Hydrothermal synthesis, characterization, and investigation of optical properties of Sb3+-doped lithium silicates nanoparticles

Synthesis of lithium metasilicate

LiNO ₃ (2 mmol) was added to a solution of silicic acid (4 mmol) in 30 mL of hot 0.01 M aqueous solution of NaOH while stirring. The resultant solution was stirred for further 15 min and then diluted to 60 mL. The obtained mixture was transferred to a 100-mL Teflon-lined stainless steel autoclave and heated for 48, 72, or 96 h at 180°C. The resulting nanomaterials of Li ₂ SiO ₃ were filtered and dried at 110°C.

Synthesis of lithium disilicate

The preparation of Li ₂ Si ₂ O ₅ was similar to that of Li ₂ SiO ₃ , except that LiNO ₃ (2 mmol) and silicic acid (6 mmol) were used. The obtained mixture was transferred to a 100-mL Teflon-lined stainless steel autoclave and heated for 48, 72, 96, or 120 h at 180°C. The resulting nanomaterials of compound Li ₂ Si ₂ O ₅ were filtered and dried at 110°C.

Synthesis of Sb3+-doped lithium metasilicate (x = 0.01, 0.0125, and 0.015)

Appropriate molar amounts of LiNO ₃ (7.9, 5.87, or 5.90 mmol), SiO ₂ .H ₂ O (15.88, 11.82, or 11.91 mmol, respectively), and Sb ₂ O ₃ (0.0387, 0.0398, or 0.0449 mmol, respectively) were dissolved in 60 mL of hot NaOH solution (0.53, 0.40, or 0.40 M, respectively) under magnetic stirring at 80°C. The resultant solution was transferred into a 100-mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 180°C for 48 h then allowed to cool naturally to room temperature, and the resulting white precipitate was recovered.

Synthesis of Sb3+-doped lithium disilicate (x = 0.005, 0.0075, and 0.01)

Appropriate molar amounts of LiNO ₃ (13.9, 9.9, or 7.92 mmol), SiO ₂ .H ₂ O (41.8, 29.81, or 23.92 mmol, respectively), and Sb ₂ O ₃ (0.035, 0.0377, or 0.0549 mmol, respectively) were dissolved in 60 mL of hot NaOH solution (1.40, 0.99, or 0.80 M, respectively) under magnetic stirring at 80°C. The resultant solution was transferred into a 100-mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 180°C for 48 h then allowed to cool naturally to room temperature, and the resulting white precipitate was recovered.

Powder X-ray diffraction analysis

The crystal phases of the synthesized samples were examined by powder X-ray diffraction technique. Figures ¹ and ² show the powder X-ray diffraction (PXRD) patterns of the Sb ³⁺ -doped lithium metasilicate and lithium disilicate, respectively. Also, the measured PXRD data for hydrothermally synthesized undoped lithium metasilicate and lithium disilicate nanoparticles are summarized in Tables ¹ and ² respectively. The measured powder XRD data are in good agreement with those of standard pure lithium metasilicate (JCPDS 29-0829) and lithium disilicate (JCPDS 15-0637), and the obtained stable phases are isostructural with Li ₂ SiO ₃ (space group Cmc2 ₁ )[ ³² – ⁴¹ ] and Li ₂ Si ₂ O ₅ (space group Ccc2)[ ⁴² – ⁴⁴ ]. Moreover, the intense sharp diffraction patterns suggest that the as-synthesized products are well crystallized. The doping limitations are 0-0.75 and 0-1.25 mole% of Sb ³⁺ for lithium metasilicate and lithium disilicate, respectively. However, for excess mole% concentration of the dopant agent in the reaction mixture, the impurity peaks were observed in the XRD patterns. The diffraction lines at 2 θ  ≈ 13°, 26° and 49° are assigned by their peak positions to the excess Sb ₂ O ₃ [ ³⁵ ]. As shown in Figures ¹ and ² , the formation of the other phases of lithium silicates and raw materials was already detected for higher mole percentage concentration of the dopant agent in the reaction mixture[ ⁴¹ , ⁴² ].

Figure 1

Figure 2

Compared to those of the nanoparticles of pure lithium silicates, the diffraction lines in the powder XRD patterns of the nanoparticles of Sb ³⁺ -doped lithium silicates shift to lower 2 θ values ( Δ 2 θ  = 26.99 (pure) – 26.81 (doped) = 0.15° and Δd = 3.322 Å (doped) − 3.304 Å (pure) = 0.018 Å for Sb ³⁺ -doped lithium metasilicate; and Δ2 θ  = 24.78 (pure) − 24.68 (doped) = 0.1°, Δd = 3.603 Å (doped) 3.589 Å (pure) = 0.014 Å for Sb ³⁺ -doped lithium disilicate are calculated via Bragg’s law ( n λ = 2d _hkl sin θ )). Since the ionic radius of the Sb ³⁺ (0.76 Å[ ⁴⁵ ]) is closer to the ionic radius of Li ⁺ (0.59 Å[ ⁴⁵ ]) rather than the Si ⁴⁺ (0.26 Å[ ⁴⁵ ]), in the Sb ³⁺ -doped lithium metasilicate and lithium disilicate, it may be expected that the dopant ion will replace with Li ⁺ ions. The larger radius of the dopant ion, as compared to the Li ⁺ , may cause an expansion of the lattice parameter in the Sb ³⁺ -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 replaces any Li ⁺ ions, where additional patterns will be observed in XRD pattern[ ²⁶ ]. However, here, the doped samples show similar powder XRD patterns without any residual or impurity phase formation. The powder XRD patterns of the doped samples suggest the fact that the dopant ions are indeed going to lattice positions rather than to interstitial positions. Moreover, when 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.

Morphology analysis

The scanning electron microscopy (SEM) images of the synthesized Li ₂ SiO ₃ nanomaterials are given in Figure ³ . After 48 h, nonuniform sheet like nanoparticles of Li ₂ SiO ₃ are obtained (Figure ³ a). The thickness, widths, and lengths of the resultant sheets are approximately 100 nm, 600 nm, and 2 μm respectively. With increasing the reaction time to 72 h, the morphology of the obtained materials has been changed to the very compact sheets with heterogeneous morphology (Figure ³ b). This is while, with the reaction time of 96 h, uniform flower like nanoparticles are obtained (Figure ³ c).

Figure 3

Figure ⁴ represents the SEM images of the synthesized Li ₂ Si ₂ O ₅ nanomaterials. After 48 h, the morphology of the obtained material is sponge-like, consisting of sheet-like and flower-like nanoparticles (Figure ⁴ a). With the increasing the reaction time to 72, 96, and 120 h, the morphology of the obtained materials has been changed to the rectangular sheets and high homogeny in the size is achieved. Using field emission scanning electron microscopy, the microstructure analysis of as-synthesized samples was investigated.

Figure 4

SEM images of the synthesized Sb ³⁺ -doped lithium metasilicates (Li _1.98 Sb _0.02 SiO ₃ ) and (Li _1.975 Sb _0.025 SiO ₃ ) are given in Figures ⁵ and ⁶ respectively. In Figure ⁵ a, with low magnification, it is revealed that the sample is composed of two dimensional continuous networks of nanosheet structures. With high magnification in Figure ⁵ b,c, it is clear that the length of the sheets are about 1 μm and the thickness of the sheets is about 100 nm. With high magnification, as shown in Figures ⁶ b,c, there are two types of flower-like structures (showing nonuniformity of the morphology). Low magnification in Figure ⁶ a,b,c shows that the size of each flower is about 3-5 μm and the thickness of the sheets is about 100 nm.

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Figure 6

Figures ⁷ and ⁸ show the SEM images of the synthesized Sb ³⁺ -doped lithium disilicate (Li _1.99 Sb _0.01 Si ₂ O ₅ ) and (Li _1.985 Sb _0.015 Si ₂ O ₅ ) respectively. Low magnification in Figure ⁷ a shows that the sample is composed of a large quantity of individual plates. High magnification images in Figure ⁷ b,c shows that the plates cut each other. The length of each plate is about 300-350 nm, and the thickness is 80-100 nm. The plates have regular multigonal shapes that are arisen from a substrate.

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Figure 8

Figure ⁸ a shows that with increasing the concentration of Sb ³⁺ from 0.5 to 0.75 mole%, the morphology of the samples change from nanoplates to nanoflower structures. The synthesized products are composed of a large quantity of individual nanobelts that form highly uniform 3-D flower-like structures. Each nanobelt has a width of about 500 nm and a length of about 2 μm. It seems that numerous nanodot particles cover all over the surface of each nanobelt.

Optical properties

The optical properties of the as-synthesized materials were measured at room temperature. Figure ⁹ shows photoluminescence spectra of the synthesized Sb ³⁺ -doped lithium metasilicate nanoflowers (λ _ex  = 232 nm). It is well known that the emission peak positions remain almost unchanged under different excitation wavelengths, showing that it is an intrinsic property of the compound[ ⁴⁴ ]. Under excitation with light at 232 nm, as shown in Figure ⁹ , the main emission peaks are located at 310 and 362 nm with a shoulder at 425 nm. The peak that appeared at 310 nm is assigned to the band edge emission. Also, the broad band with maxima at 362 nm is assigned to the trap state emission of the nanocrystals. With increasing dopant concentration, the intensity of the shoulder at 425 nm also increases. Considering that the energy gap of bulk lithium silicates is above 3.3 eV, the purple-blue photoluminescence 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[ ⁴⁶ , ⁴⁷ ]. The defects create some dangling bands. Therefore, the energy levels between conduction and valence bands (trap states) would contribute in the band structure, which result in a broad band at visible range.

Figure 9

The photoluminescence spectra of the synthesized Sb ³⁺ -doped lithium disilicate nanoflowers (λ _ex  = 227 nm) is given in Figure ¹⁰ . Compared to the described antimony-doped lithium metasilicate nanomaterials, the band edge emission of the synthesized Sb ³⁺ -doped lithium disilicate nanomaterials occurred at 310 nm upon excitation at 227 nm. Also, the trap state emission of the compound is observed as a weak band at 360 nm under this excitation wavelength. Moreover, compared to the synthesized antimony-doped lithium metasilicate nanomaterials, the emission band which is attributed to the oxygen-related defects in Sb ³⁺ -doped lithium disilicate nanoflowers (appeared as a shoulder at 425 nm) shows a decreasing intensity with the increasing dopant concentration. Electronic absorption spectra of the as-synthesized Sb ³⁺ -doped Li ₂ SiO ₃ and Li ₂ Si ₂ O ₅ nanomaterials have been given in Figure ¹¹ a,b. With increasing the dopant concentration in the structure of both of the synthesized Sb ³⁺ -doped lithium silicates, the band gap slightly shifts to lower energies. A band gap of 4.17 eV for Li _1.98 Sb _0.02 SiO ₃ and 4.13 eV for Li _1.975 Sb _0.025 SiO ₃ is calculated from the absorption spectroscopy. Also, a band gap of 4.49 eV and 4.46 eV, respectively, was measured for Li _1.99 Sb _0.01 Si ₂ O ₅ and Li _1.985 Sb _0.015 Si ₂ O ₅ from the electronic absorption spectroscopy.

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