxide, resulted in a decrease in luminescence intensity.

Fig. 2.

Luminescence spectra of the luminophors of europium, praseodymium, yttrium, and terbium oxysulfides obtained at the pyrolysis temperature: (1) 700, (2) 750, (3) 600, and (4) 650°С. λ_ex = 235 nm.

The photophysical properties of lanthanum phosphates, such as high thermal and photochemical stability, high refraction index, and weak solubility, make them ideal candidates for wide applications in different fields [10]. In LaPO₄, the La³⁺ ion can be substituted by other rare-earth ions (such as Ce, Eu, Dy, or Tb), which results in obtaining efficient luminescent materials. By means of the extraction–pyrolysis method, we obtained lanthanum-cerium phosphates (La_0.8Ce_0.15Tb_0.05PO₄ and La_0.8Ce_0.15Tb_0.05(PO₃)₃), which demonstrated intense green luminescence in the range 450–620 nm, while the intensity of the Tb³⁺ luminescence in the presence of La³⁺ and Ce³⁺ increased significantly, indicating an increase in the transfer of the excitation energy to the Tb³⁺ ion in the presence of activators lanthanum and cerium.

Doped manganites have become a subject of intensive study due to their potential applications in the field of data storage, magnetic cooling, and spintronics [11]. While lanthanum manganites doped with divalent ions of the general formula La_{1 – x}A_xMnO₃ (A = Ca, Sr, Ba, and Pb) have been known for almost 50 years, lanthanum manganites doped with alkali metal ions have been obtained recently [12]. Terbium and lanthanum manganites containing silver and potassium ions as modifying additives were synthesized by means of low-temperature pyrolysis of mixed extracts at the corresponding doping ion ratios: Tb_0.8Ag_0.2MnO₃ and La_{1 – x}K_xMnO₃, where x = 0.1, 0.15, and 0.185. The introduction of silver into terbium manganate led to the fact that the compound started exhibiting paramagnetic properties by room temperature. At a temperature decrease to 40–45 K, the studied compound transformed from a paramagnetic to a ferromagnetic state. The value of the coercive force at 10 K was 715 Oe. Thereafter, at 4 K, the transformation from the ferromagnetic to the antiferromagnetic state was observed in the Tb_0.8Ag_0.2MnO₃ sample. The study of the magnetic properties of modified lanthanum manganite showed that the increase in potassium content in the composition of La_{1 – x}K_xMnO₃ samples resulted in an increase in magnetization intensity.

Luminophors emitting in a wide range of the visible spectrum from 400 to 650 nm are also necessary for the development of advanced fields of medicines such as photodynamic therapy, which allows the painless removal of malignant neoplasms. These conditions are realized using the Eu(PO₃)₃:Eu²⁺ luminophor containing both the Eu³⁺ ion intensively luminescing in the range 600–700 nm and the Eu²⁺ ion characterized with a broad luminescence band in the range 400–500 nm obtained by extraction–pyrolysis after the annealing of precursors at the ratio Eu : TBP = 1 : 7 in a crucible at a temperature of 700–750°C for 1 h (Fig. 3).

Fig. 3.

Luminescence excitation spectra of (a) (λ_ex = 480 nm) and luminescence spectra (b) of europium phosphate Eu(PO₃)₃:Eu²⁺ (λ_ex = 353 nm) at 300 K.

The combined presence of different-valence europium ions was revealed in different luminophors during synthesis in a reducing atmosphere and sometimes without a reducing agent in vacuum [13]. However, the preparation of Eu²⁺-containing materials in air was preferable than the creation of a special reducing atmosphere. This significantly decreased the number of stages of the process. The decrease in the synthesis temperature of such materials at the application of the suggested method [14] enabled one to obtain nanoscale samples, which allowed the preparation of suspensions for use in photodynamic therapy.

CONCLUSIONS

The prospects of introducing modifying additives into functional materials by low-temperature extraction–pyrolysis have been demonstrated. Europium polytantalates (EuTa₃O₉, EuTa₅O₁₄, and EuTa₇O₁₉), terbium polytantalates (TbTa₇O₁₉), europium polyiniobates (EuNb₃O₉ and EuNb₅O₁₄), mixed yttrium (YNb_xTa_{1 – x}O₄ (x = 0.1; 0.3; 0.5; 0.7)) and gadolinium polytantalate niobates (GdNb_xTa_{1 – x}O₄ (x = 0.1; 0.3; 0.5; 0.9)), lanthanum–cerium phosphates activated by terbium (La_0.8Ce_0.15Tb_0.05PO₄ and La_0.8Ce_0.15Tb_0.05(PO₃)₃), terbium and lanthanum manganites containing silver and potassium ions as modifying additives (Tb_0.8Ag_0.2MnO₃ and La_{1 – x}K_xMnO₃ (x = 0.1, 0.15, and 0.185), and nanoluminophor (Eu(PO₃)₃:Eu²⁺) have been fabricated. It has been shown that the introduction of modifiers results in a significant improvement in the functional properties of materials.