Blue-silica by Eu²⁺-activator occupied in interstitial sites

Abstract

A blue-emitting SiO₂:Eu²⁺ compound has been successfully synthesized and characterized. The PL intensity of SiO₂:Eu_0.002²⁺ compound is about 24 times higher than that of the O-defective SiO₂ compound (without activators), which emits blue light. The valence state of the Eu ions responsible for the highly enhanced blue emission was determined to be Eu²⁺ using reference materials (EuCl₂ and EuCl₃) and XPS measurements. The Eu²⁺-activator ions occupied in the interstitial sites of the SiO₂ matrix were confirmed by FT-IR, XPS, and ²⁹Si MAS-NMR spectroscopy. Even though the void spaces formed structurally in both α-quartz and α-cristobalite can accommodate Eu²⁺ ions (ionic radius = 1.25 Å at CN = 8), SiO₂:Eu²⁺ compound fired at 1300 °C under a hydrogen atmosphere is destined to be deficient in O or Si atoms, indicating the formation of the wider void spaces in the SiO₂ crystal lattice. A sputtered depth profile of SiO₂-related compounds obtained by time-of-flight secondary ion mass spectrometry (TOF-SIMS) corroborates the O-defective SiO₂ induced by hydrogen. In particular, the interatomic potentials, depending on the interstitial positions of Eu atoms in α-cristobalite and α-quartz, are calculated based on Lennard-Jones and coulomb potentials. For α-cristobalite, the minimum potential value is −51.47 eV and for α-quartz, the value is 221.8 eV, which reveals that the Eu²⁺-activator ions more preferably enter the interstitial sites of α-cristobalite than those of α-quartz. Thanks to the stable Eu²⁺-activator ions enclosed by Si–O linkages, the SiO₂:Eu_0.002²⁺ compound emits blue light and its PL emission intensity is about 24 times higher than that of the O-defective SiO₂ compound. This phosphor material could be a platform for modeling a new phosphor and for application in the solid-state lighting field.

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1 Introduction

Silicone (27.2 wt%) is the most abundant element in the earth's crust after oxygen (45.5%), and silicone never occurs free but invariably occurs combined with 4-coordinated oxygen in nature. The [SiO₄] unit may occur as an individual group or be linked into chains, ribbons, sheets, or three-dimensional frameworks.¹ Silica, or silicon dioxide (SiO₂) naturally occurs in both crystalline and amorphous forms. The various forms of the crystalline silica are α-quartz, β-quartz, α-tridymite, β-tridymite, α-cristobalite, β-cristobalite, keatite, coesite, stishovite, and moganite.² The most abundant form of silica is α-quartz, which is the most thermodynamically stable form of the crystalline silica in ambient conditions. Quartz has been used for a long time (several thousand years) in jewelry as a gem stone, and it is used extensively in electronics as well as the optical component industries.³ It is very interesting to note that Zolensky et al. reported blue quartz phenocrysts from Llano rhyolite (llanite), Llano County, Texas, and derived its coloration from Rayleigh scattering by abundant submicrometer-sized ilmenite inclusions, without experimental evidence.⁴ According to their results, the concentrations of rare-earth element (REE) in blue quartz were determined as 0.910 ppm (Ce), 0.108 ppm (Sm), 0.016 ppm (Eu), 0.029 ppm (Tb), 0.332 ppm (Yb), and 0.064 ppm (Lu). It is well-known that in phosphor materials, various activators, such as Nd³⁺, Pr³⁺, Sm³⁺, Eu³⁺, Eu²⁺, Ce³⁺, Tb³⁺ and Yb³⁺, have been widely used for flat panel displays and optoelectronic devices.^5–8 Among them, the divalent europium ions (Eu²⁺) have been considered as a very important and useful activator, which exhibits broad emission bands between the ultraviolet (UV) and red spectral ranges, associated with the 4f⁶5d¹–4f⁷ transition.^9–11 Considering various activator ions, it is presumed that the origin of blue coloration in quartz from Llano rhyolite (llanite), Llano County, Texas, may be due to the Eu²⁺ or Ce³⁺ ions occupying the crystal lattice of quartz. Zolensky et al., however, argued that the blue quartz originates from Rayleigh scattering by the ilmenite inclusions from the large difference in the ionic radii of Si⁴⁺ (0.40 Å) and Eu²⁺ (1.31 Å), precludes direct substitution.⁴ It is very interesting that the blue emission induced by the O-related defects (without activator ions) in SiO₂-related compounds was reported by several groups.^12–18 Uchino et al. proposed that the blue-emitting center in oxidized porous silicone and nano-SiO₂ materials is a metastable defect pair consisting of =Si(O₂) and =Si. On the basis of the density functional theory calculation, which was in good agreement with the peak positions of the PLE from the blue-emitting materials.¹⁴ McCrate et al. also presented that the intrinsic oxygen vacancy defect (OVD) on planar fused silica was experimentally detected by titration with fluorescent probe molecules such as perylene-3-methanamine and 3-vinyl perylene.¹⁸ Blue emission by Ce³⁺ ions occupied in the interstitial site of the AlN structure was reported by Liu et al.¹⁹ According to their results, an octahedral interstitial site of the AlN compound with a würtzite structure was proposed to be the site for Ce³⁺ ions because the ionic size of Ce³⁺ ions is much larger than that of Al³⁺. It should be noted that there are interstitial sites formed structurally in the crystal lattice of α-quartz (or α-cristobalite) and possibilities for Ce³⁺ or Eu²⁺ activator ions to enter the interstitial sites of α-quartz (or α-cristobalite), which probably results in the blue coloration in α-quartz (or α-cristobalite). In this study, we report the synthesis and characterization of a blue silica induced by Eu²⁺-activator ions occupied in interstitial sites. In particular, the Eu²⁺-doping mechanism in the SiO₂ matrix is discussed using a photoluminescence (PL) apparatus, Rietveld refinements, Nano-Secondary Ion Mass Spectrometry (Nano-SIMS), X-ray photoelectron spectroscopy (XPS), ²⁹Si magic-angle-spinning nuclear magnetic resonance (MAS-NMR) spectrometer, and theoretical calculations of the interatomic potential based on Lennard-Jones potential and coulomb potential.

2 Experimental

The polycrystalline SiO₂:Eu compounds have been prepared from a mixture of SiO₂ (quartz) and Eu₂O₃ using NH₄Cl as a flux and lubricant under 4% H₂–Ar mixture gas between 1000 °C and 1500 °C for 6 h. Alumina boats covered with lids were used to maximize collision time among the starting materials (SiO₂, NH₄Cl, and Eu₂O₃), and to prevent the loss of unreacted NH₄Cl from a rapid out-gassing. In the firing step, each mixture was initially heated at 450 °C for 1 h, and then subsequently heated at a final temperature for 3 h (heating rate: 3 °C min⁻¹). Powder X-ray diffraction measurements of SiO₂:Eu were carried out using an X-ray diffractometer equipped with a graphite monochromator (DMAX-2200PC, Rigaku). A step scan mode was employed in a 2θ range of 10–110° with a step size of 0.02° and counting time of five second for each step. Structure refinements were carried out by the Rietveld method using the Fullprof program²⁰ with pseudo-Voïgt peak shapes and refined backgrounds. The photoluminescence (PL) spectra were obtained at room temperature using a fluorescent spectrophotometer with a 150 W xenon lamp under an operating voltage of 350 V (Fluorometer FS-2, Scinco). The reflectance spectra were obtained using a UV-visible spectrophotometer (UV-2600, Shimadzu) with BaSO₄ as a reference. Fourier-transform infrared spectroscopy (FT-IR) was performed using a FT-IR spectrophotometer (IRTracer-100, Shimadzu) with a resolution range of ±0.5 cm⁻¹ and a KBr medium prepared with 0.5 wt% of sample. The Nano-Secondary Ion Mass Spectrometry (NanoSIMS) measurements were done using a rastering Cs⁺ primary ion beam on the Cameca Ametek NanoSIMS 50L at the Arizona State University. The beam current at the sample was lowered to ∼0.48 pA by choosing a small diaphragm to obtain a fine beam. Although a low current was used for analysis, the new capillary Cs source produced a high primary beam density and high secondary ion count rates during imaging. Negative secondary ions of ²⁴Si⁻ and ¹⁶O⁻ were measured simultaneously using electron multipliers in the multi-collection mode. The measurement of NanoSIMS for all samples was performed after pre-sputtering for 5 min. All samples for the NanoSIMS measurement were prepared as follows: 500 mg of sample was dispersed in 5 mL of absolute ethanol by ultrasound sonication for 10 min. The obtained milky suspension after sonication was dripped by a pipet onto a stainless-steel substrate (1 cm diameter) and dried under ambient conditions. This was repeated until a white thick coating was obtained on the substrate. The oxidation states of the elements were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250) with a monochromatic Al K_α X-ray source (hν = 1486.6 eV) at the Busans Center of Korea Basic Science Institute (KBSI). The obtained binding energies (BEs) were calibrated with that of an adventitious carbon (C 1s) core level peak at 284.6 eV as a reference. All ²⁹Si single-pulse magic angle spinning (SP MAS) spectra were acquired using a Unity INOVA 600 MHz NMR instrument (Agilent Technologies, USA) with a cross polarization (CP) MAS probe for 5 mm zirconia rotors at room temperature, a 30° pulse corresponding to 2 μs, a pulse repetition delay of 20 s, a spectral width of 120 kHz, and a spinning rate of 10 kHz. The chemical shifts were referenced to external tetramethylsilane at 0 ppm.

3 Results and discussion

3.1 Structural characterization

Structural refinements of the samples were carried out using a Rietveld program. The background was fitted with a polynomial function and the peak shape was modelled using a pseudo-Voïgt function. The observed, calculated, and difference patterns from the Rietveld refinements of the X-ray diffraction data are shown in Fig. 1. The profile of undoped SiO₂ fired at 1300 °C under a 4% H₂–Ar atmosphere contains α-cristobalite as the major phase (90%) and quartz as the second phase (10%). The atomic coordinates of the both structures were initially selected from a data base (α-cristobalite: ICSD #75300, α-quartz: ICSD # 647436). The relative proportion of phases, final structural parameters (lattice parameters, refined atomic positions, and isotropic temperature factors for all atoms) and reliability factors are listed in Table 1. In comparison with the undoped SiO₂, the Eu-doped SiO₂ compound showed no substantial difference in the crystal structure except for the temperature factors. For instance, the isotropic temperature factors, B_iso of Si and O atoms are 1.18(3) Ų and 1.36(6) Ų for the α-cristobalite phase in undoped SiO₂, while those are 1.39(3) Ų and 1.78(6) Ų in doped SiO₂:Eu_0.002. The large temperature factors for SiO₂:Eu_0.002 imply that the Eu doping into SiO₂ leads to the increase in the structural disorder. Table 2 shows the weight percentages (%) of α-quartz and α-cristobalite as a function of firing temperature and Eu content. It is evident that the weight% of α-cristobalite is increased with increasing firing temperature and Eu content.

Fig. 1 Rietveld refinement of the powder XRD profiles for SiO₂:Eu_0.002 (a) and undoped SiO₂ (b). The diffraction pattern is composed of the peaks from α-cristobalite as a main phase and α-quartz as a minor phase. Measured data, fitted results, expected reflection positions, and the difference between measured and fitted results are expressed as red open circles, black solid lines, black vertical lines, and blue solid lines, respectively. The peak intensities over 40 degrees in 2θ have been magnified 10 times for clarity.

Table 1 Relative proportion of phases, structural parameters and reliability factors for SiO₂:Eu_0.002 and SiO₂

Compound		SiO2:Eu0.002		SiO2:No-Eu
Phase		α-Cristobalite	α-Quartz	α-Cristobalite	α-Quartz
Weight present (%)		89.2(4)	10.8(13)	90.1(5)	9.9(17)
Rwp (%)		11.1		13.0
RBragg		2.63	7.60	2.89	6.69
Rf (%)		2.98	4.53	3.17	4.61
χ^2		1.51		2.12
Space group		P41212 (no. 92)	P3221 (no. 154)	P41212 (no. 92)	P3221 (no. 154)
Lattice	a	4.9768(2)	4.9030(5)	4.9732(3)	4.9041(6)
Parameter (Å)	c	6.9490(4)	5.3831(8)	6.9362(4)	5.3851(9)
Cell volume (Å^3)		172.12(14)	112.07(3)	171.55(16)	112.16(3)

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Compound	SiO2:Eu0.002 (α-cristobalite)	SiO2:No-Eu (α-cristobalite)		SiO2:Eu0.002 (α-quartz)	SiO2:No-Eu (α-quartz)
Si, 4a			Si, 3a
x	0.2967(2)	0.2966(2)	x	0.470(2)	0.469(2)
y	0.2967(2)	0.2966(2)	y	0	0
z	0	0	z	2/3	2/3
Site occupancies	1	1	Site occupancies	1	1
Biso (Å^2)	1.39(3)	1.18(3)	Biso (Å^2)	0.5(1)	0.4(1)
O, 8b			O, 6c
x	0.2412(4)	0.2428(5)	x	0.439(2)	0.433(2)
y	0.0985(4)	0.1012(5)	y	0.279(2)	0.276(2)
z	0.1756(3)	0.1757(3)	z	0.786(2)	0.783(2)
Site occupancies	1	1	Site occupancies	1	1
Biso (Å^2)	1.78(6)	1.36(6)	Biso (Å^2)	2.1(3)	1.7(4)

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Table 2 Unit cell parameters and phase content between α-quartz and α-cristobalite in silicas fired under a 4% H₂–Ar atmosphere

Compound	α-Cristobalite (weight%)	a (Å)	c (Å)	α-Quartz (weight%)	a (Å)	c (Å)
SiO2 (Aldrich chemical)	—	—	—	100%	4.91202(13)	5.40345(17)
SiO2 (1500 °C)	100%	4.97114(19)	6.9254(3)	—	—	—
SiO2 (1300 °C)	90.03%	4.9731(3)	6.9362(4)	9.97%	4.9043(6)	5.3844(10)
SiO2:Eu0.002 (1300 °C)	89.20%	4.9767(2)	6.9491(4)	10.80%	4.9035(6)	5.38214
SiO2:Eu0.05 (1300 °C)	99.07%	4.9733(2)	6.9341(3)	0.93%	4.90(5)	5.38(9)

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3.2 Photoluminescence spectra

Fig. 2 presents the PL spectra of SiO₂:Eu_0.01 synthesized under a 4% H₂–Ar atmosphere as a function of firing temperature. The highest excitation and emission intensity are observed in the SiO₂:Eu_0.01 compound synthesized at 1300 °C that is an optimal firing temperature. The excitation spectra of SiO₂:Eu_0.01 compounds monitored at 438 nm, consist of broad bands between 220 nm and 420 nm, which may be ascribed to the allowed 4f⁷–4f⁶5d transitions of Eu²⁺.^21–23 The emission spectra monitored under the 323 nm excitation show symmetric bands centered around 438 nm, which is associated with blue-emission. The excitation and emission spectra of SiO₂:Eu_x compounds synthesized at 1300 °C as a function of Eu content are shown in Fig. 3. The PL maximum intensity is observed at x = 0.002, then decreased with increasing Eu concentration. The optimum concentration of Eu in the SiO₂:Eu compound is 0.002, which implies that SiO₂ crystal lattice is difficult to accommodate the large amount of Eu²⁺-activator ions. It should be noted that NH₄Cl is widely used as a flux and lubricant chemical in solid-state reactions because it has a relatively low melting point (340 °C) and boiling point (520 °C). In this study, NH₄Cl plays an important role in SiO₂:Eu compounds as a flux and lubricant, which forces Eu²⁺ to enter the interstitial sites of the SiO₂ matrix. As shown in Fig. 4, the PL intensity of SiO₂:Eu_0.002²⁺ is increased with the higher mole ratio between NH₄Cl and SiO₂ (mole (NH₄Cl)/mole (SiO₂)). The maximum PL intensity is observed at the mole ratio 4. It can be noted that the PL pattern of SiO₂ (No-Eu, 4 mole NH₄Cl) is similar to that of SiO₂:Eu_0.002²⁺ except the difference of the PL intensities, indicating the blue emissions induced by the O-related defects (without activator ions) in SiO₂-related compounds^12–18 as mentioned in the introduction. It is very remarkable that the PL intensity of SiO₂:Eu_0.002²⁺ is about 24 times as high as that of SiO₂ (No-Eu, 4 mole NH₄Cl) fired at 1300 °C under a 4% H₂–Ar atmosphere, which is probably due to the Eu ions occupying the SiO₂ matrix. Fig. 5 presents the PL spectra of SiO₂:Eu_x²⁺ (x = 0.002, 0.005, and 0.010) with and without NH₄Cl addition. For SiO₂:Eu_0.002²⁺ compound, it prominently shows that the emission intensity (at 438 nm) of the resulting materials after NH₄Cl addition is 1.4 times higher than that of the resulting materials without NH₄Cl addition. This result implies that NH₄Cl shows a considerable effect on the PL intensity and plays an important role as a flux and lubricant in SiO₂:Eu_x²⁺ compounds. Fig. 6 presents the diffuse reflectance spectra of SiO₂ and SiO₂:Eu_x²⁺ (x = 0.002 and 0.05). When Eu²⁺ ions are occupied into the SiO₂ matrix, the broad absorption bands are shown between 220 nm and 450 nm, whereas absorption bands are not shown in α-quartz and α-cristobalite in the region. This may indicate the broad absorption bands between 220 nm and 450 nm are associated with the 4f → 5d transition of Eu²⁺. Moreover, the absorption bands of SiO₂:Eu_0.002²⁺ are still stronger than that of SiO₂:Eu_0.05²⁺, which is consistent with the PL previous results, as shown in Fig. 3. The luminescent behavior of the SiO₂:Eu_0.002²⁺ compound synthesized at 1300 °C under a 4% H₂–Ar atmosphere with the mole ratio = 4 (mole (NH₄Cl)/mole (SiO₂)) was compared with a commercial BaMgAl₁₀O₁₇:Eu²⁺ (BAM:Eu²⁺) phosphor (obtained from Nichia corp., Japan). The excitation and emission spectra of SiO₂:Eu_0.002²⁺ and BAM:Eu²⁺ phosphor are shown in Fig. 7. The excitation and emission spectra are very similar except for the difference in the PL intensity. The relative emission intensity of SiO₂:Eu_0.002²⁺ monitored at 323 nm is about 40% compared to a commercial BAM:Eu²⁺. The Commission International de I'Eclairage (CIE) coordinates of SiO₂:Eu_0.002²⁺ and BAM:Eu²⁺ monitored under UV light at 300 nm are x = 0.145, y = 0.068 and x = 0.143, y = 0.065, respectively, as shown in the inset of Fig. 7. According to CIE coordinates, the SiO₂:Eu_0.002²⁺ compound emits deep blue.

Fig. 2 Excitation and emission spectra of SiO₂:Eu_0.01 as a function of firing temperature.

Fig. 3 Excitation and emission spectra of SiO₂:Eu_x²⁺ as a function of Eu content.

Fig. 4 Excitation and emission spectra of SiO₂:Eu_0.002²⁺ as a function of NH₄Cl mole.

Fig. 5 Excitation and emission spectra of SiO₂:Eu_x²⁺ with and without NH₄Cl (4 moles).

Fig. 6 Diffuse reflectance spectra of SiO₂-related compounds.

Fig. 7 PL spectra of SiO₂:Eu_0.002²⁺ and a commercial BaMgAl₁₀O₁₇:Eu²⁺ (BAM) phosphor (obtained from Nichia corp., Japan). The inset shows the CIE chromaticity for two phosphors depending on the excitation UV light.

3.3 Infrared spectroscopy

Fig. 8 presents the IR spectra of SiO₂-related compounds. The absorption band at 1092 cm⁻¹ in the spectra of α-quartz is associated with Si–O asymmetrical stretching vibrations, those at 799 and 780 cm⁻¹ with Si–O symmetrical stretching vibrations, that at 696 cm⁻¹ with Si–O symmetrical bending vibrations, and those at 512 and 460 cm⁻¹ with Si–O asymmetrical bending vibrations.²⁴ After firing α-quartz at 1500 °C, the IR spectrum of α-quartz is completely transformed into that of α-cristobalite with a notable new band at 621 cm⁻¹ corresponding to Si–O asymmetrical bending vibrations.²⁵ In SiO₂:Eu²⁺ compounds fired at 1300 °C under a 4% H₂–Ar atmosphere, there is no considerable wavenumber shift in IR modes except that the lower absorbance and greater broadness of IR modes compared with those of α-cristobalite. The IR modes in the SiO₂ compound (without Eu) fired at 1300 °C under a 4% H₂–Ar atmosphere reveals that the Eu ions occupied in the SiO₂ matrix might have an effect on the SiO₂ internal modes. As a consequence of the interaction, (Eu²⁺⋯[O–Si–O]⁴⁻⋯Eu²⁺), the local symmetry of the SiO₂ matrix may partially collapse, which results in the modification of the [SiO₄] internal modes, i.e., lower absorbance and greater broadness of IR modes because there is not enough Eu content to induce the chemical shift of the vibrational modes of the SiO₂:Eu compounds.²⁶ It should be noted that the variation of the IR modes by Eu ion doped in a SiO₂ matrix means that there is a pseudo-covalent bond character between the Eu ion and O ion sharing with SiO₄ tetrahedral units. Thus, this may indicate that Eu ions are occupied in the SiO₂ interstitial sites formed structurally, particularly, in wider ones formed under a 4% H₂–Ar atmosphere.

Fig. 8 Infrared spectra of SiO₂-related compounds.

3.4 X-ray photoelectron and solid NMR spectroscopy

To examine the valence state of Eu ions occupied into the SiO₂ matrix, XPS analysis was performed. All the XPS spectra were fitted after a Shirley background correction. Fig. 9 presents the wide-scan XPS spectra of EuCl₂ (Eu²⁺) and EuCl₃ (Eu³⁺) reference compounds. It should be pointed out that it is very difficult to obtain fully reduced Eu²⁺ in metal oxide-phosphor compounds based on the standard reduction potential (Eu³⁺/Eu²⁺ = −0.36 V vs. standard hydrogen electrode (SHE)),²⁷ which indicates that the reduction of Eu³⁺ to Eu²⁺ requires an annealing process at high temperature (≥1000 °C) under a reducing atmosphere such as H₂ or H₂–Ar mixture gas. In addition, the thermodynamically unstable Eu²⁺ ions are prone to be easily oxidized by foreign molecules, such as H₂O and O₂, in particular, under X-ray irradiation in XPS measurements. As shown in the inset of Fig. 9, the fact that the new band around 128 eV is observed in EuCl₂ compound supports the partial oxidation of Eu²⁺ to Eu³⁺. Fig. 10 presents Eu 3d and Eu 4d XPS spectra of EuCl₂ and EuCl₃. The Eu 4d binding energies of EuCl₃ (top at right) are assigned to the Eu³⁺ 4d_5/2 (137.3 eV) and Eu³⁺ 4d_3/2 (142.9 eV).^28–30 The spin-orbital splitting value for Eu³⁺ ion is 5.6 eV, which is in good agreement with that previously reported.²⁸ The Eu 4d binding energies of EuCl₂ (bottom at right) are assigned to Eu²⁺ 4d_5/2 (128.4 eV), Eu²⁺ 4d_3/2 (134.2 eV), Eu³⁺ 4d_5/2 (136.7 eV), and Eu³⁺ 4d_3/2 (142.3 eV), which are consistent with XPS results as previously reported.^31–36 Notably, Jiang et al. synthesized EuCl₂ nanoprisms and nanorods and presented Eu 4d XPS spectra, as follows:³⁶ Eu²⁺ 4d_5/2 (128.2 eV), Eu²⁺ 4d_3/2 (∼134 eV), Eu³⁺ 4d_5/2 (∼136 eV), and Eu³⁺ 4d_3/2 (142.2 eV), which is in good agreement with our results. Moreover, they mentioned that Eu 4d XPS spectrum showed the clear presence of Eu²⁺ (128.2 eV and ∼134 eV), although the Eu²⁺ ions on the surface were oxidized to Eu³⁺ (∼136 eV and 142.2 eV). As shown in Fig. 10 (bottom at right), the spin-orbital splitting values of Eu³⁺ and Eu²⁺ ion (for EuCl₂) are 5.6 eV and 5.8 eV, respectively. Interestingly, the Eu²⁺ 4d_5/2 peak at 128.4 eV consists of two components with the spin–orbit splitting value of 5.8 eV, which may be attributed to the different sites of the Eu²⁺ 4d_5/2 induced by the partial oxidation of Eu²⁺. From the XPS measurement for EuCl₂ and EuCl₃ reference compounds, it clearly shows that EuCl₂ (Eu²⁺) compound is more sensitive than EuCl₃ (Eu³⁺) in oxidation induced by X-ray irradiation. By the aid of the Eu 4d binding energy for EuCl₂ and EuCl₃ reference compounds, the XPS 3d binding energies of reference compounds could be precisely assigned, as presented in Fig. 10. The Eu 3d binding energies of EuCl₃ reference compound (top at left) are assigned to the Eu³⁺ 3d_5/2 (1136.5 eV) and Eu³⁺ 3d_3/2 (1166.1 eV) with the small amount of Eu²⁺ state at lower binding energy. On the other hand, the peaks of the Eu²⁺ 3d binding energies for EuCl₂ reference compound (bottom at left) are strongly intensified compared with those of the EuCl₃ compound, i.e., Eu²⁺ 3d_5/2 (1126.3 eV) and Eu²⁺ 3d_3/2 (1156.2 eV). In particular, it is reasonable that the Eu²⁺ valence state in EuCl₂ compound is prone to be easily oxidized under X-ray irradiation, as presented in Eu 3d as well as Eu 4d XPS spectra. It should be noted that the difference in 3d binding energies between Eu²⁺ and Eu³⁺ (9.7 eV) is somewhat larger than that of the 4d binding energies (8.3 eV). It can be considered that the 3d electrons are closer to the nucleus than the 4d elections and thus their binding energies are more strongly affected (ΔB.E ∝ 1/r). To examine the valence state of Eu ions occupied into the interstitial sites of SiO₂, XPS measurements were performed and its results are presented in Fig. 11. As the PL spectrum (top at right) of SiO₂:Eu_0.05²⁺ compound synthesized under 5% H₂–Ar gas mixture shows the characteristic of Eu²⁺, 4f⁶5d → 4f⁷ transition with a broad band between 380 nm and 520 nm corresponds to the blue emission. The Eu 3d XPS binding energies of SiO₂:Eu_0.05²⁺ consist of four components, Eu²⁺ 3d_5/2 (1126.1 eV), Eu³⁺ 3d_5/2 (1135.8 eV), Eu²⁺ 3d_3/2 (1155.9 eV), and Eu³⁺ 3d_3/2 (1165.4 eV). Considering the PL emission spectrum, it is evident that the Eu²⁺ ions in the SiO₂:Eu_0.05²⁺ compound is easily oxidized under X-ray irradiation. After thermal treatment of SiO₂:Eu_0.05²⁺ compound under O₂ atmosphere at 1300 °C for 3 h, the PL emission spectrum (bottom at right) was obtained. As its PL behavior shows the characteristic of Eu³⁺ with a line spectrum at 616 nm corresponding to the red emission, it is evident that the Eu²⁺ ions that occupy the SiO₂ matrix are all nearly oxidized to Eu³⁺. Thus, the Eu³⁺ 3d XPS spectrum (bottom at left) is predominantly obtained after thermal treatment of SiO₂:Eu_0.05²⁺ compound under an O₂ atmosphere at 1300 °C for 3 h. From the XPS measurement, a valuable information was obtained that the valence state of Eu ions occupied into the SiO₂ matrix is predominantly Eu²⁺, associated with the blue emission. However, XPS as a surface analysis technique, cannot determined the total presence of Eu³⁺ in the SiO₂:Eu_0.05²⁺ compound even though there exists a small amount of Eu³⁺. Thus, XPS results evidently imply that the Eu²⁺ ions stabilized on the SiO₂ matrix are easily oxidized under X-ray irradiation in the XPS measurements. ²⁹Si MAS-NMR spectra of the SiO₂ compounds are given in Fig. 12. The chemical shifts for α-quartz (a) and α-cristobalite ((b), after firing α-quartz fired at 1500 °C under 4% H₂/Ar) are −107.7 ppm and −109.3 ppm, respectively, which is in good agreement with those as previously reported.^37–39 α-Quartz (a) and α-cristobalite (b) show essentially no “side-bands” and no “chemical shifts”, indicating that there are neither different Si sites nor metal cations, as shown in Fig. 12 α-Quartz is transformed into the mixed phase (c) with 10 wt% of α-quartz and 90 wt% of α-cristobalite after firing at 1300 °C under a 4% H₂–Ar atmosphere, and the chemical shift of the mixed phase is −109.5 ppm, which is similar to that of α-cristobalite. It can be noted that in SiO₂:Eu²⁺ compounds, the spinning side-bands appearance depends on the Eu content; two weak side bands in SiO₂:Eu_0.002²⁺ fired at 1500 °C under 4% H₂–Ar (d) and SiO₂:Eu_0.002²⁺ fired at 1300 °C under 4% H₂–Ar (e), and six intense ones in SiO₂:Eu_0.05²⁺ fired at 1300 °C under 4% H₂–Ar (f). It is well-known that the spinning side-bands originated from the dipolar interactions between the nuclear spin and the unpaired electron spins of the paramagnetic ions,^40–44 in this system between ²⁹Si spin and the unpaired electron spins of the Eu²⁺ ions. Thus, the fact that there are spinning side-bands in MAS-NMR spectra of SiO₂:Eu²⁺ compounds supports that the Eu²⁺ ions are occupied with a pseudo-covalent bond character in the interstitial sites of the SiO₂ crystal lattice.

Fig. 9 Wide scan XPS spectra of reference compounds, EuCl₂ and EuCl₃.

Fig. 10 High-resolution Eu 3d and 4d XPS spectra of reference compounds EuCl₂ and EuCl₃.

Fig. 11 High-resolution Eu 3d XPS spectra and PL emission spectra of SiO₂:Eu_0.05²⁺ and SiO₂:Eu_0.05³⁺.

Fig. 12 ²⁹Si MAS-NMR spectra of SiO₂-related compounds; (a) α-quartz, (b) α-cristobalite after firing α-quartz at 1500 °C under H₂, (c) SiO₂ fired at 1300 °C under H₂, (d) SiO₂:Eu_0.002²⁺ fired at 1500 °C under H₂, (e) SiO₂:Eu_0.002²⁺ fired at 1300 °C under H₂, and (f) SiO₂:Eu_0.05²⁺ fired at 1300 °C under H₂. The spinning side-bands are marked by arrows.

3.5 Hydrogen assisted SiO₂-defects: nano-SIMS measurements

It is well-known that a reducing atmosphere plays an important role in controlling the oxygen vacancies as well as charge valences of metal ions in metal oxides. Therefore, the additional charges should be compensated for in order to incorporate metal ions of lower valence into the interstitial sites as well as to replace Si⁴⁺ with metal ions of lower valence in the SiO₂ matrix. The types of the point defects in silicon dioxide can be classified into intrinsic and extrinsic point defects. Intrinsic point defects involve vacancies caused by the missing host atoms and self-interstitials by additional host atoms at an interstitial position. Extrinsic point defects relate to foreign atoms different from the host crystal. Defects in a perfect silicon dioxide could include O (or Si) vacancies, their interstitials, Si–Si (or O–O) homobonds, and under-coordinated silicons or oxygens.⁴⁵ Many authors consider hydrogen to be an intrinsic defect because it has been commonly found in silicon dioxide.^46–50 Mysovsky et al.⁴⁷ argued that the oxygen-deficient vacancy combined a hydrogen atom, E′₄ center consists of a hydrogen substituting for an oxygen atom in α-quartz, as the following reaction:

≡Si–O–Si≡ + H → ≡Si–H + Si≡ + O (interstitial) (1)

The neutral oxygen vacancy (ODC(I)) could be converted to ≡Si–O–H groups by thermal reaction with hydrogen molecules according to the following reaction:^51,52

≡Si–Si≡ + H₂ → 2{≡Si–O–H} (2)

The formation mechanism of hydrogenic trapped-hole species in α-quartz was proposed by Nuttall et al.,⁴⁸ which indicates that the hydrogen ions (4H⁺) incorporated into α-quartz structure substitute for Si⁴⁺ as the following reaction:

4{≡Si–O}–Si (defect) + 4H⁺ → 4{≡Si–O–H} (3)

The variation of the O/Si signal ratios of SiO₂ compounds is clearly seen depending on the firing condition as presented in Fig. 13. A sputtered depth profile of SiO₂ compounds obtained by time-of-flight secondary ion mass spectrometry (TOF-SIMS) corroborates the O-defective SiO₂ induced by hydrogen as discussed above. The O/Si signal ratio of α-quartz as received (Aldrich chemical) is the highest value, whereas those of SiO₂:Eu_0.002²⁺ and SiO₂:Eu_0.05²⁺ fired at 1300 °C under a H₂ atmosphere are lower than that of commercially available α-quartz, indicating that the oxygen atoms are predominantly missing at a high temperature under a hydrogen atmosphere. Moreover, α-cristobalite obtained after firing α-quartz at 1500 °C under a H₂ atmosphere shows the constant and lowest O/Si ratio. Although our Nano-SIMS analysis does not allow for the quantification of H content in the samples, the observed O/Si signal ratios are consistent with this formation mechanism.

Fig. 13 O/Si signal ratios determined by TOF-SIMS depth profile in SiO₂-related compounds.

3.6 Interstitial and vacancy mechanism of Eu²⁺ doping in SiO₂ matrix

The incorporation of Eu²⁺ ions into a SiO₂ matrix may be explained in three different ways. The first explanation is that Eu atoms can replace silicon atoms forming a substitutional solid solution in a SiO₂ crystal. However, the direct substitution of Si⁴⁺ with Eu²⁺ is difficult because of the difference in the ionic radii of Si⁴⁺ (0.26 Å) and Eu²⁺ (1.30 Å).⁵³ Moreover, to the best of our knowledge, there has been no report on 4-coordinated Eu²⁺ (or Eu³⁺) in metal oxides as well as organometallic complexes. As a second explanation, the Eu atoms can distribute in the boundary of SiO₂ grains to form microscopic regions of europium oxide. The europium oxide can react with SiO₂ grains to generate Eu-containing silica compounds such as EuSiO₃ and Eu₂SiO₄. Several authors have reported the formation of EuSiO₃ and Eu₂SiO₄ on Eu-doped silica films in the studies.^54–56 Li et al. reported that Eu-doped SiO₂ films annealed in N₂ at a temperature higher than 800 °C exhibited a broad emission band between 400 and 800 nm, which was attributed to the formation of EuSiO₃ giving rise to an intense yellow luminescence.⁵⁴ In Eu-doped SiO₂ films, the broad emission peak centered at 610 nm between 500 and 750 nm was observed due to the Eu²⁺ ions stabilized in the Eu₂SiO₄ and EuSiO₃ crystalline structures.⁵⁶ In our Eu²⁺-doped SiO₂ compounds, the broad emission bands centered at 440 nm between 380 and 540 nm are observed, indicating blue-emission. Moreover, Rietveld analyses within the Eu-doping range from 0.002 to 0.05 show that there is no trace of impurities, which agree with the phase diagram of the EuO–SiO₂ system.⁵⁷ As a third explanation, we speculate that the Eu atoms can incorporate into the structural void spaces (interstitial sites). Both α-cristobalite and α-quartz have open structures, as shown in Fig. 14. As the void spaces formed in the α-cristobalite structure are somewhat larger than those in the α-quartz structure, it is presumed that Eu²⁺ ions preferably occupy the interstitial sites of α-cristobalite. Compared with the ionic radius between Eu²⁺ (1.25 Å at CN = 8) and Eu³⁺ (1.066 Å at CN = 8),⁵³ Eu³⁺ ions are more favorable to the void spaces of SiO₂ matrix on the condition that no other phases is formed in the EuO–SiO₂ solid-solution. However, in the Eu-doped SiO₂ compound fired at 1300 °C in air, the impurity phase of Eu₂SiO₅ (at the Eu-doping range of 0.01) is clearly observed in the XRD pattern and this compound exhibited a faint reddish-emission attributed to the formation of Eu³⁺ ions, as shown in Fig. 15, which indicates that in an air atmosphere, Eu³⁺ ions cannot occupy the interstitial sites of SiO₂ matrix and react with SiO₂ giving rise to the formation of Eu₂SiO₅ in an air atmosphere within the Eu-doping ranges from 0.002 to 0.05. In contrast, Eu²⁺ ions can enter the interstitial sites at 1300 °C under a 4% H₂–Ar atmosphere. Thus, it is presumed that Eu²⁺ ions can be stabilized in the interstitial sites of SiO₂ matrix through the oxygen vacancies and partial fragmentation of tetrahedron linkages caused by hydrogen assistance. It should be noted that Eu₂O₃ (without adding SiO₂) was fired at 1300 °C under a 4% H₂–Ar atmosphere and obtained a red-emitting Eu₂O₃ with a monoclinic structure. Based on the experimental results, the Eu²⁺ doping mechanism in SiO₂ matrix could be postulated as follows: (i) the reduction of Eu₂O₃ (Eu³⁺) to EuO (Eu²⁺) at a high temperature (≥1000 °C) under a hydrogen atmosphere, (ii) the entrance of Eu²⁺ ions in the interstitial sites of the SiO₂ matrix through the oxygen vacancies and partial fragmentation of tetrahedron linkages caused by hydrogen assistance, (iii) the block of subsequent oxidation of the Eu²⁺ ions enclosed by Si–O linkages compared with the isolated Eu²⁺ species.

Fig. 14 The schematic of the crystal structures for α-cristobalite (top) and α-quartz (bottom). Si and O atoms are represented by blue and red spheres. Structural voids are represented by grey spheres with radius of 1.25 Å (Eu²⁺ at CN = 8).

Fig. 15 XRD pattern of SiO₂:Eu_0.01³⁺ fired at 1300 °C in an O₂ atmosphere. The inset shows the PL emission spectra.

3.7 Theoretical comparison of interstitial positions of Eu atom in α-cristobalite and α-quartz: interatomic potential

We theoretically calculate the interatomic potentials depending on the interstitial positions of Eu atom in α-cristobalite and α-quartz and compare their energy values. As shown in Fig. 16, α-cristobalite and α-quartz have large interstitial void spaces. When an Eu atom is located in the void spaces, the interatomic chemical forces between atoms exert. Atoms have strong repulsion forces when their distance is shorter than sum of their atomic radii. Ions have electric repulsion or attraction, depending on their charges.

Fig. 16 Eu²⁺ positions in the interstitial site of α-cristobalite (a) and α-quartz (b). Blue, red, and purple dots are Si, O, and Eu atoms respectively.

The former can be modeled by the Lennard-Jones potential⁵⁸ and the latter can be modeled by a coulomb potential.⁵⁸ The potential can be written as follows:

where first sum is the Lennard-Jones potential of nearby ions and the second sum is the coulomb potential. Index i is related to the surrounding atoms in the crystal (ε_i: bond dissociation energy, R⃑_i: the location of i^th atom, r_i: the minimum potential distance between Eu²⁺ and i^th atom, Z_Eu: charge number of Eu²⁺, Z_i: charge number of i^th atom). There are two types of atoms around Eu, O and Si. For O, we choose ε_i as the Eu–O bond dissociation energy 473 kJ mol⁻¹ = 4.903 eV.⁵⁹ r is chosen as sum of ionic radius of Eu²⁺ (1.30 Å) and crystal radius O²⁻ (1.34 Å), Z_i for O charge number is −1 by considering the half ionic character. For Si, it does not form a bond with Eu²⁺ ions. We choose r_i as sum of ionic radius of Eu²⁺ (1.30 Å) and crystal radius of Si (0.26 Å) and determine the value ε_i as 4.892 eV, which gives same repulsion force as Eu and O Lennard-Jones force when atomic distance is compressed to 80% of their sum of radii. Z_i for Si charge number is 4, O charge number −2 and Z_Eu is 2. The local minimum point of this potential in three dimension can be found numerically, by successively following the negative gradient of potential (force), which gives the steepest descent. Starting from the initial position, search next potential minimum in the finite interval of the force direction, and repeat the procedure until it converges. In the α-cristobalite case, the minimum position is shown in Fig. 16. Crystal unit cell parameters are given by (a, b, c) = (4.978, 4.978, 6.948) (Å), (α, β, γ) = (90, 90, 120) (°) and the Eu minimum position is (0.26758, 0.25590, 0.50357) in the unit cell coordinate. Nearby oxygen coordinates are (0.1032, 0.2397, 0.8216), (0.7397, 0.3968, 0.5716), (0.2397, 0.1032, 0.1784) and (0.3968, 0.7397, 0.4284). Their distances from Eu atoms are 2.504, 4.325, 2.769 and 4.293 Å. Thus, the calculated minimum potential value is −51.47 eV. In the α-quartz case, the minimum position is shown in Fig. 16. Crystal unit cell parameters are given by (a, b, c) = (4.912, 4.912, 5.404) (Å), (α, β, γ) = (90, 90, 120) (°) and the Eu minimum position is (−0.0301, 0.0014, 0.6674) in unit cell coordinates. Nearby oxygen coordinates are (0.4136, 0.2676, 0.7857), (0.1460, −0.2676, 0.5476), (−0.4136, −0.1460, 0.8801), and (−0.2676, 0.1460, 0.4524). Their distances from Eu are 2.005, 2.014, 2.008 and 2.011 Å. Minimum potential value is calculated to be 221.8 eV. The results suggest that it is more difficult to put Eu atoms into α-quartz than α-cristobalite. In the α-cristobalite case, the minimum potential is negative as well as the distances between atoms at the minimum point are large. Only two oxygens around 2.5 and 2.7 Å and next nearest oxygen distances are larger than 4 Å. In the α-quartz case, the potential value is positive, which implies instability. Moreover, the nearby oxygen distances are almost identically 2 Å at a minimum potential point, showing a quite regular and closely packed structure. It is reasonable to infer that the Interstitial positioning of Eu is more easily done in the α-cristobalite case than the α-quartz case.

4 Conclusions

Blue-emitting silica by Eu²⁺-activator ion occupied in the interstitial sites of SiO₂ matrix has been successfully synthesized and characterized. The PL excitation spectra of SiO₂:Eu_0.01 compounds monitored at 438 nm consist of broad bands between 220 nm and 420 nm, which may be ascribed to the allowed 4f⁷–4f⁶5d transitions of Eu²⁺, which is in good agreement with XPS results. The emission spectra, monitored under the 323 nm excitation, show symmetric bands centered around 438 nm, which is associated with blue-emission. From FT-IR spectra of SiO₂ compounds with and without Eu²⁺-activator ions, it is clearly observed that the lower absorbance and greater broadness in the IR modes of SiO₂:Eu²⁺ compounds compared with those of α-quartz and α-cristobalite without Eu²⁺-activator ions indicate that the Eu²⁺-activator ions are well located in the interstitial sites of the SiO₂ matrix with a pseudo-covalent bond character between Eu ion and O ion sharing with SiO₄ tetrahedral units. ²⁹Si MAS-NMR spectra corroborate the IR results interpretation. The fact that there are spinning side-bands in MAS-NMR spectra of SiO₂:Eu²⁺ compounds supports that the Eu²⁺ ions are occupied with a pseudo-covalent bond character in the interstitial sites of the SiO₂ crystal lattice. The interatomic potentials depending on the interstitial positions of the Eu atom in α-cristobalite and α-quartz are calculated using Lennard-Jones potential and coulomb potential. For α-cristobalite, the minimum potential value is −51.47 eV, and for α-quartz, the value is 221.8 eV, which reveals that the Eu²⁺-activator ions more preferably enter the interstitial sites of α-cristobalite than those of α-quartz. Due to the Eu²⁺-activator ions occupied in the SiO₂ matrix, the PL intensity of the SiO₂:Eu_0.002²⁺ compound is about 24 times as high as that of the O-defective SiO₂ compound and the relative emission intensity of SiO₂:Eu_0.002²⁺ monitored at 300 nm is about 40% compared to a commercial BAM:Eu²⁺ (obtained from Nichia Co. Ltd in Japan). This phosphor material could be a breakthrough for modeling a new phosphor and for applications in the solid state lighting field.

Acknowledgements

This research was financially supported by the Ministry of Education (MOE) and National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation (2014H1C1A1066859). One of the authors, S. J. Kim acknowledges the support from the National Research Foundation (NRF) of Korea (Grant No. 2009-0094046). Dr Seen Ae Chae at the KBSI is acknowledged for carrying out NMR experiments. We would like to thank NSFs Major Research Instrumentation (NSFARRA award #0960334) and Arizona State University for acquisition and installation of the Nano-SIMS 50L.

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