1. Introduction

In recent years, the remarkable effort has been devoted to study the rare earth materials due to their practical applications, such as luminescent phosphors, catalysts, magnets, and other functional devices [1–5]. Among the various rare earth materials, luminescent properties of lanthanide ions (Ln³⁺) have attracted more attention because these Ln³⁺ ions can serve as activator centers, yielding characteristic fluorescence with high luminescent efficiency under UV light excitation. Thus, Ln³⁺-doped luminescent materials have found applications in full-color display, cell imaging, LEDs, biomedical labeling, etc [6–10]. The traditional host materials ABO₄ (A = Ca²⁺, Y³⁺, Lu³⁺, La³⁺; B = V⁵⁺, W⁶⁺, Mo⁶⁺) can exhibit broad and intense UV absorptions originating from the oxygen to metal charge transfer within the BO₄ group [11–14]. In studying the efficiency of rare-earth luminescence in inorganic host materials, the primary excitation energy usually induces luminescence through the following steps in these materials: (1) the absorption of excitation by the host lattice, (2) energy transfer through the lattice to the neighborhood of the activator, (3) transfer from the lattice to the activator, and (4) radiative emission of the energy by the activator [15].

To achieve efficient multicolor tunable emissions of the Ln³⁺ ions, host sensitization via energy transfer (ET) from the host to the activator Ln³⁺ ions is one of the effective ways, which could get over the low absorptions of the parity forbidden 4f-4f transitions of Ln³⁺ ions [16–18]. For example, Yu et al. presented RE³⁺ (RE = Eu, Sm, Dy, Tb)-doped CaMoO₄ nanofibers with high luminescent efficiency and stability through a facile supersaturated recrystallization process [19]. With regard to CaWO₄:Ln³⁺ (Ln = Eu, Tb) nanoparticles, they can exhibit strong red and green emission upon UV excitation, due to the efficient energy transfer from WO₄ groups to the activator ions [20]. Particularly, high-efficiency phosphors can be achieved in lanthanide orthovanadate systems by doping different kinds of rare earth ions. For instance, Song et al. formulated LuVO₄:Ln³⁺ (Ln = Tm, Er, Sm, Eu) nanoparticles, which showed the characteristic transition of Ln³⁺ ions, generating bright blue, green, orange-red, and red emission [21]. Especially, YVO₄ is as a typical host material, which merits excellent optical stability, higher quantum efficiency, larger stokes transition properties and good biocompatibility [22]. For example, Ningthoujam RS et al. fabricated silica-coated YVO₄:Ln³⁺ (Ln = Eu, Dy, Tm) nanoparticles with strong emission and color tunable emission by a microemulsion method [1]. The relevant research suggests that the multicolor emissions could be realized by controlling the types of activator ions incorporated into host crystal lattice [23]. The emission spectra of RE ions-doped YVO₄ phosphors consist of multicolor emissions: the main red emission for Eu³⁺, yellow green for Dy³⁺, and blue for Tm³⁺ activator ions. However, the research on (Ln = Pr, Tb, Ho, Er) ions-doped YVO₄ is still elusive. It is urgent to explore and understand the energy transfer (ET) process for improved optical properties of (Ln = Pr, Sm, Eu, Tb, Dy, Ho, Er)-doped YVO₄ matrix materials.

On the other hand, the design and synthesis of inorganic nanocrystals with well-defined morphologies and tunable sizes remain one of the fundamental issues in the materials science [24,25]. This is because the properties and performance of one material are closely related to its microstructures such as the morphology, dimensionality, integrity (lattice and surface defects), and size as well as its crystal structure [26,27]. For instance, well-dispersed mesoporous nanophosphors can be directly utilized as the targeting tools for drug delivery and disease therapy in the biomedical and bioengineering fields [28,29]. To the best of our knowledge, the systematical synthesis of YVO₄:Ln³⁺ (Ln = Pr, Sm, Eu, Tb, Dy, Ho, Er) with mesoporous nanostructure for optical research still faces a big challenge. Compared with the synthesis implemented in the complex organic solvents, a water-based system could pave a new way for green chemistry alternative to fabricate various nanomaterials [30]. In addition, a water-based system also merits the simplicity, safety, convenience, and the potential for scale-up production.

Herein, a series of single-crystalline YVO₄:Ln³⁺ (Ln = Pr, Sm, Eu, Tb, Dy, Ho, Er) nanoparticles with mesoporous cell-like nanostructure were prepared through initiating homogeneous precipitation followed by a hydrothermal treatment. The crystal structure and luminescence properties of the YVO₄:Ln³⁺ nanoparticles were investigated in detail. Furthermore, the band gap energy E_g dependence property on the doping Ln³⁺ has been investigated.

2. Experimental section

2.1. Synthesis

YVO₄ and YVO₄:Ln³⁺ (Ln = Pr, Sm, Eu, Tb, Dy, Ho, Er) nanocrystallines were fabricated via initiating homogeneous precipitation followed by a hydrothermal treatment, in which the dopant concentration of Ln³⁺ ions was kept the same at 2 mol%. The synthetic procedure of the samples can be briefly described as follows. Firstly, solutions of 0.2 mol/L Y(NO₃)₃ (9.8 mL), 0.01 mol/L Ln(NO₃)₃ (4 mL), and 0.2 mol/L NH₄VO₃ (10 mL) were added to form a suspension mixture and the pH value was carefully adjusted to 10 by a given concentrated NaOH solution. Secondly, a light yellow homogenous suspension was formed by stirring for 30 min, which was then transferred to a Teflon autoclave with a filling capacity of 70%, and heated at 180 °C for 6 h. Thirdly, after naturally cooling to room temperature, the precipitate was collected by centrifugation, washed 3 times with deionized water, and dried at 80 °C in air for 12 h for further characterization.

2.2. Characterization

Phase purity of all samples was characterized using a Rigaku Miniflex II apparatus equipped with Cu-Kα radiation (λ = 0.15418 nm). The average grain sizes were calculated from the intense diffraction peak (200) using the Scherrer formula. The full width at half maximum (FWHM) β, which is used to calculate the average crystallites size D, is defined as the half width after subtracting the instrument broadening. The lattice parameters were refined by a least-squares method employing Rietica Rietveld program, in which Ni powder serves as an internal standard for peak positions calibration. The fitted parameters, a, c, unit cell volume V, and axial ratio c/a were compiled in Table 2 (Appendix) and Table 3 (Appendix). Among these parameters, the data within parentheses represent the relative errors of refinement. Technically, the smaller they are and the higher reliability of curve fitting is.

The morphologies were characterized by a field emission scanning electron microscopy (FE-SEM) (JEOL JSM-6700) with an acceleration voltage of 10 kV and a transmission electron microscopy (TEM) on a JEM-2010 apparatus with an acceleration voltage of 200 kV. UV-vis diffuse reflectance spectra of the samples were measured on a Varian Cary 500 scan UV-vis spectrophotometer with BaSO₄ as the background and in the range of 200-800 nm.

2.3. Optical testing

Luminescent spectra and lifetime decay curves were collected by a Varian Cary Eclipse Fluorescence Spectrometer. For comparison, the testing parameters like slit width, intensity of light source, and specimen mass were kept the same. The “eye-visible luminescence pictures” were collected through the following procedures: (i) 0.05 g samples were well dispersed in 100 mL ethanol solution under the ultrasonic; (ii) the eye-visible luminescence pictures were taken in the dark room through directly illuminating the corresponding solution samples in a quartz with a UV lamp (λ_ex = 254 nm).

3. Results and discussion

3.1. Structure and morphology of YVO₄:Ln³⁺ with mesoporous cell-like nanostrcture

The phase purity, crystallinity, and average particle size of the obtained samples were characterized by XRD. Figure 1 shows the XRD patterns of YVO₄:Ln³⁺, in which the dopant concentration of Ln³⁺ ions was kept the same at 2 mol%. The XRD patterns of samples indicate that all the diffraction peaks can be readily indexed to the pure tetragonal YVO₄ phase (JCPDS No. 17-0341). No traces of additional peaks from the doped components or other secondary phase were detected, indicating the phase purity of all synthetic specimens with the limitation of detection of our XRD instrument. The crystallinity of samples is high as demonstrated by sharp diffraction peaks. The average crystallite sizes of the YVO₄:Ln³⁺ nanocrystals calculated from peak broadening of the (200) line using the Scherrer formula was approximately 15∼17 nm (Table 2), being further confirmed by the following TEM characterization.

 

Fig. 1 XRD patterns of YVO₄:Ln³⁺ samples with mesoporous cell-like nanostructure:(a) Ln = Y, Pr, Sm, Eu, and (b) Ln = Tb, Dy, Ho, Er. Vertical bars at the bottom denote the standard data for a tetrahedral zircon structure of bulk YVO₄ (JCPDS No. 17-0341). Symbols “*” denote the internal standard of Ni.

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To further study the structural details of the synthesized samples, the lattice parameters of YVO₄:Ln³⁺ nanoparticles were calculated by performing the full-profile Rietveld refinement. Figure 2 shows the XRD pattern of YVO₄:Eu³⁺ nanoparticles. The “×” marks represent the experimental diffraction data, the red solid curve shows the calculated diffraction data, and the straight bars show the standard diffraction data for bulk YVO₄ and Ni, respectively. The green solid line located at the bottom of the diffraction data represents the deviation between the calculated and the experimental values. As indicated in Fig. 2, the simulated XRD pattern matched well the experimental one, as indicated by the data fit parameters R_p = 9.208%, R_wp = 12.621%, and χ² = 1.321. The fitted parameters of all YVO₄:Ln³⁺ samples with the different dopant Ln³⁺ ions were listed in Table 2 and Table 3. Among these parameters, R_p and R_wp, represent the regression sum of relative errors and the regression sum of weighted squared errors, respectively. Generally, the smaller they are and the higher reliability of curve fitting is.

 

Fig. 2 Rietveld data fit on powder X-ray diffraction data for YVO₄:Eu³⁺ nanocrystals with mesoporous cell-like nanostructure. Simulated and experimental data and their difference pattern are also shown. Symbols “*” denote the internal standard of Ni.

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The fitted parameters, a, c, unit cell volume V, and axial ratio c/a were compiled in Table 3. As shown in Fig. 3, the lattice parameters of single-crystalline YVO₄:Ln³⁺ nanocrystals with mesoporous cell-like nanostructure are as a function of Ln³⁺ ionic radius. As seen in Figs. 3(a) and 3(b), lattice parameter a increased linearly with increasing Ln³⁺ ionic radius. For lattice parameter c, there is just a slight change of c as Ln³⁺ ionic radius ranging from 1.004 Å and 1.027 Å, and then increased linearly with further increasing Ln³⁺ ionic radius from 1.040 Å and 1.126 Å. In addition, the axial ratio c/a slightly fluctuates around the value of 0.8830. When Ln³⁺ ions were incorporated into YVO₄ nanocrystals, the unit cell volume V increases linearly with increasing Ln³⁺ ionic radius as theoretically predicted by Vegard’s law [31]. Upon the corporation of rare earth Ln³⁺ ions into YVO₄ crystal, an observed linear lattice expansion in the lattice parameters could provide strong evidence for the successful substitution of the dopant Ln³⁺ ions for Y³⁺ in the luminescent host.

 

Fig. 3 Lattice parameters: a, c, c/a, and V of single-crystalline YVO₄:Ln³⁺ nanocrystals with mesoporous cell-like nanostructure are as a function of Ln³⁺ ionic radius.

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The detailed morphology and particle size of YVO₄:Ln³⁺ nanocrystals with mesoporous cell-like nanostructure were studied by SEM, TEM, and HRTEM. As shown in Fig. 10 (Appendix), the samples are composed of well-dispersed nanocrystals. However, the detailed microstructures of the samples are beyond the limitation of detection (LOD) for SEM study. Hence, the microstructures were further characterized by TEM and HRTEM. As shown in Fig. 4(a), as a representative of Eu³⁺ doping YVO₄ nanocrystals, a low-magnification TEM image revealed that the samples are constituted of well dispersed and a regular shape with a relatively narrow size distribution. Selected-area TEM image (Fig. 4(b)) clearly showed that the diameter of these nanocrystals is about 20 nm, with two or more internal mesoporous structure with about 2∼10 nm. High-resolution TEM observations (Fig. 4(c)) demonstrated that YVO₄:Eu³⁺ nanocrystals showed a single crystalline nature of a single nanocrystal with mesoporous cell-like nanostructure. The lattice spacing was calculated to be 0.363 nm between two adjacent lattice fringes, which could be readily indexed to (200) of zircon-type YVO₄. This is consistent with the results of XRD study. Therefore, the YVO₄:Ln³⁺ nanocrystals were not an open hollow structure, but an internal closed cavity structure of a single nanoparticle. The high crystallinity of the mesoporous nanoparticles is also confirmed by the SAED image (Fig. 4(d)), in which a clear diffraction pattern is observed besides some diffraction rings arising from the polycrystalline nature of the sample. Therefore, a single nanoparticle exhibited characteristic of a tiny single crystal, and the YVO₄:Eu³⁺ nanocrystals were not the boundary mesoporous structure through aggregated nanoparticles, but an internal mesoporous structure of a single nanoparticle. Furthermore, the other samples also showed the similar morphology and crystallinity as single-crystalline mesoporous YVO₄:Eu³⁺ nanocrystals. This can be attributed to the similar preparative conditions and the low dopant concentration of Ln³⁺ ions. The microstructure of these mesoporous nanocrystals is quite different from those micro-particles obtained by a microwave synthesis and those nanophosphors acquired by the sol-gel method in literature, respectively [32–34]. The as-prepared YVO₄:Ln³⁺ with mesoporous cell-like nanostructure could be anticipated to show special optical properties compared with those reported somewhere else.

 

Fig. 4 YVO₄:Eu³⁺ nanocrystals with mesoporous cell-like nanostructure: (a) Surveyed TEM image, (b) Partial enlarged drawing of TEM image, (c) Partial enlarged HRTEM image of single nanoparticle, and (d) SAED image.

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3.2. Optical properties of YVO₄:Ln³⁺ with mesoporous cell-like nanostructure

The optical properties of the samples of YVO₄:Ln³⁺ (Ln = Pr, Sm, Eu, Tb, Dy, Ho, Er) with mesoporous cell-like nanostructure were tested at the room temperature. For the convenient discussion, the emission spectra of doped rare earth ions are divided into two groups, one group is doped rare earth ions Ln = Dy, Eu, Sm, Er, while the other group is doped rare earth ions Ln = Y, Tb, Pr, Ho. According to literature reports [35,36], theoretical research has showed that nanocrystals with mesoporous nanostructure could yield better luminescence performance due to multiple light scattering inside the nanocrytals, further improving the absorption efficiency of exciting light and generating far more excitation and emission sites. Therefore, it should be pointed out that the samples with mesoporous nanostructure have an improved optical property compared with the nanoparticles without mesoporous structure.

Figure 5 (left) shows the excitation of the doped rare earth ions YVO₄:Ln³⁺ (Ln = Dy, Eu, Sm, Er) samples (first group). Insets are the digital photographs of the corresponding samples dispersed in an ethanol solution under UV lamp irradiation at 254 nm. The excitation spectrum is measured in the range 230∼420 nm by monitoring the strongest emission peak according to the characteristic emission of Ln³⁺ ions. The excitation spectra showed a very strong broad band at 310 nm ranging from 230 nm to 350 nm mainly belonging to V−O charge transfer transition of VO₄^3- groups. Moreover, some line peaks in the longer wavelength region were observed, which can be assigned to f→f electronic transitions of Ln³⁺ ions [21,37]. It is clearly observed in the spectra that the V−O absorption band is much dominant over the f→f absorptions of the Ln³⁺ ions. This indicates the occurrence of a strong energy transfer from V−O to Ln³⁺.

 

Fig. 5 Optical excitation (left) and emission spectra (right, λ_ex = 310 nm) of single-crystalline YVO₄:Ln³⁺ (Ln = Dy, Eu, Sm, Er) with mesoporous cell-like nanostructure. Insets are photographs of the corresponding samples under UV lamp irradiation at 254 nm.

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Figure 5 (right) showed the emission spectra of the YVO₄:Ln³⁺ nanocrystals by monitoring the VO₄^3- group at 310 nm. As for Dy³⁺ ions, the emission peaks were centered at 483 nm (⁴F_9/2→⁶H_15/2) and 574 nm (⁴F_9/2→⁶H_13/2), which could give the yellow-green light of YVO₄:Dy³⁺ phosphors [38,39]. The emission spectrum of YVO₄:Eu³⁺ arose from the transitions ⁵D₁→⁷F₁, ⁵D₁→⁷F₂ and ⁵D₀→⁷F_J (J = 1, 2, 3, 4) of Eu³⁺ at 538, 558, 593, 618, 650, and 698 nm, which is the characteristic of red phosphors [37,40]. In addition, the emission peak of VO₄^3- did not appear in the emission spectra for Dy³⁺ and Eu³⁺ ions. As for Sm³⁺ ions, the emission spectrum of YVO₄:Sm³⁺ consisted of four main peaks at 568, 602, 647, and 700 nm, which corresponded to ⁴G_5/2→⁶H_5/2, ⁴G_5/2→⁶H_7/2, ⁴G_5/2→⁶H_9/2 and ⁴G_5/2→⁶H_11/2 transitions, respectively [6]. The characteristic emission peaks of Er³⁺ were at 524 nm (⁴H_11/2→⁴I_15/2) and 553 nm (⁴S_3/2→⁴I_15/2) [21]. Furthermore, a significant overlap can be observed comparing the broad emission band of VO₄^3- intrinsic emission at 400∼650 nm and the sharp lines associated with the characteristic transitions from Sm³⁺ and Er³⁺ ions, which could produce the orange-red and blue-green light emission, respectively. For the same doping concentration of Ln³⁺ ions, the emission intensity of single-crystalline YVO₄:Ln³⁺ nanocrystals follows such a sequence: Dy³⁺ > Eu³⁺ > Sm³⁺ > Er³⁺. This indicated that an efficient energy transfer from YVO₄ matrix to the rare earth ions of Dy³⁺, Eu³⁺, Sm³⁺ and Er³⁺ also occurred upon irradiation conditions [14]. The energy transfer from VO₄^3- absorption to the exited states of activator(s) (Ln = Dy, Eu, Sm and Er) can be explained on the basis of the overlapping of the electric dipole fields of the sensitizer (VO₄^3-) and the activator(s) [15]. The first is absorption of UV radiation by VO₄^3- groups, during this, many electrons are promoted from the ground state to the excited state and thus an equal number of holes are generated in the ground state. When the source is removed, electron−hole recombination takes place and gives broad emission from 370 to 650 nm with a maximum at ∼480 nm [14]. Therefore, the excited photons from VO₄^3- emission are absorbed by an activator (Ln³⁺) because the excitation energy levels of Ln³⁺ fall on the emission range of VO₄^3- in 370∼650 nm.

Compared with the optical properties of the first group, the excitation and emission spectra of YVO₄:Ln³⁺ (Ln = Y, Tb, Pr, Ho) samples (second group) are quite different, as shown in Fig. 6. The luminescence intensity of the second group is not only very weak, but the characteristic emission of doped rare earth ions is difficult to observe. In order to facilitate the discussion, the intensity of excitation and emission spectra of the samples were enlarged by 10 times. Under excitation at 310 nm, the emission spectrum of the un-doped YVO₄ exhibited a broad emission band around 400∼650 nm located at 480 nm, corresponding to the intrinsic emission of VO₄^3- groups [41]. For Ho³⁺ ions, the intrinsic emission of VO₄^3- groups and the characteristic emission of Ho³⁺ ions can be observed, including the ⁵S₂→⁵I₈ and ⁵F₅→⁵I₈ transitions of Ho³⁺ ions at 542, 650 nm [42]. Moreover, the intrinsic emission intensity of VO₄^3- groups is slightly weaker than that of the un-doped YVO₄, probably due to partial energy transfer to Ho³⁺ ions through the VO₄^3- groups. After doped with Pr³⁺ or Tb³⁺ ions, the intrinsic emission intensity of VO₄^3- groups is only about 30% compared to the un-doped sample. Meanwhile, a very weak (³P₀→³H₆) electronic transition of Pr³⁺ ions appeared at 608 nm [43], and no electronic transition of was observed in terms of Tb³⁺ ions.

 

Fig. 6 Optical excitation (left) and emission spectra (right, λ_ex = 310 nm) of single-crystalline YVO₄:Ln³⁺ (Ln = Y, Tb, Pr, Ho) with mesoporous cell-like nanostructure. The intensity of excitation and emission spectra of the samples were both enlarged 10 times.

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Figure 7 shows photoluminescence decay curves of Ln³⁺ in single-crystalline YVO₄:Ln³⁺ (Ln = Dy, Eu, Sm, Er) nanocrystals, which can be well-fitted by a double-exponential function [44,45], I_t = A₁exp(-t/τ₁) + A₂exp(-t/τ₂), except for Er³⁺ions. The effective lifetime is defined as [τ] = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂). The τ₁ and τ₂ are the long and short decay times for the exponential components, respectively. The activator Ln³⁺ ions occupied in inner part has a longer lifetime than the less ordered layer near surface. Due to the impurities (e.g., surface hydroxyl groups) at particle surfaces, the excited energy could be easily transferred to the traps as the non-radiative recombination centers. The effective lifetimes were calculated to be 1.190, 0.850 and 0.234 ms for Eu³⁺, Sm³⁺, Dy³⁺, respectively. The external quantum efficiency was recorded at the excitation wavelength of 310 nm on the Edingburgh Instruments FLS920 spectrofluorometer equipped with a xenon lamp as an excitation source [46]. The external quantum efficiency of the YVO₄:Eu³⁺ crystals with mesoporous cell-like nanostructure was 32.13%, which was significantly higher than 1.94% of YVO₄:Eu³⁺ submicrometer crystals assembled by polyacrylamide hydrogel [47].

 

Fig. 7 Optical decay curve of Ln³⁺ in single-crystalline YVO₄:Ln³⁺ (Ln = Dy, Eu, Sm, Er) with mesoporous cell-like nanostructure.

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3.3. Electronic structure of YVO₄:Ln³⁺ with mesoporous cell-like nanostructure

Figure 8(a) showed the absorption spectra of YVO₄:Ln³⁺ (Ln = Dy, Eu, Sm, Er) nanocrystals. All samples were nearly transparent in the visible region, which indicated that all of the as-prepared samples have an extremely low concentration of oxygen vacancies. Moreover, all samples exhibited an intense band-to-band absorption originated from the contribution of ¹A₁→¹T₁ charge transition around 300 nm overlapped with ¹A₁→¹T₂ charge transition at 266 nm of VO₄^3- groups [48]. After normalizing the absorption intensity of the peak at 266 nm, the absorption spectra showed an obvious red shift, which has the consistent changes related to the following band gap values. Interestingly, both the relative intensity of the shoulder (300 nm) and the main peak (266 nm) change with the type of the dopant Ln³⁺ ions and follows such a sequence: Eu³⁺ > Dy³⁺ > Sm³⁺ > Er³⁺. Moreover, the intensity of the shoulder at 300 nm is above the intensity of the main peak at 266 nm for Eu³⁺ ions. According to literature reports [49,50], this extra peak at round 300 nm is assigned to the charge-transfer (CT) band of O^2-→Eu³⁺ resulting from an electron transfer from the ligand O^2- (2p⁶) orbitals to the empty states of 4f⁶ for the Eu³⁺ configuration. Therefore, the broad band around 300 nm is attributed to the overlap of VO₄^3- absorption and CT transition between Eu³⁺ and O^2-, which may partially overlap with the energy levels of the VO₄^3- group. However, for the YVO₄:Ln³⁺ (Ln = Tb, Pr, Ho) samples, the relative intensity of the shoulder (300 nm) and the main peak (266 nm) is almost unchanged compared to the un-doped YVO₄ sample (Fig. 8(b)). Specifically, the absorption spectrum of the YVO₄:Ho³⁺ sample is almost the same as that of the un-doped YVO₄ sample. It should be pointed out that both YVO₄:Ln³⁺ (Ln = Tb, Pr) samples showed a relatively weak broad absorption band at ∼400 nm, which was totally absent for the un-doped YVO₄ sample. Analysis proved that this visible-light absorption (> 360 nm) of color centers is mainly due to the intrinsic defect levels from the V⁴⁺ centers as evidenced by the EPR study reported in the previous literatures [51,52].

 

Fig. 8 Absorbance spectra of single-crystalline YVO₄:Ln³⁺ with mesoporous cell-like nanostructure: (a) Ln = Dy, Eu, Sm, Er, and (b) Ln = Y, Tb, Pr, Ho.

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YVO₄ has a direct optical band gap based on both theoretical calculations and experimental measurements. The top of the valence band is dominated by O 2p state. The bottom of the conduction band is mainly composed of V 3d states [53,54]. Due to no 4f⁰ electrons of Y³⁺ ions, so Y³⁺ ions do not contribute to the valence band top and the bottom of the conduction band of the YVO₄ nanocrystal. Therefore, the relative intensity changes of the absorption peak of the YVO₄:Ln³⁺ samples should be taken into account the following factors: (1) The energy level of the VO₄^3- group is related to the degree of matching with the electron energy level of doping Ln³⁺ ions. Their luminescence properties depend strongly on the location of the 4f energy levels of the Ln³⁺ dopants relative to the valence band (VB) and the conduction band (CB) of the host [48]. The luminescence emission of Ln³⁺ doped YVO₄ is due to energy transfer from VO₄^3- to the excited states (ES) of the activators (Ln³⁺) [55]. Therefore, from the viewpoint of the energy transfer efficiency, the better the level of energy matching between them, the easier is to generate characteristic emission of rare earth ions. (2) In terms of the change of valence state of rare earth ions such as Pr³⁺and Tb³⁺ ions, which are easily oxidized to Pr⁴⁺ and Tb⁴⁺ ions. In the case of dopant Tb³⁺ and Pr³⁺ ions in YVO₄ nanocrystals, valence states change are relatively easier to switch, resulting in the low-cost vanadium and oxygen vacancy defects. Therefore, the color centers formed in YVO₄:Ln³⁺ (Ln = Tb, Pr) samples both showed a light yellow color, which can be ascribed to the reduction of V⁵⁺ to V⁴⁺, further forming of V⁴⁺ defects and oxygen vacancies. Therefore, the formation of V⁴⁺ defects is closely associated with redox between Ln³⁺ (Ln = Tb, Pr) ions and different types of vanadium ions.

Figure 9 depicts the photon energy dependence of (F(R)*hν)² for the corresponding YVO₄:Ln³⁺ (Ln = Y, Eu,Tb) samples. According to the Kubelka-Munk equation, the bandgap energy E_g of mesoporous cell-like YVO₄:Ln³⁺ nanocrystals could be calculated from plots of (αhν)² versus energy (hν), which are listed in Table 1. The band gap was 4.00 ± 0.08 eV for the un-doped YVO₄. Interestingly, a significant difference was observed about the effect of different types of rare earth ions on the band gap of YVO₄:Ln³⁺ nanocrystals, even though the dopant Ln³⁺ ions was kept the same doping concentration at 2 mol%. For instance, the band gap of YVO₄:Tb³⁺ nanocrystals was 3.98 ± 0.07 eV, showing almost the same value within the error range to the undoped YVO₄. This indicated that Tb³⁺ ions doping slightly contributes to the band gap structure of YVO₄ nanocrystals because the energy level structure of Tb³⁺ ions do not match the energy level of the VO₄^3- group. However, the bandgap energy E_g showed an obvious red-shift, reducing to 3.67 ± 0.08 eV for YVO₄:Eu³⁺ nanocrystals, which is due to an increase in the covalent bonding of V−O [56]. Eu³⁺ ions have partially filled 4f⁶ orbitals, leading to an electron transferring from O^2- (2p⁶) orbital to the empty 4f⁶ orbitals, further resulting in the broad band (< 360 nm) overlapping of the VO₄^3- host absorption and charge transfer transition between O^2- and Eu³⁺ [50]. Therefore, the contribution of 4f⁶ electrons of Eu³⁺ either to the valence or conduction band could dramatically increase the covalent bonding of V−O and further lead to a reduction of E_g. That is to say, the energy level of Eu³⁺ ions well matches the energy level of VO₄^3- group, facilitating an effective energy transfer from the VO₄^3- groups to the excited states of Eu³⁺ ions.

 

Fig. 9 Energy dependence of (αhν)² for single-crystalline YVO₄:Ln³⁺ (Ln = Y, Eu, Tb) with mesoporous cell-like nanostructure.

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Table 1. Band gap (E_g) of single-crystalline YVO₄:Ln³⁺ (Ln = Pr, Sm, Eu, Tb, Dy, Ho, Er) and un-doped YVO₄ host with mesoporous cell-like nanostructure.

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For YVO₄:Ln³⁺ (Ln = Dy, Eu, Sm, Er, Ho) samples, the efficient energy transfer from VO₄^3- absorption to the exited states of activator(s) (Ln = Dy, Eu, Sm, Er, Ho) is due to the right match between the energy level of VO₄^3- groups and the activator(s) [15]. The broad band around 300 nm is attributed to the overlap of VO₄^3- absorption and CT transition between Ln³⁺ and O^2-, which may partially overlap with the energy levels of the VO₄^3- group. On the other hand, the difference in the energy level of these rare earth ions brings about a different contribution to the reduction of the band gap of the YVO₄:Ln³⁺ sample. As listed in Table 1, the band gaps of the YVO₄:Ln³⁺ samples were calculated to be 3.67 ± 0.08 eV for Eu³⁺ ions, 3.78 ± 0.07 eV for Dy³⁺ ions, 3.81 ± 0.07 eV for Sm³⁺ ions, 3.94 ± 0.07 eV for Er³⁺ ions, and 3.99 ± 0.08 eV for Ho³⁺ ions, respectively. Therefore, the contribution of rare earth ions to the band gap reduction of YVO₄:Ln³⁺ samples has the following trend: Eu³⁺ > Dy³⁺ > Sm³⁺ > Er³⁺ > Ho³⁺, which is consistent with the the relative intensity changes of the shoulder (300 nm) and the main peak (266 nm) in the absorption spectra of the YVO₄:Ln³⁺ samples.

For YVO₄:Ln³⁺ (Ln = Tb, Pr) samples, the band gaps of the YVO₄:Ln³⁺ samples are calculated to be 3.98 ± 0.07 eV for Tb³⁺ ions, and 4.02 ± 0.07 eV for Pr³⁺ ions, respectively, which are almost equal to that for the un-doped YVO₄. Moreover, the relative intensity of the shoulder at 300 nm and the main peak at 266 nm in the absorption spectra remained nearly unchanged. It is indicated that the energy level of these rare earth ions poorly matches the energy level of the VO₄^3- groups, which showed a slight effect on the band gap of the YVO₄:Ln³⁺ samples. From the emission spectra in Fig. 6 (right), it is hard to observe the characteristic emission of the corresponding rare earth Tb³⁺ and Pr³⁺ ions. Based on the literature reports [49,57], due to the tendency to be oxidized to the tetravalent state in Tb³⁺ and Pr³⁺ ions, an intervalence charge transfer (IVCT) transition occurs in these two ions, which contributes to quench the luminescence of YVO₄:Ln³⁺ (Ln = Tb, Pr).

4. Conclusions

New synthetic strategies have been applied to prepare a series of single-crystalline Ln³⁺-doped YVO₄ (Ln = Pr, Sm, Eu, Tb, Dy, Ho, Er) with mesoprous nanostructrue. The synthetic samples showed a slight variation of crystallite size, ranging from 15 to 17 nm. The unit cell volume showed a linear increase trend with increasing the ionic radius of Ln³⁺ ions. For YVO₄:Ln³⁺ (Ln = Dy, Eu, Sm, Er, Ho) samples, the characteristic emission of rare earth ions can be clearly observed because of an effective energy transfer from the VO₄^3- groups to the excited states of Ln³⁺ ions. However, the characteristic emission is quenching in Ln³⁺ ions (Ln = Tb, Pr). The bandgap energy E_g showed an obvious red-shift, reducing to 3.67 ± 0.08 eV for YVO₄:Eu³⁺ nanocrystals, which is likely due to an increase in the covalent bonding of V−O. The contribution of rare earth ions to the band gap reduction of YVO₄:Ln³⁺ samples has a sequence: Eu³⁺ > Dy³⁺ > Sm³⁺ > Er³⁺ > Ho³⁺, bringing about the difference in the matching degree of the energy levels between the VO₄^3- groups and Ln³⁺ (Ln = Dy, Eu, Sm, Er, Ho) ions. Due to the tendency to be oxidized to the tetravalent state in Tb³⁺ and Pr³⁺ ions, an intervalence charge transfer (IVCT) transition occurs in these two ions, which contributes to quench the luminescence of YVO₄:Ln³⁺ (Ln = Tb, Pr).

Appendix

 

Fig. 10 The SEM images of the un-doped YVO₄ and YVO₄:Eu³⁺ samples.

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Table 2. The fit parameters are by the full-profile Rietveld refinement for YVO₄:Ln³⁺ (Ln = Pr, Sm, Eu, Tb, Dy, Ho, Er) and un-doped YVO₄ host with mesoporous cell-like nanostructure.

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Table 3. The lattice parameters and crystallite size of YVO₄:Ln³⁺ (Ln = Pr, Sm, Eu, Tb, Dy, Ho, Er) and un-doped YVO₄ host with mesoporous cell-like nanostructure.

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Funding

National Natural Science Foundation of China (No. 21663021), the Natural Science Key Project of Jiangxi Province (No. 2017ACB20040), and the Natural Science Foundation of Jiangxi Province (No. 20161BAB213058).

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