Controllable Synthesis and Size-Dependent Luminescent Properties

Jul 29, 2010 - As a consequence, the intensity ratio of 5D0−7FJ to 5D1−7FJ .... One-step synthesis of luminescent YVO 4 :Eu 3+ / γ -Al 2 O 3 nano...
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J. Phys. Chem. C 2010, 114, 14018–14024

Controllable Synthesis and Size-Dependent Luminescent Properties of YVO4:Eu3+ Nanospheres and Microspheres Wen Xu, Yu Wang, Xue Bai, Biao Dong, Qiong Liu, Jiansheng Chen, and Hongwei Song* State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin UniVersity, 2699 Qianjin Street, Changchun 130012, China ReceiVed: May 27, 2010; ReVised Manuscript ReceiVed: July 14, 2010

Uniform, monodispersed, and size-controllable YVO4:Eu3+ nano- and microspheres ranging of 20-1200 nm in diameter were successfully synthesized under the solvothermal condition with N,N-dimethylformamide as the solvent, poly(vinyl pyrrolidone) (PVP) and cetyltrimethyl ammonium bromide (CTAB) as surfactants, meanwhile, introducing HCl acid in the reaction. They were characterized by various techniques, including field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), X-ray power diffraction (XRD), and Fourier transform infrared (FTIR) spectra. The results indicate that the assistant of PVP played an important role in the formation of uniform and monodispersed YVO4: Eu3+ spheres, which effectively prevented aggregation of crystallites. The amount of acid can adjust particle size, while the coassistant of CTAB is helpful of forming spherelike particles. A possible formation mechanism of YVO4:Eu3+spheres was proposed. Furthermore, size-dependent luminescent properties were studied. It is interesting to observe that the excitation bands originating from VO43- groups became narrow on the red side with the decreasing particle size, which is attributed to the size confinement effect to energy transfer. As a consequence, the intensity ratio of 5D0-7FJ to 5D1-7FJ increased and the initial population ratio of 5D0-5D1 increased. A detailed model was proposed to explain the size-dependent photoluminescence. I. Introduction Recent years, phosphors used for fluorescence probes in biological fields have attracted a great deal of attention, which can be divided into three major categories: organic dyes, quantum dots (QDs), and the rare-earth (RE) doped nanophosphors.1–3 The commonly used fluorescent nanoprobes are organic dyes and quantum dots (QDs),4 because they can be very small in size and have high aqueous solubility. However, organic dyes and QDs have their intrinsic disadvantages, such as a broad emission band, chemical instability, background noise, and toxicity to living organisms.5,6 Very recently, RE doped biological nanocrystals (NCs) have attracted improving attention due to their sharp emission lines, long lifetime, especially, the controllable frequency conversion models including downconversion and up-conversion.7,8 RE doped NCs used for fluorescence probes in biological fields mainly include fluorides and oxides. Fluorides have low phonon energy and high upconversion efficiency, however, because of their adsorption bands located in the region of vacuum ultravoilet (VUV), usually they can not generate efficient down-conversion emissions under UV or visible excitation. In addition, they have some other disadvantages, such as chemical instability, low water solubility, and toxicity, which significantly limit their application in the biological field. Therefore, it is important to search for efficient RE-doped oxide nanophosphors, which can safely be used in biological applications. In various oxide matrixes, yttrium vanadate (YVO4) is an excellent one, which has very strong absorption in the UV region and can emit a variety of colors with high efficiency. The bulk YVO4:RE powders or single crystals have been extensively used in lighting and display9 and laser materials.10 Consequently, the luminescent properties of * To whom correspondence should be addressed. Phone and fax: +86431-85155129. E-mail: [email protected].

nanocrystalline YVO4:Eu3+ powders have aroused great interest.11,12 A lot of synthesis methods have been employed in the preparation of YVO4:Eu3+ nanophosphors, for example, thermal pyrolysis, coprecipitation technique, hydrothermal or solvothermal method, and so on.13–15 Despite, controllable synthesis of uniform and monodispersed YVO4 nanocrystals (NCs) is still a challenge, which is significant to further explore size-dependent luminescent properties of nanophosphors and realize various biological functions. Herein, we present the controllable preparation and characterization of uniform and monodispersed YVO4:Eu3+ nanospheres and microspheres with DMF as solvent under the solvothermal condition. Also, size-dependent luminescent properties were studied with an emphasis. It is interesting to observe that with decreasing particle size, the excitation bands of VO43groups gradually became narrow on the red side, and the initial population proportion of 5D0-5D1 increased. A model was given to explore the size-dependent luminescent properties. II. Experimental Section A. Materials. Yttrium nitrate (Y(NO3)3 · 5H2O) and europium nitrate (Eu(NO3)3 · 5H2O) were received from the National Engineering Research Centre of Rare Earth Metallurgy and Function Materials. Sodium citrate (Na3VO4 · 12H2O), cetyltrimethyl ammonium bromide (CTAB), and polyvinyl pyrrolidone (PVP K30) were obtained from Tianjing Chemical Factory. The solvent N,N-dimethylformamide (DMF) was purchased from Beijing Chemical Plant (Beijing, P. R.China). All chemicals were used as received. B. Preparation. All the YVO4:Eu3+ nanospheres and microspheres were prepared via the solvothermal method using DMF as solvent. In a typical synthesis, 0.25 g of yttrium nitrate Y(NO3)3 · 5H2O, 0.01 g of europium nitrate Eu(NO3)3 · 5H2O were first dissolved in DMF (10 mL), then a given amount of CTAB

10.1021/jp1048666  2010 American Chemical Society Published on Web 07/29/2010

YVO4:Eu3+ Nanospheres and Microspheres

Figure 1. The XRD patterns of YVO4:Eu3+ solvothermal products prepared with the assistance of different surfactants: (a) no surfactant, (b) CTAB, (c) PVP, and (d) PVP + CTAB.

and PVP were added with stirring for 1 h, and then 1 mL 2 mol/L HCl aqueous solution and 0.22 g of Na3VO4 · 12H20 were added subsequently with stirring for another 12 h until the solution became homogeneous. After that, the mixing solution was sealed in a Teflon-lined stainless steel autoclave. The autoclaves were heated at 150 °C for 24 h under static condition and then allowed to cool naturally. YVO4:Eu3+ precipitates were collected with centrifugation and washed three times with alcohol, then air-dried at 60 °C for 24 h. All of the above-mentioned manipulations were performed in ambient atmosphere. In our synthesis process, we also control the morphology and particle size of YVO4:Eu3+ NCs by introducing an HCl-water solution (1 mL) with different molar weights, 1, 1.4, 1.8, 2, 4, 6 M, respectively, and fixing surfactants of PVP and CTAB with the same conditions. C. Characterizations. The surface morphology of the asprepared products were measured with a JEOL JSM-7500 field emission scanning electron microscope (FE-SEM) at an accelerating voltage of 15 kV. The high-resolution-transmission electron microscope (HR-TEM) images were recorded on a JEOL-2100F HR-TEM under a working voltage of 200 kV. The

J. Phys. Chem. C, Vol. 114, No. 33, 2010 14019 phase structure and purity of the as-prepared samples were characterized by X-ray power diffraction (XRD) with a Rigaku D/max 2550 X-ray diffractometer, using a monochromatized Cu target radiation resource (λ ) 1.54 Å). The Fourier transform infrared (FTIR) spectra were measured by a Shimadzu DT-40 model 883 IR spectrophotometer. Ultraviolet-visible (UV-vis) absorption spectra were measured with a Shimadzu UV-3101PC UV-vis scanning spectrophotometer with a range of 200-1100 nm. The excitation and emission spectra were recorded at room temperature using a Hitachi F-4500 spectrophotometer. The luminescent dynamics were pumped by a laser-system consisting of a Nd:YAG pumping laser (1064 nm), the third-order Harmonic-Generator (355 nm), and a tunable optical parameter oscillator (OPO, Continuum Precision II 8000). It was with the pulse duration of 10 ns, repetition frequency of 10 Hz, and line width of 4-7 cm-1. III. Results and Discussion A. Structure, Morphology of YVO4:Eu3+ Spheres. First of all, the structure and morphology of the samples prepared with different surfactants were investigated. Figure 1 shows the XRD patterns of the YVO4:Eu3+ products prepared with assistance of different surfactants. It can be seen that all of the samples were exactly in agreement with the corresponding standard cards, JCPDS 17-0341 for tetragonal YVO4. No impurity peaks appear, implying that the samples prepared with different surfactants are all YVO4:Eu3+ in pure tetragonal phase. The crystallite size of the samples can be estimated from Scherrer’s equation,

D ) 0.89 λ/β cos θ where D is the average grain size, the factor 0.89 is characteristic of spherical objects, λ is the X-ray wavelength (0.154 05 nm), and β and θ are the full-width at half-maximum and diffraction angle of an observed peak, respectively. On the basis of the strongest peak (200) at 2θ ) 25.06°, the crystallite sizes of the samples a, b, c, and d were determined to be 14, 14, 17.5, and 18.0 nm, respectively. Figure 2 presents the morphologies of the YVO4:

Figure 2. FE-SEM images of the YVO4:Eu3+ solvothermal products prepared under different surfactants: (a) no surfactant, (b) CTAB, (c) PVP, and (d) PVP + CTAB.

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Figure 3. TEM, SAED, and HR-TEM images of the YVO4:Eu3+ spheres corresponding to the sample in Figure 2d.

Eu3+ samples prepared with different surfactants. It can be seen that the YVO4:Eu3+ NCs prepared without surfactant (see Figure 2a) and with only the assistance of CTAB (see Figure 2b) are irregular and aggregate together. The NCs prepared with only PVP yield monodispersed nanoparticles, with a mean size of ∼100 nm; however, they are not ideal spheres (see Figure 2c). The NCs with the coassistants of CTAB and PVP yield uniform and monodispersed nanospheres, with an average size of ∼170 nm (see Figure 2d). Therefore, we can conclude that in the preparation of solvothermal YVO4:Eu3+ NCs with DMF solvent, the existence of PVP surfactant can efficiently prevent aggregation of the nanoparticles and is an important key to obtain monodispersed NCs. The coexistence of CTAB is helpful for forming ideal nanospheres of YVO4:Eu3+. Therefore, we employ PVP and CTAB as mixing surfactants and change other conditions in the following synthesis. The morphology of the YVO4:Eu3+ spheres with the coassistance of PVP and CTAB (corresponding to Figure 2d) were further examined by HR-TEM images, as shown in Figure 3. Figure 3a,b further shows that the YVO4:Eu3+ NCs are uniform and monodispersed spheres, with an average diameter of ∼170 nm. The inset selected area electron diffraction (SAED) pattern recorded from the central YVO4:Eu3+ spheres in Figure 3c reveals that it consists of only one set of diffraction points aligning in a tetragon, which corresponds to the tetragonal YVO4:Eu3+. Despite, we still suggest that the YVO4:Eu3+ NCs are polycrystals, because the crystalline sizes calculated by XRD patterns are much smaller than the practical sizes of the NCs.16 The typical HR-TEM image (see Figure 3d) of YVO4:Eu3+ spheres clearly shows that in one nanoparticle the lattices almost arrange along one direction, and lattice fringes are with an interplanar spacing of 0.36 nm, which correspond to the [200] plane of YVO4. Figure 4 shows the FTIR spectra of the samples prepared with different surfactants. In all the samples, the transmittance band around 821 cm-1 is ascribed to the V-O stretching vibration in the VO4 tetrahedron.17 The bands at ∼1600 and ∼3500 cm-1 are assigned to the OH- bending and stretching modes, respectively. In the NCs prepared with the assistance of CTAB (see Figure 4b), some new vibration bonds occur at

Figure 4. FTIR spectra of the YVO4:Eu3+ spheres under different surfactants: (a) no surfactant, (b) CTAB, (c) PVP, and (d) PVP + CTAB.

the bands of ∼2980, 2880, 1460, 1380, 1100, and 1050 cm-1, which can be attributed to the ∼CHx and amine groups arising form the starting CTAB. Relative to the NCs without surfactant, the NCs prepared with PVP (see Figure 4c) shows a new vibration bond at ∼1650 cm-1, which should be assigned to the CdO group arising from the starting material (PVP).18 In the NCs prepared with the coassistants PVP and CTAB (see Figure 4d), the characteristic vibration bands of the starting material PVP appear, but the characteristic vibration bands of the starting material CTAB are not detected. This suggests that in the presence of PVP, CTAB cannot be adsorbed on the surface of the YVO4:Eu3+ NCs. The morphology and size were also controlled by introducing the acid with different weights of the coassistants of PVP and CTAB. Figure 5a-f shows the FE-SEM images of the YVO4: Eu3+ products corresponding to different molar weights of HCl water solution (1 mL) in the preparation, 1, 1.4, 1.8, 2, 4, and 6

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Figure 6. Absorption spectra of YVO4:Eu3+ particles corresponding to the samples characterized in Figure 5.

Figure 5. FE-SEM images of the YVO4:Eu3+ spheres prepared under different amounts of acid: (a) 1, (b) 1.4, (c) 1.8, (d) 2, (e) 4, and (f) 6 M.

M, respectively. In Figure 5a-f, the mean diameters of the NPs corresponding to the HCl contents of 1, 1.4, 1.8, 2, 4, and 6 M were determined to be ∼20, 60, 100, 170, 800, and 1200 nm, respectively. (The crystalline sizes of them were determined to be 11.5, 15.4, 17.5, 18.0, 17.9, 18.2 nm, respectively; see the Supporting Information, Fig. S2). Obviously, the average particle size increases with the increase of HCl content. It can be also seen that when the molar weight of HCl is 1 M, the YVO4:Eu3+ NCs tend to adhere. When the HCl content approaches to 6 M, some small particles appear beside the spherical YVO4:Eu3+. Our further results also demonstrate that when the HCl content exceeds 6 M, the average diameter of YVO4:Eu3+ spheres do not increase continuously. Therefore, we can conclude that the proper concentration of HCl is also an important key for the formation of sizecontrollable, uniform, and monodispersed YVO4:Eu3+ nano- or microspheres. It should be noted that we also prepared YVO4:Eu3+ in different alkaline conditions with the coassistants CTAB and PVP, which demonstrated that whatever the contents of NaOH were added, the morphology of YVO4:Eu3+ products hardly changed and was still in dreadful disorder. Actually, we also tested the influence of solvent temperature on the morphology and size of the YVO4:Eu3+ products ranging from 100 to 180 °C. The results demonstrate that when the temperature was lower than 150 °C, the as-prepared samples processed uniform nanospheres. However, when the temperature reached 180 °C, large amounts of small particles appeared and the size was not uniform again (see the Supporting Information, Fig. S1). B. Formation Mechanism of YVO4:Eu3+ Nanospheres or Microspheres. Now, let us discuss the formation mechanism of YVO4:Eu3+ nanospheres or microspheres in the DMF solvent with the assistants of acid, PVP, and CTAB. First, we choose DMF as solvent instead of water because it can decrease the hydrolysis rate of RE nitrate salts, so that the reaction process is easier to be controlled. In the formation of YVO4:Eu3+, PVP was adsorbed on the surface of the NCs, which was confirmed by the FTIR spectra. We suggest that the amount of PVP coordinated onto the Y3+ ions of YVO4:Eu3+ crystal surfaces through the carboxyl groups and

efficiently prevented aggregation of crystallites with the mixing amount of acid.18 When the acid increased, more carboxyl groups of PVP were protonized, leading to less adsorption of PVP on the surface of YVO4:Eu3+, as a consequence, the particles were easier to aggregate and the particle size gradually increased.19 The amount of CTAB may undergo a dissolving process to produce positive ionic species such as CTA+ under solvothermal conditions, which could facilitate the stabilization of the initial YVO4:Eu3+ nucleation, and the coexistence of CTAB is helpful for forming ideal nanospheres of YVO4:Eu3+. Meanwhile the carboxyl groups of the PVP have interaction with CTA+, which reduces the coordinate abilities of PVP in the solvothermal process.18 Thus, PVP does not thoroughly protect the nuclei, which induces the nuclei to aggregate to give rise to forming larger spheres, and it can explain the reason why employing PVP as a surfactant can get smaller particles compared with employing mixing surfactants with a fixed amount of acid. C. Size-Dependent Photoluminescence Spectra of YVO4: Eu3+ Spheres. The absorption, excitation, and emission spectra of YVO4:Eu3+ nano- and microspheres with different sizes were studied in order to better understand the size-dependent photoluminescence behavior.22,23 Figure 6 shows absorption spectra of the samples with different sizes. A band peaking at about 276 nm can be observed for all the samples, corresponding to the charge transfer transition from the oxygen to the central vanadium atoms inside the VO43-. Also, the peak location changes little with changing the particle size, implying that the energy gap between the ground and excited states of VO43spaces in different samples change little. Figure 7 shows the excitation and emission spectra of YVO4: Eu3+ spheres with different sizes, corresponding to the samples characterized in Figure 5. In the excitation spectra, broad bands ranging from 220 to 350 nm are observed in all the samples, corresponding to absorption of vanadate bands resulting from a charge transfer from the excited ligands to the central vanadium atom inside the VO43- ions. The mechanism of this energy transfer (ET) is well-known as the subject of some investigations.11,21 In the excitation spectra, the weak lines (∼395 nm) corresponding to the f-f transitions within the 4f6 configuration of the Eu3+ ions can also be observed, but their intensity is weak relative to that of the ET transitions from VO43- to Eu3+, implying that ET transitions from VO43- to Eu3+ is more efficient. It is interesting to observe that excitation bands of vanadate become narrow gradually on the red side as the size of the YVO4:Eu3+ spheres decreases from 1200 to 20 nm. Because the energy levels for the VO43- spaces changed little in different samples, we attributed the variations of excitation

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Figure 7. Normalized excitation (λem ) 620 nm) and emission (λex ) 270 nm) spectra of YVO4:Eu3+ spheres with different sizes, corresponding to the samples characterized in Figure 5.

spectra to the size-confinement effect to ET. For YVO4:Eu3+ phosphors, efficient ET through adjacent vanadate species has been observed to dominantly contribute to its high photoluminescence and has the average migration distance as far as ∼9 nm, until the excitation reaches the Eu3+ ions.21 Besides, the absorption coefficient (R) for YVO4 at the peak location is as high as ∼200 000 cm-1,11c so that the effective excitation depth of UV light maybe only tens of nanometers. This means that the photoluminescence is dominated by the emissions of Eu3+ in the surface layer around tens of nanometer, even in the bulk material. As the excitation energy migrates from one vanadate to the others, a part of excitation energy will be lost due to electron-phonon coupling before it is further transferred to a Eu3+ ion. As a consequence, the excitation bands shift to red. As the particle size decreases, the ET among different vanadates will be confined due to the boundary effect, leading to energy migration among different vanadates to decrease and the excitation bands on the red side of vanadates to disappear. In the emission spectra of Figure 7, the green emissions of 5 D1-7F1 and the red 5D0-7FJ (J ) 0-3) transitions are observed, as labeled in the figure. It should be pointed out that the spectral configurations of the emission spectra in different samples do not change obviously. The intensity ratios of ∑5D0-7FJ to

Xu et al. ∑5D1-7FJ in the samples of 20, 60, 100, 170, 800, and 1200 nm are determined to be 21.1, 18.7, 17.0, 17.5, 17.1, and 16.9, respectively. This indicates that the ratio quickly decreases when the size increases from 20 to 100 nm, and it remains nearly constant as the size increases continuously. D. Size-Dependent Luminescent Dynamics of YVO4:Eu3+ Spheres. In order to further investigate the size-dependent photoluminescent properties, the luminescent dynamics of YVO4:Eu3+ spheres with different particle sizes were also studied. Figure 8 shows the luminescence rising and decay dynamics of 5D0-7F2 transitions in different YVO4:Eu3+ samples at room temperature. As can be seen, all of the 5D0-7F2 transitions increase initially, approach a maximum, and then decay slowly. Also, the smaller the particle size, the smaller is the relative contribution of the rising process. Under 270 nm excitation, electrons will first be excited from ground states to the excited states of vanadate groups, and then the excited vadanate groups transfer energy quickly to different excited states of Eu3+. There exist two main paths for populating the 5 D0 level, the VO43- feeding and the 5D1-5D0 relaxation. The transitions from VO43- excited states to Eu3+ ions are very fast. In Figure 8, the rise process in the time scale of several tens of microseconds should be caused by the nonradiative relaxation of 5D1-5D0. The studies on steady-state emission spectra indicate that the emissions from levels higher than 5D1 are negligible, thus the rate equations governing the level populations of the two states can be set up:20

dN1(t) ) -W10N1(t) - R1N1(t) dt

(1)

dN0(t) ) W10N1(t) - R0N0(t) dt

(2)

where N1(t) and N0(t) represent the populations of 5D1 and 5D0 at any time, respectively, R0 or R1 is the total radiative transition rate of ∑5D0-7FJ or ∑5D1-7FJ, and W10 is the nonradiative

Figure 8. Room-temperature luminescent dynamics of 5D0-7F2 transitions at 620 nm in YVO4:Eu3+ spheres with different sizes (λex ) 270 nm).

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TABLE 1: Variation of Decay (τ0), Rising (τ1) Lifetime Constants, Relative Contribution (I0/I1) on Particle Size and the Steady-State Emission Intensity Ratios of ∑5D0-7FJ to ∑5D1-7FJ (I0′/ I1′) size (nm) τ0 (µs) τ1 (µs) I0/I1 I0′/I1′

20

60

100

170

800

1200

267 60 2.21 21.1

302 73 1.84 18.7

310 82 1.41 17.0

255 70 1.27 17.5

267 65 1.54 17.1

325 83 1.35 16.9

relaxation rate of 5D1-5D0. On the basis of eqs 1 and 2, it can be deduced that

N0(t) ) I0 e-t/τ0 - I1 e-t/τ1

(3)

Figure 9. Dependence of I0/I1 and N0(0)/N1(0) on the particle size of YVO4:Eu3+ spheres (I0/I1 and N0(0)/N1(0) are defined in the text).

with

τ0 )

1 , R0

τ1 )

I0 ) N0(0) + N1(0)

I1 ) N1(0)

1 R1 + W10

W10 R1 + W10 - R0

W10 R1 + W10 - R0

where N0(0) or N1(0) represents the electron populations of D0 or 5D1 immediately after one excitation pulse terminates (t ) 0). In Figure 8, all the luminescent dynamics are well fitted by eq 3, and the obtained values of τ0, τ1, and I0/I1 are listed in Table 1. It is worth mentioning that the ratio of I0/I1 gradually increases with the decreasing particle size, and the lifetime constants τ0 and τ1 vary nearly independent of the particle size. On the basis of our previous results, the population ratio of 5 D0 to 5D1 under the excitation of a continuous light has the following relationship with N0(0)/N1(0),20 5

N0′ (W10 + R1) N0(0) W10 ) + N1′ R0 N1(0) R0

(4)

In order to further deduce the population ratio of N0(0)/N1(0), the following equations are set up based on eqs 3 and 4:

R1 )

I0 N0(0) W10 + R1 - R0 ) +1 I1 N1(0) W10

(5)

I0′ W10(W10 + R1) N0′η0 N0(0) (W10 + R1)2 R2 ) ≈ + ) I1′ N1′η1 N1(0) R0R1 R0R1 (6) where I0′ and I1′ represent the steady-state emission intensity of 5D0-7FJ and 5D1-7FJ, respectively, the ratio of I0′/I1′ was shown in Table 1, η0 and η1 present the luminescent quantum efficiency of 5D0 and 5D1. The energy gap between 5D0 and its nearest down level 7F6 is as high as 12 000 cm-1, the nonradiative transition rate for 5D0 can be neglected, thus

Figure 10. Schematic illustration of energy transfer and luminescence processes of YVO4:Eu3+ nano- and microspheres.

we have assumed that η0 ≈ 1. On the basis of eqs 5 and 6, we have

N0(0) (R1 - 1)R2 ) N1(0) (R1τ0 /τ1) - (R2τ1 /τ0) + R2 - 1

(7)

In eq 7, all the parameters on the right side have been determined by experiments, the values of N0(0)/N1(0) in different samples can be deduced. Figure 9 shows dependence of N0(0)/N1(0) as well as I0/ I1 on particle size. It is interesting to observe that as the particle size varies from 1200 to 100 nm, N0(0)/N1(0) as well as I0/I1 are nearly reserved as a constant. As the particle size decreases from 100 to 20 nm, N0(0)/N1(0) as well as I0/I1 quickly increases. This means that as the particle size of YVO4:Eu3+ is larger than ∼100 nm, about 2/3 of the excited energy of the VO43- groups is transferred to 5D1, while only 1/3 of excited energy is directly transferred to 5D0 if the excited energy transferred to the other higher excited states is neglected. On the other hand, as the particle size deceases to ∼20 nm, half of the excitation energy of the VO43- groups is transferred to 5D1 or 5D0. In order to better understand the size-dependent photoluminescence, the schematic illustration of energy transfer processes of YVO4:Eu3+ from VO43- to 5D1 and 5D0 is drawn in Figure 10.24,25 In Figure 10, we assume that the quantitative configuration coordinate of the excited VO43- in the nanosized particle is narrower than that in the bulk because of the limited energy migration among VO43- groups due to the size confinement effect to ET. In addition, when the electrons are excited from the ground state to the excited state, the balance positions of

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both the Eu3+ ions and VO43- groups should change. Also, the change of balance position for Eu3+ ions should be smaller than that for VO43- groups due to its feature of the 4f configuration. Note that in Y2O3:Eu3+, Konrad et al. also proposed a similar model to explain the blue shift of the excitation band of nanocrystalline Y2O3 relative to the bulk. They attributed this fact to improved electron-phonon coupling in NCs.25 The quantitative configurational coordinates of the excited states 5D0 and 5D1 of Eu3+ ions crossover with the quantitative coordinates of bulk and nanocrystalline VO43- groups at the points A, B, C, and D, respectively. The energy gaps between A, B, C, D, and the potential minimum E of the VO43- coordinate are marked as ∆E1, ∆E2, ∆E3, and ∆E4, respectively. It is not difficult to deduce the population ratio of N0(0)/N1(0) in the bulk is proportional to e-(∆E3 - ∆E1)/kT, while that in the NCs is proportional to e-(∆E4 - ∆E2)/kT. Because ∆E4 - ∆E2 > ∆E3 ∆E1, the value of N0(0)/N1(0) in the NCs increases in contrast to that in the bulk. IV. Conclusions Size-controllable YVO4:Eu3+ spheres with a diameter of ∼20 to ∼1200 nm were prepared with DMF as the solvent under solvothermal conditions and with the assistance of the surfactants CATB, PVP, and HCl acid. The results demonstrate that the size of the YVO4:Eu3+ spheres are closely related to the amount of acid. Also, the coexistence of PVP is an important key for the formation of uniform and monodispersed YVO4:Eu3+ spheres in the DMF solvothermal solution. In addition, sizedependent luminescent properties were studied. It is important to observe that (1) as the particle size decreases to tens of nanometers, the excitation bands of VO43- on the red side become narrow considerably. These facts can be both attributed to the boundary effect of nanoparticles, which hinders the ET among VO43- groups. (2) As the particle size decreases to tens of nanometers, the initial population ratio of N0(0)/N1(0) increases in contrast to that in the microsized particles, which can be well explained by the quantitative configuration coordinates of YVO4:Eu3+. Acknowledgment. The authors would like to acknowledge the National Talent Youth Science Foundation of China (Grant No. 60925018), the High-Tech Research and the National Natural Science Foundation of China (Grant Nos. 50772042, 20971051, and 10704073), and the Development Program of China (863) (Grant No. 2007AA03Z314).

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