Synthesis of cubic and hexagonal ordered mesoporous YVO₄:Eu³⁺ and their photoluminescence properties

Abstract

Cubic and hexagonal ordered mesoporous Eu³⁺-doped yttrium vanadate (YVO₄:Eu³⁺) have been synthesized successfully through the nanocasting route with a Y(NO₃)₃/Eu(NO₃)₃/NH₄VO₃/HNO₃/ethanol system as a guest unit and KIT-6 or SBA-15 silica as the hard template host, and were characterized by X-ray diffraction (XRD), transmission electronic microscopy (TEM) and low-temperature nitrogen adsorption. The prepared YVO₄:Eu³⁺ samples have the characteristic cubic (Ia3d) (or hexagonal (p6mm)) ordered mesostructure based on different hard templates, and possess high surface area, large pore volume and uniform pore size distribution. Photoluminescence (PL) measurement shows that the main red emission peaks of two mesoporous YVO₄:Eu³⁺ samples appear at 618 nm, and different mesostructures lead to different optimum concentrations of Eu³⁺ dopant and different PL intensities. For the hexagonal mesoporous YVO₄:Eu³⁺, when Eu³⁺ dopant is 5(mol)% the highest PL intensity can be reached; for the cubic one, optimized Eu³⁺ amount is 8 mol%. The interaction between Eu³⁺ ions in hexagonal (p6mm) mesoporous YVO₄:Eu³⁺ is more active than that in cubic (Ia3d) mesoporous YVO₄:Eu³⁺, thus the absorbed energy is dissipated by nonradiation; while the cubic mesoporous YVO₄:Eu³⁺ has more Eu³⁺ luminescent sites, compared with the hexagonal mesostructure one.

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Introduction

Rare earth doped vanadate is an important solid-state luminescent material, and it has been used widely as the phosphors and fluorescent probes,^1–4catalysts,^5–8 polarizers and laser host materials.^9,10 In the previous work, most studies of vanadate focused on the size and morphology of the particle,^11–13 and little was reported the preparation of mesoporous vanadate as porous-structural solid-state luminescent materials. With the development of the preparation techniques for mesoporous materials, multifunctional materials with mesoporous structures, photoelectric and magnetic properties, have been paid more attention. Various kinds of highly ordered mesoporous metals,¹⁴ metal oxides,^15,16 carbides,¹⁷nitrides,^18,19 and sulfides²⁰ have been synthesized, which have wide potential applications. However, the studies on the synthesis and property of mesoporous solid-state luminescent materials are limited, because of the difficulty from their multi-component complexity and easy precipitation of precursors. Therefore, highly ordered mesoporous vanadate compounds with different mesoporous network topologies have not been reported yet.

For mesoporous materials, numerous previous works are related to preparation of replicas (metal, metal oxides, etc.) with various mesoporous network topologies, while their characteristics (optical, electric, and magnetic properties) and applications are usually touched upon only for a single compound with only certain topological mesostructures. Few papers are related to the comparative study of multi-topological macrostructure compounds, we think, which is an important research project for the studies and applications of multi-functional mesoporous materials. Among various topological mesostructure materials, the most commonly porous network topologies are two-dimensional (2D) hexagonal (p6mm) and three-dimensional (3D) cubic (Ia3d). The mesoporous network topology of 2D hexagonal (p6mm) shows highly symmetrical tubular channels, and ones with three-dimensional (3D) cubic (Ia3d) present an interpenetrating bi-continuous network with highly branched channels. If inorganic materials with various pore network topologies can be obtained, more potentially commercial requirements may emerge. For instance, a perfect material combining the features of a “cavity” structure for the storage/delivery of specific species and photoluminescence (PL) for simultaneous tracking or monitoring these species should be designed and developed, in which the absorption amount, storage ability, and release rate could be controlled or adjusted through the different mesoporous network topologies, and the luminescent properties would vary with different internal structures. This material also could avoid steric hindrance for guests, compared with the post-loaded PL nanoparticles in the mesoporous silica or carbon.^13,21–27 Therefore, a kind of multifunctional mesoporous vanadate luminescent material is vividly portrayed.

In this paper, we have firstly synthesized 2D hexagonal (p6mm) and 3D cubic (Ia3d) ordered mesoporous YVO₄:Eu³⁺, and compared the difference of PL properties between YVO₄:Eu³⁺ samples with different pore network topologies. YVO₄:Eu³⁺ shows strong red emission excited by UV light by an efficient energy transfer from excited VO₄³⁻ complex anions to Eu³⁺ activator ions.¹ It is an excellent red emitting material. Nevertheless, it is hard to find appropriate soluble precursors to prepare the highly ordered mesoporous YVO₄:Eu³⁺ materials.

Herein, a strong acid was added to the precursor to make a homogeneous liquid solution (Y(NO₃)₃/Eu(NO₃)₃/NH₄VO₃/HNO₃/ethanol). Mesoporous 2D hexagonal (p6mm) and 3D cubic (Ia3d) YVO₄:Eu³⁺ were synthesized by using mesoporous SBA-15 and KIT-6 hard templates, and the effect of pore network topology on their PL properties was investigated. It is interesting, when the highest PL intensity of YVO₄:Eu³⁺ was reached, the concentration of Eu³⁺ dopant was 5 mol% for the hexagonal mesoporous one and 8% for the cubic one. It suggests that there are large numbers of Eu³⁺ luminescent sites in the 3D cubic mesoporous host, due to the cubic (Ia3d) mesoporous YVO₄:Eu³⁺ with more open mesostructure channels having a larger effective surface compared with the hexagonal (p6mm) mesostructure.

Experimental section

Preparation of samples

The mesoporous silica (KIT-6 and SBA-15) hard template was prepared according to the established procedures.^28,29 Ordered mesoporous YVO₄:Eu³⁺ (Y:Eu = 0.92 : 0.08) was synthesized as follows: 0.352 g (0.92 mmol) Y(NO₃)₃·6H₂O (Aldrich), 0.036 g (0.08 mmol) Eu(NO₃)₃·6H₂O (Aldrich), 3 ml concentrated nitrate acid, and 0.117 g (1 mmol) NH₄VO₃ (Aldrich) were added in sequence in 10 ml ethanol under magnetic stirring till a yellow clear solution was obtained. Then, 0.15 g KIT-6 or 0.2 g SBA-15 hard template was added to the above solution. This mixture solution was further stirred until a completely dry powder was obtained. Subsequently, the samples were heated slowly to 873 K and calcined at 873 K for 4 h to form crystalline YVO₄:Eu³⁺ in the channel of the silica template. The obtained YVO₄:Eu³⁺/silica composite sample was treated with hot 2 M NaOH solution 3 times to remove the silica template, followed by washing it with de-ionized water several times, and it was dried at 373 K.

Characterization of samples

Powder X-ray diffraction (XRD) patterns were recorded on a Panalytical X'Pert with Cu Kα radiation (λ = 0.15346 nm). N₂ adsorption–desorption isotherms were recorded at 77 K on a Micromeritics ASAP 2020 instrument, and the samples were degassed at 523 K and 10⁻⁶ Torr for 10 h prior to measurement. The Brunauer–Emmett–Teller (BET) method was used to calculate the specific surface area of sample, and the pore size distribution was calculated by Barrett–Joyner–Halanda (BJH) method. The total pore volume (Vp) of sample was estimated based on the amount adsorbed at the relative pressure of p/p₀ = 0.99. Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2100 microscope operated at 200 kV, and the sample to be measured was first dispersed in ethanol and then collected using copper grids covered with carbon film. Energy dispersive X-ray spectroscopy (EDS) patterns were performed on a Philips instrument. Photoluminescence spectra were recorded on a Varian Cary Eclipse Fluorescence Spectrophotometer at room temperature equipped with a 150 W Xe lamp in the range 200–800 nm with a resolution of 1 nm. The quantum yield data were recorded on a fluoromax-4 spectrofluorometer of Horiba Scientific.

Results and discussion

Powder XRD

Cubic and hexagonal ordered mesoporous YVO₄:Eu³⁺ were fabricated by a nanocasting route and employed a homogenous Y(NO₃)₃/Eu(NO₃)₃/NH₄VO₃/HNO₃/ethanol solution system as a guest and mesoporous silica KIT-6 or SBA-15 as the hard template host. Fig. 1 shows the small angle X-ray diffraction (SAXRD) patterns of mesoporous silica KIT-6 and cubic ordered mesoporous YVO₄:Eu³⁺ materials. It can be seen that the mesoporous silica KIT-6 template exhibits several obvious diffraction peaks, characteristic of a highly ordered cubic Ia3d structure. The mesoporous YVO₄:Eu³⁺ cast from the mesoporous KIT-6 silica shows a SAXRD pattern with the same diffraction peak (211) position which was less resolved in comparison with its parents.

Fig. 1 Small-angle XRD patterns of (a) cubic ordered mesoporous KIT-6 silica, (b) cubic ordered mesoporous YVO₄:Eu³⁺, and wide-angle XRD pattern of (c) cubic ordered mesoporous YVO₄:Eu³⁺. The standard data for phase YVO₄:Eu³⁺ (JCPDS 17-0341) is also presented in the figure.

Likewise, the SAXRD pattern of the hexagonal ordered mesoporous YVO₄:Eu³⁺ material in Fig. 2 shows the same diffraction peak (100) positions and is less resolved compared with the hard template of hexagonal mesoporous SBA-15. These results suggest that the homogenous Y(NO₃)₃/Eu(NO₃)₃/NH₄VO₃/HNO₃/ethanol solution can be efficiently infiltrated into the silica cavity, and the mesostructure of the sample can be maintained during the thermal treatment process. The unit cell parameter of mesoporous silica KIT-6 calculated from the (211) diffraction is 22.5 nm and SBA-15 (100) is 10.7 nm, and the unit cell parameter of corresponding mesoporous YVO₄:Eu³⁺ is 20.3 nm and 10.0 nm, respectively. A slight shrinkage occurred and is attributed to a high degree of condensation of silicate during calcination to form the YVO₄:Eu³⁺ mesostructure. Consequently, it can be confirmed that the mesoporous YVO₄:Eu³⁺ products are negative replicas of mesoporous silica templates.

Fig. 2 Small-angle XRD patterns of (a) hexagonal ordered mesoporous SBA-15 silica, (b) hexagonal ordered mesoporous YVO₄:Eu³⁺, and wide-angle XRD pattern of (c) hexagonal ordered mesoporous YVO₄:Eu³⁺. The standard data for phase YVO₄:Eu³⁺ (JCPDS 17-0341) is also presented in figure.

In the wide angle XRD patterns of mesoporous YVO₄:Eu³⁺ samples in Fig. 1c and 2c, there are the same diffraction peaks of the tetrahedral phase YVO₄ (JCPDS 17-0341), that is to say, the mesoporous YVO₄:Eu³⁺ material with 3D cubic mesostructure exhibits the same crystalline structure as the sample with 2D hexagonal mesostructure. Also, because of the small amount of Eu³⁺, the wide angle XRD patterns of YVO₄:Eu³⁺ are the almost same as that of the tetrahedral phase YVO₄ and the diffraction peaks of Eu³⁺ compounds cannot be observed, suggesting that Eu³⁺ ions have been homogeneously incorporated or dispersed into the YVO₄ host lattice.³⁰

TEM

TEM images of cubic and hexagonal mesoporous YVO₄:Eu³⁺, prepared with KIT-6 or SBA-15 hard templates, are shown in Fig. 3. The representative TEM images along [531] and [111] directions³¹ (Fig. 3A, B) reveal that this YVO₄:Eu³⁺ sample consists of large ordered domains of cubic Ia3d structures which is a good replication of mesoporous KIT-6. The high-resolution TEM (HRTEM) image (Fig. 3C) shows the structures and crystal walls of the pores, which can also be confirmed by the wide angle XRD results (Fig. 1c). The spacing of the lattice fringe is ∼0.35 nm, which can be attributed to [200] direction (d = 0.3559 nm).

Fig. 3 TEM images (A) along [531] and (B) [111] direction) and (C) HRTEM image of the cubic (Ia3d) ordered mesoporous YVO₄:Eu³⁺ prepared by the nanocasting route with KIT-6 silica template; TEM images ((D) along [110] direction), (E) HRTEM image and (F) EDS spectrum of the hexagonal (P6mm) ordered mesoporous YVO₄:Eu³⁺ prepared with the SBA-15 silica template.

The TEM images (Fig. 3D) along the [110] direction show the hexagonal P6mm ordered mesoporous structure of YVO₄:Eu³⁺ prepared with the SBA-15 hard template, and in the HRTEM image (Fig. 3E), we can clearly see the lattice fringes of the walls and strip channels, which is different from the cubic Ia3d structure (Fig. 3C). The spacing of the lattice fringe is ∼0.34 nm, considering the measurement error, this datum is also close to d = 0.3559 nm ([200] direction in Fig. 2c). The spectrum of energy dispersive X-ray spectroscopy (EDS) (Fig. 3F) shows that, after leaching SiO₂ with hot 2 M NaOH three times, only a small amount of silica can be detected in the surface (this situation of using KIT-6 template is similar to that of using SBA-15), due to the strong interaction between the host silica template and heavy rare earth element guest.^32–34 The Eu content is 1.15% (atom), Y is 17.48%, V is 16.43%, and other elements include O and a small number of Si (< 4%) and Au (this sample was treated by gold sputtering).

N₂ adsorption–desorption isotherms

Fig. 4 shows the N₂ adsorption–desorption isotherms and pore diameter distribution curves of hard templates and mesoporous YVO₄:Eu³⁺ replicas, and the BET surface areas, total pore volumes, pore diameters and cell parameters of these samples are listed in Table 1. The isotherms of KIT-6 and SBA-15 templates are the typical IV-type curves with an H1-type hysteresis loop of mesoporous materials. The mesoporous silica KIT-6 has a little larger BET surface area (737 m² g⁻¹), higher pore volume (1.44 cm³ g⁻¹) and lager pore size (9.5 nm), in comparison with the SBA-15 template (the BET surface area of 663 m² g⁻¹, pore volume of 0.93 cm³ g⁻¹ and pore size of 7.7 nm). As a replica of SBA-15, hexagonal ordered mesoporous YVO₄:Eu³⁺ exhibits a slightly higher BET surface area (127 m² g⁻¹), pore volume (0.22 cm³ g⁻¹) and pore size (6.4 nm) than the cubic mesoporous YVO₄:Eu³⁺ (the BET surface area of 104 m² g⁻¹, pore volume of 0.20 cm³ g⁻¹ and pore size of 4.2 nm). The results above mean that the replica of KIT-6 cubic mesoporous YVO₄:Eu³⁺ has a thicker wall, a slightly smaller pore size and pore volume than the hexagonal one; a homogenous solution can more easily infiltrate and fill the 3D cubic mesostructure silica template with larger pore size and pore volume than the 2D hexagonal template.

Fig. 4 Nitrogen adsorption–desorption isotherms of (A) KIT-6 hard template and cubic ordered mesoporous YVO₄:Eu³⁺ prepared with KIT-6 hard template, (B) SBA-15 hard template and hexagonal ordered mesoporous YVO₄:Eu³⁺ prepared with SBA-15 hard template, and (inset) the pore-size distribution curves of samples.

Table 1 Textural properties of mesoporous KIT-6, SBA-15, cubic and hexagonal ordered mesoporous YVO₄:Eu³⁺

Sample	SA (m^2 g^−1)	Pore vol. (cm^3 g^−1)	Pore size (nm)	a o ^a (nm)	d hkl (nm)
a The cell parameter, cubic a0 = d211(6)^0.5; hexagonal a0 = d100(4/3)^0.5.
KIT-6	737	1.44	9.5	22.5	9.2 (d211)
Meso YVO4:Eu^3+ (cubic)	104	0.20	4.2	20.3	8.3 (d211)
SBA-15	663	0.93	7.7	10.7	9.3 (d100)
Meso YVO4:Eu^3+ (hexagonal)	127	0.22	6.4	10.0	8.7 (d100)

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Cubic mesoporous YVO₄:Eu³⁺ also displays the IV-type isotherm of mesoporous material. However, the isotherm of hexagonal mesoporous YVO₄:Eu³⁺ displays two steps at p/p₀ = 0.5∼0.7 and 0.8∼1.0. The former step corresponds to the uniform mesopores replicated from SBA-15 and the latter the voids, possibly due to the incomplete filling of precursor inside the long-narrow channels of SBA-15.^31,35,36

These results above suggest that well-ordered mesoporous YVO₄:Eu³⁺ can be generated inside the mesoporous silica hard template by employing a new guest unit, in which the Y(NO₃)₃/Eu(NO₃)₃/NH₄VO₃/HNO₃/ethanol homogenous solution can be infiltrated firstly into the mesopore silica hard template through capillary and host–guest interactions,^37,38 subsequently confined growth inside the channels of the KIT-6 or SBA-15 host by the evaporation of the solvent, resulting in the formation of a YVO₄:Eu³⁺/silica composite. After the silica template is removed from the composite, the ordered mesoporousYVO₄:Eu³⁺ can be obtained. In the process of preparing the sample, we have found that the channels of cubic mesoporous KIT-6 are more favorable to an infiltration of the homogenous solution than that of hexagonal SBA-15.

Photoluminescence properties

Mesoporous YVO₄:Eu³⁺ materials exhibit tunable and intense optical properties by adding an appropriate amount of Eu³⁺. PL spectra of cubic and hexagonal ordered mesoporous YVO₄:Eu³⁺ have been tested at room temperature and under the same conditions, and the results are presented in Fig. 5–7.

The excitation spectra of cubic and hexagonal ordered mesoporous YVO₄:Eu³⁺ (8%), taken from the ⁵D₀–⁷F₂ emission at λ_em = 618 nm, are shown in Fig. 5 (dashed line). A broad band at 200∼350 nm can be observed in the excitation spectra, ascribed to the charge transfer from the oxygen ligands to the central vanadium ions inside the VO₄³⁻groups.^39,40 The general f–f transitions within Eu³⁺ (4f⁶ electron configuration) in the longer wavelength region can hardly be observed due to their weak intensity relative to that of VO₄³⁻.^40,41 Hence, an excitation of the Eu³⁺ ions is mainly caused by the energy transfer from the VO₄³⁻groups to Eu³⁺ ions.

Fig. 5 Excitation spectra (dashed line) and emission spectra (solid line) of (a) cubic and (b) hexagonal ordered mesoporous YVO₄:Eu³⁺ (Y/Eu = 92/8, mol) materials.

Fig. 6 Effect of the Eu³⁺ dopant concentration on PL of cubic ordered mesoporous YVO₄: Eu³⁺ material and (inset) on the intensity of peak at 618 nm.

Fig. 7 Effect of the Eu³⁺ dopant concentration on PL of hexagonal ordered mesoporous YVO₄: Eu³⁺ material and (inset) on the intensity of peak at 618 nm.

The emission spectra (solid line), taken at an excitation wavelength of 309 nm, shows that the mesoporous YVO₄:Eu³⁺ materials behave the characteristic lines from low energy level ⁵D₀ to ⁷F_J (J = 1, 2, 3, 4) of Eu³⁺⁴² at ∼590 nm, 618 nm, 652 nm and 698 nm, respectively. And the peaks at 537 nm and 558 nm are from ⁵D₁ to ⁷F_J (J = 1, 2). The presence of emission lines from higher excited states of Eu³⁺ (⁵D₁, ⁵D₂ and ⁵D₃) is attributed to the low vibration energy of VO₄³⁻groups (823 cm⁻¹). The multiphonon relaxation of VO₄³⁻ is not completely able to bridge the gaps between the higher energy levels (⁵D₁, ⁵D₂ and ⁵D₃) and ⁵D₀ level of Eu³⁺, resulting in the weak emission from these levels.⁴³ The highest intensity at 618 nm in the PL spectra of cubic and hexagonal mesoporous YVO₄:Eu³⁺ should attribute to the transition of ⁵D₀–⁷F₂. This suggests that the electric-dipole transition emission (⁵D₀–⁷F₂) is dominant, which results from an absence of an inversion symmetry at the Eu³⁺ lattice sites (D_2d symmetry) for both mesoporous YVO₄ hosts.⁴⁴ The results in Fig. 5 also show that the positions of peaks in both spectra are of indifference, which means that the essential environment of the Eu³⁺ ions in the cubic mesoporous YVO₄ host is almost the same as one in the hexagonal YVO₄.

However, the intensity of cubic mesoporous YVO₄:Eu³⁺ (8%) at 618 nm is higher than that of the hexagonal one (Fig. 5), and the quantum yield of cubic ordered mesoporous YVO₄:Eu³⁺ (8%) is 47.29% and that of hexagonal one is 36.04%. This is because when the concentration of Eu³⁺ ions is 8%, the cubic ordered mesoporous YVO₄:Eu³⁺ get the optimum concentration of Eu³⁺, and for the hexagonal ordered mesoporous YVO₄:Eu³⁺, it is a quenching concentration of Eu³⁺ ions, which affect its quantum yield.

Interestingly, an obviously difference of the relative intensity with the concentration of Eu³⁺ ions between two YVO₄:Eu³⁺ samples can be observed. The results in Fig. 6 and 7 show that the PL intensity of YVO₄:Eu³⁺ varies considerably with a change of Eu³⁺concentration, and the highest PL intensity of the cubic mesoporous YVO₄:Eu³⁺ at 8 mol% Eu³⁺ and the highest PL intensity of the hexagonal one at 5% can be observed. It can be explained by the interaction between rare earth ions because of the typical concentration quenching of lanthanide-doped system due to a mutual interaction between Eu³⁺ ions.^45–48 The interaction between Eu³⁺ ions in hexagonal mesoporous YVO₄:Eu³⁺ is stronger than that in cubic mesoporous YVO₄: Eu³⁺, which dissipates the absorbed energy by nonradiation. Their difference is attributed to different mesoporous topology structures. The formed different surface defaults and channel arrangements make the surface available for absorbing UV light and the distance of mutual interaction Eu³⁺ ions can be different, which affects the number of luminescent centres in the respective emitting levels, resulting in different optimum concentrations of Eu³⁺ dopant.⁴⁹

Conclusions

In summary, Eu³⁺-doped yttrium vanadate (YVO₄:Eu³⁺) with cubic (Ia3d) and hexagonal (p6m) ordered mesopores have been synthesized successfully through the nanocasting route by employing a Y(NO₃)₃/Eu(NO₃)₃/NH₄VO₃/HNO₃/ethanol solution system as a guest unit and KIT-6 or SBA-15 silica as the hard template host. The cubic or hexagonal ordered YVO₄:Eu³⁺ materials possesses the typical mesoporous features, high surface area, large pore volume and uniform pore size distribution, which are the fine replication of KIT-6 or SBA-15 hard templates.

Both cubic and hexagonal mesoporous YVO₄:Eu³⁺ materials display a good luminescence property and the same characteristic lines from the low energy level ⁵D₀ to ⁷F_J (J = 1, 2, 3, 4) of Eu³⁺. However, there are different quenching concentrations for two cubic and hexagonal mesoporous YVO₄:Eu³⁺ samples, the highest PL intensity of the cubic mesoporous YVO₄:Eu³⁺ at 8 mol% Eu³⁺ and the highest PL intensity of the hexagonal one at 5 mol%, which suggest the different mesoporous channels of the cubic and hexagonal mesoporous YVO₄:Eu³⁺ lead to different defaults and available surfaces for absorption of the UV light. The studies on the synthesis and comparison of various mesostructure fluorescent materials might lead to better ideas on the further development of solid-state luminescent materials and other related materials (such as catalysis and energy materials), and the multifunctional YVO₄:Eu³⁺ materials will promise a good candidate for the applications in combination of carrier and tracer.

Acknowledgements

This project was supported financially by the National Basic Research Program of China (2010CB732300), and the National Natural Science Foundation of China (20971087/B0101), the Science and Technology Commission of Shanghai Municipality (08ZR1418800) and the Education Commission of Shanghai Municipality (J51503).

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