A series of single-crystalline Ln3+-doped YVO4 (Ln = Pr, Sm, Eu, Tb, Dy, Ho, Er) nanoparticles with mesoporous cell-like nanostructure were prepared via initiating homogeneous precipitation followed by a hydrothermal treatment. All synthetic samples with mesoporous cell-like nanostructure showed a slight variation of crystallite size, ranging from 15 to 17 nm. TEM analysis demonstrated that the diameter of the internal mesoporous structure is about 2∼10 nm. The calculated unit cell volume showed a linear increase trend with increasing the ionic radius of the doped Ln3+ ions. Under UV irradiation, the characteristic emission of rare earth ions can be clearly observed for YVO4:Ln3+ (Ln = Dy, Eu, Sm, Er, Ho) samples. Moreover, the mesoporous cell-like nanostructure has exhibited an enhanced optical property compared with the nanoparticles without mesoporous nanostructure. However, for Ln = Tb and Pr, the characteristic emission of rare earth ions is difficult to observe, even quenching. Furthermore, the bandgap energy Eg showed an obvious red-shift, reducing to 3.67 ± 0.08 eV for YVO4:Eu3+ nanoparticles. The band gap reduction of YVO4:Ln3+ samples possessed the sequence: Eu3+ > Dy3+ > Sm3+ > Er3+ > Ho3+ whereas the band gaps of the YVO4:Ln3+ (Ln = Tb, Pr) samples remained unchanged, almost equal to the YVO4 host. Moreover, 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).
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
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 .
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 . 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 . 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 . Especially, YVO4 is as a typical host material, which merits excellent optical stability, higher quantum efficiency, larger stokes transition properties and good biocompatibility . 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 . The relevant research suggests that the multicolor emissions could be realized by controlling the types of activator ions incorporated into host crystal lattice . 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 . 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 Eg dependence property on the doping Ln3+ has been investigated.
2. Experimental section
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.
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 (λex = 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.
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 Rp = 9.208%, Rwp = 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, Rp and Rwp, 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.
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 . 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.
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.
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 VO43- 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+.
Figure 5 (right) showed the emission spectra of the YVO4:Ln3+ nanocrystals by monitoring the VO43- group at 310 nm. As for Dy3+ ions, the emission peaks were centered at 483 nm (4F9/2→6H15/2) and 574 nm (4F9/2→6H13/2), 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→7FJ (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 VO43- 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 4G5/2→6H5/2, 4G5/2→6H7/2, 4G5/2→6H9/2 and 4G5/2→6H11/2 transitions, respectively . The characteristic emission peaks of Er3+ were at 524 nm (4H11/2→4I15/2) and 553 nm (4S3/2→4I15/2) . Furthermore, a significant overlap can be observed comparing the broad emission band of VO43- 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 . The energy transfer from VO43- 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 (VO43-) and the activator(s) . The first is absorption of UV radiation by VO43- 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 . Therefore, the excited photons from VO43- emission are absorbed by an activator (Ln3+) because the excitation energy levels of Ln3+ fall on the emission range of VO43- 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 VO43- groups . For Ho3+ ions, the intrinsic emission of VO43- 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 . Moreover, the intrinsic emission intensity of VO43- groups is slightly weaker than that of the un-doped YVO4, probably due to partial energy transfer to Ho3+ ions through the VO43- groups. After doped with Pr3+ or Tb3+ ions, the intrinsic emission intensity of VO43- 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 , and no electronic transition of was observed in terms of Tb3+ ions.
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], It = 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 . 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 .
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 VO43- groups . 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 O2-→Eu3+ resulting from an electron transfer from the ligand O2- (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 VO43- absorption and CT transition between Eu3+ and O2-, which may partially overlap with the energy levels of the VO43- 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].
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 VO43- 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 . The luminescence emission of Ln3+ doped YVO4 is due to energy transfer from VO43- to the excited states (ES) of the activators (Ln3+) . 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 Eg 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 VO43- group. However, the bandgap energy Eg 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 . Eu3+ ions have partially filled 4f6 orbitals, leading to an electron transferring from O2- (2p6) orbital to the empty 4f6 orbitals, further resulting in the broad band (< 360 nm) overlapping of the VO43- host absorption and charge transfer transition between O2- and Eu3+ . 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 Eg. That is to say, the energy level of Eu3+ ions well matches the energy level of VO43- group, facilitating an effective energy transfer from the VO43- groups to the excited states of Eu3+ ions.
For YVO4:Ln3+ (Ln = Dy, Eu, Sm, Er, Ho) samples, the efficient energy transfer from VO43- 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 VO43- groups and the activator(s) . The broad band around 300 nm is attributed to the overlap of VO43- absorption and CT transition between Ln3+ and O2-, which may partially overlap with the energy levels of the VO43- 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 VO43- 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).
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 VO43- groups to the excited states of Ln3+ ions. However, the characteristic emission is quenching in Ln3+ ions (Ln = Tb, Pr). The bandgap energy Eg 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 VO43- 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).
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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