Hydrothermal synthesis and investigation of optical properties of Nb⁵⁺-doped lithium silicate nanostructures

Abstract

The hydrothermal synthesis and optical properties of Nb⁵⁺-doped lithium metasilicate and lithium disilicate nanomaterials were investigated. The microstructures and morphologies of the synthesized Li_2−2x Nb_2x SiO_{3+ δ} and Li_2−2x Nb_2x Si₂O_{5+ δ} nanomaterials were studied with powder X-ray diffraction and scanning electron microscopy techniques, respectively. The synthesized niobium-doped lithium metasilicate and lithium disilicate nanomaterials, respectively, are isostructural with the standard bulk Li₂SiO₃ (space group Cmc2₁) and Li₂Si₂O₅ (space group Ccc2) materials. Photoluminescence spectra of the synthesized materials are studied. The measured optical properties show dependence on the dopant amounts in the structure.

Introduction

Lithium ceramics are of research interest because of their technological applications. Among these ceramics, Lithium silicates have been investigated as breeder materials for nuclear fusion reactors and as carbon dioxide absorbents in addition to other more well-known applications such as in thermal expansion glass–ceramics used in ceramic hobs [1–6]. The tetrahedral silicate ion (SiO₄ ²⁻), in the structure of silicates, provides good mechanical resistance and stability for the phosphor [7–11]. Lithium metasilicate and lithium disilicate, therefore, are suitable pyroelectric materials and used also in optical waveguide devices [12].

Synthesis of lithium silicate doped with La³⁺, Sm³⁺, Gd³⁺, Ho³⁺, Dy³ [19–22], Nd³⁺ [23], Na⁺ [24], Eu³⁺, Ce³⁺ and Tb³⁺ [25] ions has been reported previously. Also, Cu²⁺-doped [26], Cr⁴⁺-doped [27], Al³⁺-doped [28], Cr³⁺- and Tm³⁺-doped [29], V³⁺-, V⁴⁺- and V⁵⁺-doped [30] lithium silicates have been synthesized.

Recently, we have reported the hydrothermal synthesis and optical properties of Sb³⁺-doped lithium metasilicate and lithium disilicate nanomaterials [31]. However, to the best of our knowledge, no work has been devoted to niobium-doped lithium silicates. Doping of Nb⁵⁺ causes conductivity [13] and generates metallic behavior in insulators [14], increases electrical resistivity and enhances hysteresis squareness and fatigue behavior [16, 17], decreases the dielectric constant maximum and Curie point [18] and so on. Also, Nb can be considered as a donor dopant for PZT materials [15].

In this research work, we report the synthesis and optical properties of Li_2−2x Nb_2x SiO_{3+ δ} and Li_2−2x Nb_2x Si₂O_{5+ δ} nanomaterials under hydrothermal conditions. Also we have studied the effect of dopant amount on the morphology of the synthesized nanomaterials, while keeping other conditions unchanged. The effect of the dopant concentration on the morphology of the synthesized materials is investigated. Moreover, optical properties of the synthesized Li_2−2x Nb_2x SiO₃ and Li_2−2x Nb_2x Si₂O₅ nanomaterials are studied. The synthesized materials’ optical and catalytical properties were improved by doping Nb⁵⁺ in lithium silicates so they are applicable in fabrication of optical devices and also as catalysts.

Methods

All the reagents used in the experiments were of analytical grade, and used as received without further purification. Nb⁵⁺-doped lithium metasilicate and lithium disilicate nanomaterials are synthesized in a one-step hydrothermal process.

Synthesis of niobium-doped lithium metasilicate (Li_2−2x Nb_2x SiO_3+δ ) (x = 0.0025, 0.005)

Appropriate molar amounts of LiNO₃ (MW = 68.95 g mol⁻¹) (10 and 11.9 mol, respectively), SiO₂·H₂O (MW = 96.11 g mol⁻¹) (20 and 23.92 mol, respectively) and Nb₂O₅ (MW = 265.815 g mol⁻¹) (0.0263 and 0.06 mol, respectively) were dissolved in 60 mL of hot NaOH solution (0.67 and 0.80 M solution, respectively) under magnetic stirring at 80 °C. The resultant solution was transferred and sealed in a Teflon-lined stainless steel autoclave of 100 mL capacity, under autogenous pressure and heated to 180 °C for 96 h. The autoclave was then allowed to cool naturally to room temperature and the resulting white precipitate was recovered.

Synthesis of niobium-doped lithium disilicate (Li_2−2x Nb_2x Si₂O_5+δ ) (x = 0.005, 0.0075 and 0.01)

Appropriate molar amounts of LiNO₃ (MW = 68.95 g mol⁻¹) (11.9, 10 or 9.9 mol, respectively), SiO₂·H₂O (MW = 96.11 g mol⁻¹) (35.9, 30.22 or 30 mol, respectively) and Nb₂O₅ (MW = 265.815 g mol⁻¹) (0.06, 0.073 or 0.1 mol, respectively) were dissolved in 60 mL of hot NaOH solution (1.20, 1.0 and 1.0 M solution, respectively) under magnetic stirring at 80 °C. The resultant solution was transferred and sealed in a Teflon-lined stainless steel autoclave of 100 mL capacity, under autogenous pressure and heated to 180 °C for 96 h. The autoclave was then allowed to cool naturally to room temperature and the resulting white precipitate was recovered.

Results and discussion

Powder X-ray diffraction analysis

Phase identifications were performed on a powder X-Ray diffractometer Siemens D5000 using Cu-K_α radiation. The morphology of the obtained materials was examined with a Philips XL30 Scanning Electron Microscope equipped with energy-dispersive X-ray (EDX) spectrometer. Absorption and photoluminescence spectra were recorded on a Jena Analytik Specord 40 and a Perkin Elmer LF-5 spectrometer, respectively.

Figure 1a, b, respectively, shows the EDX spectra of the synthesized Nb⁵⁺-doped lithium metasilicate and lithium disilicate nanomaterials, which verify the doping and the compositional analysis of Nb⁵⁺ in the nanoparticles of lithium silicates.

Fig. 1

EDX spectra of the hydrothermally synthesized a Li_1.995Nb_0.001SiO_3+δ and b Li_1.985Nb_0.003Si₂O_5+δ nanoparticles

The crystal phases of the synthesized materials were examined by powder X-ray diffraction technique. Figures 2 and 3 show the powder XRD patterns of the Nb⁵⁺-doped lithium metasilicate and lithium disilicate, respectively. The measured powder XRD data are in good agreement with those of corresponding undoped lithium metasilicate or lithium disilicate nanomaterials [31] and the obtained stable phases are, respectively, isostructural with Li₂SiO₃ (space group Cmc21) [31–41] and Li₂Si₂O₅ (space group Ccc2) [31, 42–44]. The measured data are in agreement with the respective Joint Committee on Powder Diffraction Standards (JCPDS) card for Li₂SiO₃ (JCPDS 29-0829) (a = 9.3808 Å, b = 5.3975 Å and c = 4.6615 Å) and for Li₂Si₂O₅ (JCPDS 15-0637) (a = 5.825 Å, b = 14.56 Å and c = 4.796 Å). The standard crystallographic data for lithium metasilicate (JCPDS 29-0829) and lithium disilicate (JCPDS 15-0637) and the powder XRD data for respective hydrothermally synthesized undoped nanomaterials [31] are summarized in Tables 1 and 2, respectively. Also, the powder XRD data for respective hydrothermally synthesized Nb-doped lithium metasilicate and Nb-doped lithium disilicate are summarized in Tables 3 and 4 for comparisons. Moreover, the intense sharp diffraction patterns suggest that the as-synthesized products are well crystallized.

Fig. 2

PXRD patterns of the hydrothermally synthesized Li_2−2x Nb_0.4x SiO_3+δ nanomaterials where a x = 0.0025, b x = 0.005 and c x = 0.01

Fig. 3

PXRD patterns of the hydrothermally synthesized Li_2−2x Nb_0.4x Si₂O_5+δ nanomaterials where a x = 0.005, b x = 0.0075, c x = 0.01

Table 1 Crystallographic data of the hydrothermally synthesized Li₂SiO₃ nanomaterials obtained after 96 h at 180 °C

Table 2 Crystallographic data of the hydrothermally synthesized Li₂Si₂O₅ nanomaterials obtained after 120 h at 180 °C

Table 3 Crystallographic data of the hydrothermally synthesized Nb⁵⁺-doped Li₂SiO₃ nanomaterials obtained after 96 h at 180 °C

Table 4 Crystallographic data of the hydrothermally synthesized Nb⁵⁺-doped Li₂Si₂O₅ nanomaterials obtained after 96 h at 180 °C

The doping limitations are 0–0.25 and 0–0.75 mol% of Nb⁵⁺ for lithium metasilicate and lithium disilicate, respectively. Excess mol% concentration of the dopant agent in the reaction mixture, as shown in Figs. 2 and 3, results in impurity peaks in the XRD patterns. The diffraction line at 2θ ≈ 49° is assigned by its peak position to the excess Nb₂O₅ [43]. Moreover, the formation of other phases of lithium silicates and raw materials was already detected for higher mol% concentration of the dopant agent in the reaction mixture (Figs. 2, 3) [31, 41, 42, 48].

Compared to those of the nanomaterials of undoped lithium silicates, the diffraction lines in the powder XRD patterns of the Nb⁵⁺-doped lithium silicates nanomaterials shift to lower 2θ values and, therefore, to larger d values. For the most intensive diffraction line (200) a diffraction line shift of ∆2θ = 18.881° (pure)−18.80° (doped) = 0.081° (∆d = 4.7206 Å (doped)−4.7005 Å (pure) = 0.0201 Å) for Nb⁵⁺-doped lithium metasilicate and for the most intensive diffraction line (040) a diffraction line shift of ∆2θ = 24.78° (pure)−24.70° (doped) = 0.08° (∆d = 3.600Å (doped)−3.589 Å (pure) = 0.011 Å) for Nb⁵⁺-doped lithium disilicate are calculated via Bragg’s law. Tables 5 and 6 show the crystal sizes of the Nb-doped materials in different dopant amounts via Debye–Scherrer equation.

Table 5 Debye–Scherrer data information for pure and Nb⁵⁺-doped Li₂SiO₃ nanomaterials

Table 6 Debye–Scherrer data information for pure and Nb⁵⁺-doped Li₂Si₂O₅ nanomaterials

Since the ionic radius of the Nb⁵⁺ (0.64Å [46]) is closer to the ionic radius of Li⁺ (0.59Å [46]) rather than the Si⁴⁺ (0.26Å [46]), in the Nb⁵⁺-doped lithium metasilicate and lithium disilicate, it may be expected that the dopant ion will replace with Li⁺ ions in the structure. The larger radius of the dopant ion, compared to the Li⁺, may cause an expansion of the lattice parameter in the Nb⁵⁺-doped lithium silicate nanomaterials. Since both ionic radii and charges are not the same for the dopant and Li⁺ ions, it is also possible that the dopant ion takes an interstitial position in lattice rather than replacing any Li⁺ ions, where additional patterns will be observed in XRD pattern [47]. However, here, the powder XRD data measured for the doped samples are in accordance with those of the undoped materials without any residual or impurity phase formation. The powder XRD patterns of the doped samples, therefore, suggest the fact that the dopant ions are indeed going to lattice positions rather than interstitial positions.

Moreover, on replacing Li⁺ ions, the dopant ions are bound to create some oxygen-related defect centers or Li⁺ vacancies for charge compensation. Therefore, it is believed that the dopant ions will be in a structurally disordered environment.

Cellref version 3 was used to refine the cell parameters from the measured powder XRD data of the synthesized doped nanomaterials. Compared to the standard crystallographic data for lithium metasilicate (JCPDS 29-0829) and lithium disilicate (JCPDS 15-0637), the refined unit cell parameters of the synthesized Nb-doped lithium metasilicate and lithium disilicate nanomaterials are a = 9.3702 Å, b = 5.3994 Å, c = 4.6643 Å and a = 5.826 Å, b = 14.6168 Å, c = 4.878 Å, respectively.

Microstructure analysis

SEM images of the pure lithium metasilicate and lithium disilicate are present in our previous work [31]. Figure 4 shows typical SEM images of the synthesized Li_1.995Nb_0.001SiO_{3+ δ} nanoparticles. The synthesized sample is composed of multi-ply sheets (thickness and length of about 100 nm and 5 μm, respectively) join together to form nano-flowers. Typical SEM images of the synthesized Li_1.99Nb_0.002Si₂O_{5+ δ} and Li_1.985Nb_0.003Si₂O_{5+ δ} are given in Figs. 5 and 6, respectively. The synthesized Li_1.99Nb_0.002Si₂O₅ nanomaterial is composed of plate-like nanoparticles with homogenous dispersion (Fig. 5b, c). The length of the nano-plates is approximately 0.7–0.8 μm. As shown in Fig. 6, with increasing the dopant concentration in the structure to x _Nb = 0.0075, the resultant nano-plates assemble to each other to form nano-flower-like structures. The length and thickness of the nano-plates are estimated to be 500 and 80–100 nm approximately.

Fig. 4

SEM images of the hydrothermally synthesized Li_2−2x Nb_2xSiO_3+δ (x = 0.0025) nano-flowers

Fig. 5

SEM images of the hydrothermally synthesized Li_2−2x Nb_2x Si₂O_5+δ (x = 0.005) nanoparticles

Fig. 6

SEM images of the hydrothermally synthesized Li_2−2x Nb_2x Si₂O_5+δ (x = 0.0075) nano-flowers

Optical properties

The emission spectra of pure Li₂SiO₃ and Li₂Si₂O₅ are shown in Figs. 7 and 8. In the excitation spectrum of the synthesized Li₂SiO₃ and Li₂Si₂O₅ nanomaterials, a band is observed with maxima at 360 and 250 nm, respectively. Accordingly, in the emission spectrum of the synthesized Li₂SiO₃ nanomaterials, an intense peak appears at 410.03 nm. In comparison, an intense peak at 291.45 nm is observed in the emission spectrum of the synthesized Li₂Si₂O₅ nanomaterials. With increasing in the reaction time, no shift is observed in the emission spectrum of the obtained Li₂SiO₃ and Li₂Si₂O₅ nanomaterials. However, increasing band intensities in the emission spectra of both compounds are observed with increasing reaction time. In the emission spectrum of Nb⁵⁺-doped lithium metasilicate nano-flowers (Fig. 9), under excitation with light at 234 nm, the main emission band is located at 360 nm with shoulders at 310, 340 and 425 nm. The shoulder appeared at 310 nm is assigned to the band edge emission. Also, the broad band with maxima at 360 nm and the shoulder at 340 nm are assigned to the trap state emission of the nanoparticles. Considering that the energy gap of bulk lithium silicates is above 3.3 eV, the purple-blue photoluminescence appeared as a shoulder at 425 nm (approximately 2.92 eV) is probably due to a triplet to ground state transition of a neutral oxygen vacancy defect, as suggested by ab initio molecular orbital calculations for many other well-studied metal oxides. Also, the emission band related to the Nb(V) centers in the structure is expected to be superimposed on the shoulder at 425 nm [44]. In comparison, the synthesized Nb⁵⁺-doped lithium disilicate nanoparticles exhibit an intense broad emission band (λ _ex = 229 nm) at 420 nm (~2.95 eV) (Fig. 10) assigned to the oxygen-related defects and Nb⁵⁺ centers in the structure, which shows an increasing intensity with increasing the dopant concentration in the structure [45].

Fig. 7

Emission spectrum of the hydrothermally synthesized Li₂SiO₃ nanomaterial (λ _ex = 332 nm)

Fig. 8

Emission spectra of the hydrothermally synthesized Li₂Si₂O₅ nanomaterials (λ _ex = 231 nm)

Fig. 9

Emission spectrum of the hydrothermally synthesized Li_2−2x Nb_2x SiO_3+δ (x = 0.0025) nano-flowers (λ _ex = 234 nm)

Fig. 10

Emission spectra of the hydrothermally synthesized Li_2−2x Nb_0.4x Si₂O₅ nanomaterials where a x = 0.005 (λ _ex = 229 nm), b x = 0.0075 (λ _ex = 229 nm)

Conclusion

In summary, nano-plates and nano-flowers of Nb⁵⁺-doped lithium metasilicate and lithium disilicate were synthesized successfully by employing a simple hydrothermal method. The molar ratio of Li:Si and the dopant concentration in the reaction mixture affect the crystal phase and morphology of the final product, respectively. The synthesized Nb-doped stable phases are isostructural with the corresponding undoped Li₂SiO₃ or Li₂Si₂O₅ materials. The synthesized nanomaterials exhibited emerging PL optical properties in the UV–visible region which shows dependence on the dopant amounts in the structure. These materials are expected to have potential application in light-emitting devices and as catalysts.