Submicrometer-Sized Hierarchical Hollow Spheres of Heavy

Dec 5, 2013 - (22, 23) Hence, it is still a great challenge to generalize the sacrificial template method to fabricate hollow spheres of LnVO4. In our...
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Submicrometer-Sized Hierarchical Hollow Spheres of Heavy Lanthanide Orthovanadates: Sacrificial Template Synthesis, Formation Mechanism, and Luminescent Properties Xiaoyan Yang,† Lin Xu,† Zheng Zhai,† Fangfang Cheng,§ Zhenzhen Yan,∥ Xiaomiao Feng,∥ Junjie Zhu,§ and Wenhua Hou*,†,‡ †

Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China ‡ Nanjing University−Yangzhou Institute of Chemistry and Chemical Engineering, Yangzhou 211400, P. R. China § Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210008, P. R. China ∥ Key Laboratory for Organic Electronics & Information Displays, Institute of Advanced Materials, School of Materials Science & Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210046, P. R. China S Supporting Information *

ABSTRACT: Hollow spheres of heavy lanthanide orthovanadates (LnVO4, Ln = Tb, Dy, Er, Tm, Yb, Lu) and yolk−shell structures of Ho(OH)CO3@HoVO4 have been successfully prepared by employing Ln(OH)CO3 colloidal spheres as a sacrificial template and NH4VO3 as a vanadium source. In particular, the as-obtained LuVO4 hollow spheres are assembled from numerous hollow-structured elliptic nanoparticles, and their textural parameters such as the inner and outer diameters, shell thicknesses, and number of shells could be finely tuned through introducing different amounts of NH4VO3 and employing Lu(OH)CO3 templates with different sizes. The possible mechanisms for the formation of hollow spheres and yolk−shell structures, and also the hollow-structured elliptic nanoparticles of LuVO4, i.e., building blocks of LuVO4 hollow spheres, are proposed and discussed in detail. Under ultraviolet excitation, the obtained LuVO4:Eu3+ hollow spheres show strong red emissions located in the saturated color region, and the modulation of emission intensity and color purity could be realized by tuning the textural parameters of the obtained hollow spheres. It was found that the nanostructure of the building blocks of LuVO4:Eu3+ hollow spheres also had an effect on the luminescent properties of the as-obtained materials. Moreover, the quantum efficiency could be affected by the textural parameters of the as-obtained LuVO4:Eu3+ hollow spheres, and the doubleshelled LuVO4:Eu3+ hollow sphere has the highest quantum efficiency. In addition, the excellent biocompatibility indicates the potential biological applications of LuVO4 hollow spheres.

1. INTRODUCTION

Among various morphologies, hollow nano/microspheres with shells composed of nanoparticles are currently attracting continuous interest. Besides the conventional properties of hollow structures, e.g., low density, large specific area, and encapsulation ability, these hollow spheres are also endowed with some new properties (e.g., surface permeability) which enables their potential applications in catalysis,14,15 lithium batteries,16 water treatment,17 drug delivery,18 biotechnology,19−21 and so on. It was reported that GdVO4 hollow spheres could be prepared by using Gd(OH)CO3 colloid spheres as sacrificial template and NH4VO3 as the vanadium source with the introduction of HCl aqueous solution.18 Considering the similar ionic radius of lanthanide ions, it is

Owing to their unusual magnetic characteristics, excellent luminescent and electronic properties, lanthanide orthovanadates (LnVO4) have been found with potential applications as catalysts,1,2 polarizers,3 laser host materials,4 and phosphors.5,6 Since the dimensional and structural characteristics generally endowed materials with a wide range of applications, the fabrication and synthesis of LnVO4 nano/micromaterials with well-defined morphologies and tunable sizes become one of the most efficient approaches to improve the properties and to develop the potential applications of LnVO4. Up to now, various morphologies of different sizes, dimensions, and structures such as nanoparticles,7−9 nanocrystals,5,10 micropancakes,8 microdoughnut,8 spherical aggregates,8 nanorods,11,12 sheaves,11 spherulites,11 and spindle-,13 persimmon-,9 and cube-like shapes9 of LnVO4 have been synthesized via different approaches. © XXXX American Chemical Society

Received: September 12, 2013 Revised: November 22, 2013

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40 mL Teflon-lined autoclave, and kept at 200 °C for 12 h. The solid products were collected through centrifugation, washed with distilled water and alcohol for several times, and dried at 80 °C for 5 h in the air. The textural parameters such as the inner and outer diameters (Din and Dout), shell thicknesses (Rshell), and number of shells of the resulted samples are listed in Table S2. 2.4. In Vitro Cytotoxicity of LuVO4 Hollow Spheres and Cell Viability. To test the potential biological application of the resulted samples, the cytotoxicity against human cervices carcinoma cells (HeLa cells) was measured by taking LuVO4 hollow spheres (sample S1) as an example and using a 3-(4,5-dimethylthiazol-2-yl)-2,5diphenyltetrazolium bromide (MTT) method (details about the method can be found in Note S1). 2.5. Characterization. The phase purity and crystallinity of the products were examined by XRD on a Philip-X’Pert X-ray diffractometer equipped with a Cu Kα radiation source (λ = 1.5418 Å). The general morphology and structure of the as-synthesized products were characterized by scanning electron microscopy (SEM JEOL JEM-6300F). Transmission electron microscopy (TEM), highresolution transmission electron microscopy (HRTEM) images, and selected area electron diffraction (SAED) patterns were obtained by using a JEOL JEM-200CX microscope operating at an accelerating voltage of 200 kV. For TEM observation, the sample was dispersed in a mixture of ethanol and distilled water by ultrasonic treatment and then dropped on carbon−copper grids. The PL excitation and emission spectra were obtained on a Hitachi F-4600 luminescence spectrometer at room temperature. N2 adsorption−desorption isotherms were measured on a Micromeretics ASAP 2020 apparatus at liquid nitrogen temperature. The zeta potential was measured on a zeta potential analyzer (Malvern, Nano-Z). The size distributions were measured on a dynamic light scattering (DLS) instrument (Brookhaven, BI-200SM).

expected that the sacrificial template method can also be used to fabricate hollow spheres of other lanthanide orthovanadates (LnVO4). However, only spindle-like and hollow ellipsoid-like morphologies of YVO4 were obtained by employing Y(OH)CO3 colloidal spheres as sacrificial template and NH4VO3 as the vanadium source.22,23 Hence, it is still a great challenge to generalize the sacrificial template method to fabricate hollow spheres of LnVO4. In our previous work, by adopting an appropriate amount of Y(OH)CO3 template, monodispersed YVO4 hollow spheres with hierarchical nanoshells were successfully prepared via a similar sacrificial template method without introduction of HCl aqueous solution.24 Until now, there is still no report on the successful fabrication of other lanthanide orthovanadates hierarchical hollow spheres. Moreover, a general preparation method still needs to be developed. From the successful fabrication of YVO4 hollow spheres, it was found that the amount of Y(OH)CO3 template introduced was a critical parameter in obtaining YVO4 hollow spheres. In this paper, series heavy lanthanide orthovanadates (LnVO4, Ln = Tb, Dy, Er, Tm, Yb, Lu) hierarchical hollow spheres and Ho(OH)CO3@HoVO4 yolk−shell structure were prepared by adopting an appropriate amount of Ln(OH)CO3 colloid spheres as sacrificial templates and employing NH4VO3 as the vanadium source. The effects of the amount of NH4VO3 and size of Lu(OH)CO3 template on the textural parameters of the as-obtained LuVO4 samples were investigated, and the possible formation mechanisms were proposed and discussed in detail. Moreover, the luminescent properties and the influences of the textural parameters of the as-obtained hollow LuVO4:Eu3+ samples were studied. Finally, for possible biological applications, the biocompatibility of LuVO4 hollow spheres was also evaluated.

3. RESULTS AND DISCUSSION 3.1. Characterization of LuVO4 Hierarchical Hollow Spheres. Figure 1a shows the SEM image of the as-prepared

2. EXPERIMENTAL SECTION 2.1. Materials. All reagents were of analytical grade and used as received without further purification. Distilled water was used throughout. Ln(NO3)3 (Ln = Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu) aqueous solution (0.1 M) was prepared by dissolving the corresponding lanthanide oxide in HNO3 solution (3.0 M) under heating with agitation. 2.2. Ln(OH)CO3 (Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu) Colloidal Spheres. Monodispersed colloidal spheres of Ln(OH)CO3 (Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu) were prepared through a urea-based homogeneous precipitation process.25 First, Ln(NO3)3 (Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu) aqueous solution (0.10 M) and urea [CO(NH2)2] were dissolved in distilled water, forming an aqueous solution in which the total concentration of Ln3+ was kept constant at 0.015 M while that of urea 0.50 M. Then, the resulted solution was stirred at room temperature for 2 h. After that the solution was heated at 90 °C for 2 h in the oil bath. Finally, the resultant Ln(OH)CO3 (Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu) colloidal spheres were collected by centrifugation, washed with distilled water for several times, dried at 60 °C, and kept for further use. The Lu(OH)CO3:5% Eu3+ colloidal spheres with different diameters were also prepared by changing the reaction time at 90 °C via the similar synthetic process. The average diameters of the as-prepared Ln(OH)CO3 (Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu) colloidal spheres are listed in Table S1. 2.3. LnVO4 (Ln = Tb, Dy, Er, Tm, Yb, Lu) Hierarchical Hollow Spheres and Ho(OH)CO3@HoVO4 Yolk−Shell Structure. In a typical synthesis process, 0.602 mmol of Ln(OH)CO3 (Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu) colloidal spheres was first dispersed into 10 mL of distilled water by ultrasonic treatment for 15 min. A total of 0.07 g of NH4VO3 was dissolved in 20 mL of distilled water under heating, and then the resulted NH4VO3 solution was added into the above suspension of Ln(OH)CO3 colloidal spheres. After that, the resultant mixture was stirred for 30 min at room temperature, transferred into a

Figure 1. SEM images of (a) the as-prepared Lu(OH)CO3 colloidal spheres (T1) and (b) the resulted sample S1 after hydrothermal treatment of template T1 with NH4VO3.

Lu(OH)CO3 precursor (T1). It is obviously that the precursor is consisted of uniform submicrospheres with a smooth surface. The average size distribution of T1 is ∼290 nm (see Figure S1a). After hydrothermal treatment with NH4VO3 at 200 °C for 12 h, the resulted sample S1 still maintains a morphology of uniform spheres, yet the average diameter is slightly decreased to ∼272 nm (see Figure S1c) and the surface becomes relatively rougher (Figure 1b). In addition, it can also be observed that there are numerous nanoparticles on the surface of spheres, and the hollow nature of the obtained sample can be affirmed from some of the broken spheres. The XRD pattern of the as-obtained Lu(OH)CO3 precursor is illustrated in Figure 2A. From the broad and weak peaks, the amorphous nature of Lu(OH)CO3 colloidal spheres was confirmed. After hydrothermal treatment of Lu(OH)CO3 B

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aggregate of numerous elliptic nanoparticles with major and minor axis of ∼20 and ∼15 nm, respectively. In addition, some nanoparticles are hollow-structured. The SAED pattern taken from the shell (inset in Figure 3c) indicates that the shell is attributed to the tetragonal LuVO4 phase, and the observed diffraction rings demonstrate the polycrystalline nature of the shell. As disclosed by the corresponding HRTEM image of the area marked with a white frame in Figure 3c, the interplanar distance was determined to be 3.513 Å, corresponding to the (200) plane of LuVO4 phase (see Figure 3d). From the above analyses, it is confirmed that the as-obtained sample S1 is constituted of LuVO4 hollow spheres. After annealing at 700 °C for 3 h, the resulted sample S1-700 still maintains a similar morphology as sample S1 (Figure 3e). From the high-resolution TEM image of a single sphere (Figure 3f), it is noted that most of elliptic nanoparticle subunits are hollow-structured and the hollow cavity is larger than that of S1, indicating an increased inner diameter and a decreased shell thickness of the subunits after annealing. Several interplanar distances corresponding to the (200), (112), and (101) planes of LuVO4 can be clearly observed from the HRTEM image of the shell of hollow spheres (Figure 3g), indicating that the crystallinity of sample S1 was improved after annealing. In addition, diffraction fringes in the center of an elliptic nanoparticle are the same as those in the shell of nanoparticles, confirming that the pale centers of elliptic nanoparticle are closed hollow cavities rather than open holes. The diffraction rings in the SAED pattern of the shell (inset in Figure 3h) demonstrate that the resulted sample is still of polycrystalline nature after annealing sample S1 at 700 °C. Therefore, from the above analyses, it is concluded that the nanostructure, i.e., the inner diameter and shell thickness of the subunits, and also the crystallinity of the as-obtained sample could be changed and improved after annealing. As confirmed by N2 adsorption−desorption isotherms (see Figure S2 and Note S3), the resulted sample S1 before and after annealing at 700 °C (S1-700) have a mesoporous structure. 3.2. Modulation of the Textural Parameters of LuVO4 Hollow Spheres. It was found that the textural parameters of YVO4 hollow spheres, such as Rshell, Din, Dout, and number of shells, could be greatly affected by the amount of NH4VO3 introduced into the reaction system.24 Here, by taking LuVO4 hollow spheres as an example, the effect of the amount of

Figure 2. XRD patterns of (A) template T1, (B) sample S1, and (C) sample S1 annealing at 700 °C (S1-700). The standard data of tetragonal LuVO4 (JCPDS # 17-0880) are presented at the bottom as a reference.

colloidal spheres with NH4VO3 at 200 °C for 12 h, all diffraction peaks of the resulted sample S1 can be readily indexed to the tetragonal phase of LuVO4 (JCPDS # 17-0880) (Figure 2B), indicating the formation of LuVO4 phase after the hydrothermal process. To further determine its chemical composition, the as-obtained hydrothermal product was annealed at 700 °C for 3 h, and no other diffraction peaks except those from LuVO4 were observed, indicating a complete consumption of the sacrificial template Lu(OH)CO3. Otherwise, Lu2O3 would be formed from unreacted Lu(OH)CO3 upon calcination. To further reveal the structure and morphology of the hydrothermal product, TEM and SAED characterizations were employed. As shown in Figure 3a, sample S1 has a uniform morphology of monodispersed spheres with a diameter of ∼200 nm, being in accordance with the SEM image (see Figure 1b). The strong contrast between the dark edge and pale center further confirms the hollow nature of the as-obtained sample S1, and all spherical structures have a hollow cavity with a diameter of ∼130 nm and the shell thickness is about ∼35 nm. From the high-resolution TEM images of a single sphere (Figure 3b,c), it can be found that the shell of the sphere is

Figure 3. TEM images of sample S1 (a) low-magnified image, (b) image of a single sphere, (c) high-magnified image of the shell (inset is SAED pattern of the shell), (d) HRTEM image of the area marked with a black frame in (c), and sample S1 after annealing at 700 °C (S1-700) (e) lowmagnified image, (f) image of a single sphere (inset is SAED pattern of the shell), and (g) HRTEM image of the area marked with a black frame in (f). C

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time. As shown in Table S1 and insets of Figure 4a,e, the average diameter of Lu(OH)CO3 colloidal spheres is decreased from ∼290 to ∼246 nm as the reaction time is decreased from 2 h to 30 min (see Figure S1a,b). By employing template T2 with a diameter of ∼246 nm, the as-obtained sample S3 has a slightly bigger average diameter (∼280 nm) with a bigger average inner diameter (∼240 nm) and a thinner shell thickness (∼20 nm) (see Figure 4e,f, Figure S1e, and Table S2) than that of sample S1 from template T1 which has a larger diameter of ∼290 nm (see Figure 4a,b and Table S2). It can be concluded that the shell thickness of the resulted hollow spheres is increased accordingly with the size of template spheres, while the average diameter is decreased with the amount of NH4VO3 and the sizes of template spheres. 3.3. Characterization of LnVO4 (Ln = Tb, Dy, Er, Tm, Yb) Hierarchical Hollow Spheres and Ho(OH)CO3@ HoVO4 Yolk−Shell Structures. Owing to the similar ionic radii of the lanthanide elements, the synthesis method for LuVO4 hollow spheres was also extended to fabricate the hollow sphere structures of other lanthanide orthovanadates. A series of Ln(OH)CO3 (Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu) colloidal spheres were prepared, and the diameter evolution of the as-obtained Ln(OH)CO3 colloidal spheres is described in details in Note S2. Figure S4 shows TEM images of the resulted samples after hydrothermal treatment of Ln(OH)CO3 (Ln = Tb, Dy, Ho, Er, Tm, Yb) colloidal spheres with NH4VO3 at 200 °C for 12 h. It is observed that most of the resulted samples are hollow spheres assembled from numerous elliptic nanoparticles (Figure S4a−e) except that of Ho(OH)CO3, which has a yolk− shell structure (Figure S4f). In addition, it can also be observed that the subunits of the resulting samples, i.e., elliptic nanoparticles, are solid-structured, not hollow-structured as in the case of LuVO4 hollow spheres. Figure S5a shows the XRD patterns of the resulted samples from Ln(OH)CO3 (Ln = Tb, Dy, Er, Tm, Yb) before and after annealing. All diffraction peaks are well indexed to the corresponding tetragonal phases of LnVO4 (Ln = Tb, Dy, Er, Tm, Yb) (JCPDS # 17-0340, 16-0870, 17-0199, 18-1379, 170338), indicating the formation of pure LnVO4 (Ln = Tb, Dy, Er, Tm, Yb) phase after hydrothermal process and the complete consumption of Ln(OH)CO3 (Ln = Tb, Dy, Er, Tm, Yb) template. These results are just in accordance with the hollow structures of these samples shown in the corresponding TEM images (Figure S4a−e). Whereas, for the sample S7 obtained from Ho(OH)CO3 template, as shown in Figure S5b(A,B), besides those from the tetragonal phase of HoVO4 (JCPDS # 15-0764), one new peak attributed to the hexagonal phase of Ho2O3 (JCPDS # 19-0554) emerged after annealing at 700 °C for 3 h. As Ho(OH)CO3 would be thermally decomposed into Ho2O3 upon annealing at a high temperature, it can be suggested that the as-obtained sample S7 is composed of HoVO4 and Ho(OH)CO3. Combined with the TEM image (Figure S4f), it can be concluded that the as-obtained sample S7 from Ho(OH)CO3 template has a yolk−shell structure of Ho(OH)CO3@HoVO4. From the TEM (Figure S4 and Figure 3) and XRD (Figure S5 and Figure 2) results, it is observed that Ln(OH)CO3 (Ln = Tb, Dy, Er, Tm, Yb, Lu) templates are all completely converted into their corresponding LnVO4 hollow spheres except that Ho(OH)CO3 template is partially converted, and thus a yolk− shell structure of Ho(OH)CO3@HoVO4 is formed. 3.4. Formation Mechanism of LnVO4 (Ln = Tb, Dy, Er, Tm, Yb, Lu) Hollow Spheres and Ho(OH)CO3@HoVO4

NH4VO3 and also the size of Lu(OH)CO3 colloidal spheres were investigated. The TEM images of the as-obtained samples after hydrothermal treatment of Lu(OH)CO3 (T1) with different amounts of NH4VO3 are shown in Figure 4. When the amount of

Figure 4. TEM images of the resulted samples (S1 and S2) after hydrothermal treatment of Lu(OH)CO3 template T1 with different amounts of NH4VO3 and S3 from T2 with 0.07 g of NH4VO3: (a, b) 0.07 g (S1, nLu:nV = 1:1) (inset in (a) is the TEM image of T1), (c, d) 0.28 g (S2, nLu:nV = 1:4), and (e, f) 0.07 g (S3, nLu:nV = 1:1) (inset in (c) is the TEM image of T2).

NH4VO3 was 0.07 g (nLu:nV = 1:1), the resulted sample S1 was consisted of uniform hollow spheres with an inner diameter of ∼192 nm and a shell thickness of ∼40 nm, respectively (see Figure 4a, Figure S1c, and Table S2). From the magnified image of a single sphere, it is observed that the shell is single layered (Figure 4b). As the amount of NH4VO3 was increased to 0.28 g (nLu:nV = 1:4), the resulted sample S2 was also composed of uniform hollow spheres, yet the average diameter was decreased to ∼215 nm while the inner diameter was decreased to ∼115 nm and the shell thickness was increased to ∼50 nm (see Figure 4c, Figure S1d, and Table S2). Moreover, it can also be observed that most hollow spheres are doubleshelled, the thickness of the exterior shell being ∼30 nm while that of the interior shell being ∼20 nm (see Figure 4d and Table S2). As the amount of NH4VO3 was further increased to 0.35 g (nLu:nV = 1:5), the obtained sample S4 was also doublelayered, whereas the exterior shells of some spheres were too loose to be maintained and a large amount of individual nanoparticles were existing in the obtained samples (see Figure S3). From the analyses above, it can be concluded that as the amount of NH4VO3 is increased, the inner diameter of the asobtained hollow spheres is decreased, while the shell thickness and number of shells are increased. To investigate the effect of template size on the textural parameter of the resulted LuVO4 hollow spheres, two Lu(OH)CO3 colloidal templates (T1 and T2) with different diameters were prepared at 90 °C by changing the reaction D

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Figure 5. TEM images of (a) Lu(OH)CO3 colloidal spheres, (b, c) the resulted products of Lu(OH)CO3 template with NH4VO3 at room temperature for different times (b: 1 min, c: 30 min), and (d, e) resulted products after hydrothermal treatment of Lu(OH)CO3 template with NH4VO3 at 200 °C for different times (d: 5 min; e: 12 h).

hydrothermal treatment at 200 °C for 5 min, the shell grew thicker while the core became smaller, giving rise to a clear yolk−shell structure (Figure 5d) and an increased peak intensity in the corresponding XRD pattern (Figure 6D). As the hydrothermal treatment was extended to 12 h, the remained Lu(OH)CO3 template was completely consumed, generating highly crystalline tetragonal LuVO4 hollow spheres which were assembled by numerous hollow-structured elliptic nanoparticles (Figures 5e and 6E). It is also observed that the full width at half-maximum (fwhm) of the observed peaks is decreased with the hydrothermal reaction time (Figure 6). According to the Debye−Scherrer equation,26 it indicates that the size of nanoparticle subunits is increased with the reaction time, being consistent with the size variations shown in the TEM images (Figure 5). On the basis of the above-mentioned results, a possible mechanism can be proposed for the formation of LnVO4 (Ln = Tb, Dy, Er, Tm, Yb, Lu) hollow spheres and Ho(OH)CO3@ HoVO4 yolk−shell structures. As illustrated in Scheme 1, when Ln(OH)CO3 (Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu) colloidal template was dispersed into water, Ln3+ ions were ionized from the spherical template (see eq 1) and located around the template surface. These Ln3+ ions would react with VO43− ions once NH4VO3 was introduced, forming many irregular nanoparticles of LnVO4 (Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu) around the external surface of Ln(OH)CO3 spheres (eq 2). With the extension of stirring, more and more LnVO 4 nanoparticles were genetated, wrapping around Ln(OH)CO3 template and thus a core−shell structure was formed.

Yolk−Shell Structures. To elucidate the formation mechanism of LnVO4 (Ln = Tb, Dy, Er, Tm, Yb, Lu) hollow spheres and Ho(OH)CO3@HoVO4 yolk−shell structures, a series of time-dependent experiments were carried out before and after hydrothermal treatment by taking Lu(OH)CO3 (T1) as a representative template, and the amount of NH4VO3 was kept as 0.07 g. Figure 5a shows the TEM image of Lu(OH)CO3 template. It can be seen that Lu(OH)CO3 template is constituted of uniform solid spheres with smooth surfaces, which is in accordance with the SEM image (see Figure 1a). The broad and weak peaks in the corresponding XRD pattern demonstrate the amorphous nature of Lu(OH)CO3 colloidal spheres (Figure 6A). Once NH4VO3 was introduced into the aqueous

Ln(OH)CO3(s) ⇌ Ln 3 +(aq) + OH−(aq) + CO32 −(aq) Figure 6. XRD patterns of (A) Lu(OH)CO3 colloidal spheres, (B, C) the resulted products of Lu(OH)CO3 template with NH4VO3 at room temperature for different times (B: 1 min; C: 30 min), and (D, E) samples obtained after hydrothermal treatment of Lu(OH)CO3 template with NH4VO3 at 200 °C for different times (D: 5 min; E: 12 h). The standard data of tetragonal LuVO4 (JCPDS # 17-0880) was used as reference.

(1)

Ln 3 +(aq) + VO4 3 − (aq) ⇌ LnVO4 (s)

(2)

The as-formed LnVO4 shell would prevent a direct chemical reaction between Ln3+ and VO43−.24 The LnVO4 shells generated under stirring were constituted of numerous nanoparticles with a poor crystallinity, and these nanoparticles would be dissolved and recrystallized to form high crystalline nanoparticles under hydrothermal conditions due to the driving force of solubility difference.27 The reversible reaction of eq 2 occurred both interior and exterior of the as-formed LnVO4 shell. From the above experiment results, it is known that the solubility product (Ksp) of Ln(OH)CO3 (Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu) is larger than that of the corresponding LnVO4, and thus more Ln3+ ions would be ionized from Ln(OH)CO3 template than from the LnVO4 shell. Therefore, more VO43− ions were consumed inside the shell than outside the shell according to Le Chatelier’s principle, leading to a net consumption of VO43− inside the shell, which seems like VO43− ions diffused inward through the shell. Hence, it is concluded that the further consumption of VO43− ions after the

solution of Lu(OH)CO3 template, many irregular nanoparticles immediately formed on the smooth surface of Lu(OH)CO3 template spheres (Figure 5b). Correspondingly, a weak peak attributed to (200) plane of the tetragonal LuVO4 (JCPDS # 17-0880) phase was observed in the XRD pattern (Figure 6B), indicating the newly formed nanoparticles are of LuVO4 phase. After keeping stirring the reaction mixture at room temperature for 30 min, more and more LuVO4 nanoparticles were generated, wrapping Lu(OH)CO3 sphere and thus forming a core−shell structure (Figure 5c). This variation can also be observed from the corresponding XRD pattern (Figure 6C). As the number of LuVO4 nanoparticles was greatly increased, the peaks attributed to the tetragonal LuVO4 (JCPDS # 17-0880) phase were fully appeared correspondingly. After a further E

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Scheme 1. Schematic Illustration for the Formation of LnVO4 (Ln = Tb, Dy, Er, Tm, Yb, Lu) Hierarchical Hollow Spheres and Ho(OH)CO3@HoVO4 Yolk−Shell Structures

Figure 7. (a) PL excitation (left) and emission (right) spectra of samples Eu-S1, Eu-S2, Eu-S3, and Eu-S1-700 (insets are the corresponding photographs under 254 nm UV light irradiation and TEM images of a single sphere of the samples). (b) Plots of the ratio of the magnetic dipole transition of 5D0 → 7F1 (601 nm) to the electric dipole transition of 5D0 → 7F1 (619 nm) (I601/I619) (black line) and quantum efficiencies of samples Eu-S1, Eu-S2, Eu-S3, and Eu-S1-700 (red dashed line).

of colloidal sphere template, resulting in the generation of a yolk−shell structure of Ho(OH)CO3@HoVO4. As for the possible formation mechanism of hollowstructured nanoparticles, i.e., subunits of LuVO4 hollow sphere, since the Ksp value of Ln(OH)CO3 (Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu) is increased with the decreased ionic radius of Ln3+ ions, it is easy to conclude that Lu(OH)CO3 has the largest Ksp value among these heavy lanthanide basic carbonates. Hence, the amount of Lu3+ ions ionized from the Lu(OH)CO3 template was the most, resulting in the fastest generation of LuVO4 nanoparticles with the poorest crystallinity. Under hydrothermal treatment, the dissolution−recrystallization process of LuVO4 nanoparticles occurred due to the driving force of solubility difference.27 Hence, LuVO4 nanoparticles with a poor crystallinity have a great tendency to be dissolved and recrystallized to form a crystal phase with a high crystallinity. Whereas, owing to the larger Ksp value of Lu(OH)CO3 than that of LuVO4, more Lu3+ ions were ionized from Lu(OH)CO3 template than from LuVO4 shell, leading to the fact that the formation rate of LuVO4 was faster than the dissolution rate of LuVO4 (eq 2). Therefore, the remained LuVO4 nanoparticles with a poor crystallinity would be wrapped by the newly

formation of LnVO4 shell is mainly controlled by the dissolution−recrystallization process. With the proceeding of the reaction under hydrothermal conditions, more Ln(OH)CO3 template was dissolved while more LnVO4 nanoparticles inside and outside the shell underwent a dissolution− recrystallization process, leading to the gradual decrease of Ln(OH)CO3 core, increase of interior void, and also improvement of the LnVO4 shell crystallinity. Eventually, hollow structures of LnVO4 (Ln = Tb, Dy, Er, Tm, Yb, Lu) with a high crystallinity were obtained after hydrothermal treatment at 200 °C for 12 h, except that the resultant sample from Ho(OH)CO3 was a yolk−shell structure of Ho(OH)CO3@HoVO4. For the formation of yolk−shell structure of Ho(OH)CO3@ HoVO4, it can be explained as follows: for a single Ln(OH)CO3 sphere, the bigger the diameter, the more amount of Ln(OH)CO3 it contains. Since the as-prepared Ho(OH)CO3 template has the greatest diameter, it is not easy for the template to be completely converted into HoVO4 shell during the dissolution−recrystallization process under the same reaction conditions with those with a much smaller diameter F

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generated LuVO4 with a higher crystallinity, forming a core− shell structure. As the hydrothermal reaction went on, the dissolution and recrystallization process was continued in the cores of the as-formed core−shell structures. Finally, hollowstructured nanoparticles were formed. On the other hand, when the concentration of NH4VO3 is large enough, the formed concentration gradient and also the diffusion rate of VO43− ions would be greatly increased. Thus, a large amount of VO43− ions could quickly get through the justforming porous LnVO4 shell, leading to the formation of the secondary liquid phase between Ln(OH)CO3 core and LnVO4 shell. Therefore, VO43− ions in the secondary liquid phase would react with the surface Ln3+ ions ionized from the Ln(OH)CO3 core to generate the second shell of LnVO4. Therefore, the further hydrothermal treatment still relied on the dissolution and recrystallization of the as-formed LnVO4 shell, leading to the final formation of hollow spheres with double shells. 3.5. Luminescence Properties. Here, in order to realize the modulation of luminescent properties of LuVO4:Eu3+ hollow spheres, three samples (Eu-S1, Eu-S2, and Eu-S3) with different textural parameters such as Rshell, Din, Dout, and number of shells were prepared and taken as the representative samples. In addition, to investigate the effect of the nanostructure of the building blocks (i.e., hollow nanoparticles) on the luminescent properties of the as-obtained LuVO4:Eu3+ samples, the PL excitation and emission spectra of Eu3+ ions doped S1-700 (Eu-S1-700) were also measured. Figure 7a shows the PL excitation and emission spectra of Eu-S1, Eu-S2, Eu-S3, and Eu-S1-700. It is observed that each excitation spectrum consists of a single strong absorption band maximized at ∼299 nm (Figure 7a, left), which is attributed to the charge transfer from the oxygen ligands to the central vanadium ions inside VO43− groups.13,28,29 No peaks attributed to f−f transition of Eu3+ ions is observed, indicating that the excitation of Eu3+ ions is mainly through the VO43− groups for all four samples. Five emission lines between 539 and 700 nm were observed in the emission spectrum of sample Eu-S1 (Figure 7a, right). These lines can be ascribed to the 5D1 → 7F1 and 5D0 → 7FJ (J = 1, 2, 3, and 4) transitions of Eu3+ ions, respectively, in which the red 5D0 → 7F1 (601 nm) magnetic dipole transition is the dominated line and its intensity is much stronger than that of the 5D0 → 7F1 (619 nm) electric dipole transition (see Figure 7a, right). Since the magnetic dipole transition of 5D0 → 7F1 is allowed when the Eu3+ ion occupies a site with an inversion center and is not sensitive to the local environment, it can be suggested that Eu3+ ion in the as-obtained LuVO4 lattice occupies a site with a high symmetry. As the shell thickness of the as-obtained LuVO4:Eu3+ hollow spheres is decreased, the emissions of sample Eu-S3 are still dominated by the magnetic dipole transition of 5D0 → 7F1, indicating the similar crystal lattice of S1 and S3 (Figure 7a, right). Nevertheless, the intensities are decreased to a great extent, which might be attributed to the fact that spheres with a thinner shell and a bigger diameter have more defects.24 On the other hand, as the shell thickness and number of shells of the as-obtained LuVO4:Eu3+ hollow spheres are increased, the magnetic dipole transition of 5D0 → 7F1 is still the dominant absorption in Eu-S2, although it has a slightly weaker intensity than that of Eu-S1. In addition, for the emission spectrum of sample Eu-S2, besides the similar weak absorption in 5D1 → 7F1 (593 nm) and 5D0 → 7F3 (653 nm), other transitions of 5D0 →

FJ (J = 1, 2, and 4) are all observed with weaker intensities than those of Eu-S1 (see Figure 7a (right) and Figure S6). The decreased intensities transitions can be attributed to the obviously increased light absorption in the longer wavelength region for spherical nanoshells with the increased shell thickness and layers of shell.30 On the other hand, as illustrated in Figure 7a (right), Figure S6 and the intensity ratio plot (Figure 7b (black line)) of the magnetic dipole transition of 5 D0 → 7F1 (601 nm) to the electric dipole transition of 5D0 → 7 F1 (619 nm) (I601/I619), the intensity of the red emission 5D0 → 7F1 (601 nm) in Eu-S2 is far stronger than that of other emissions, indicating a high color purity of sample Eu-S2. Therefore, it can be concluded that the modulation of PL emission intensities and color purity could be realized through tuning the textural parameters of the as-obtained hollow spheres. It is known that after annealing at a high temperature, the phosphors are usually endowed with a higher crystallinity, which is beneficial to the enhancement of emission intensity. From XRD (Figure 2) and TEM (Figure 3) results, it is observed that the crystallinity of S1 is really improved after annealing. Moreover, Eu-S1-700 is observed with a larger I601/ I619 ratio than Eu-S1, demonstrating that Eu3+ ions occupied a site with a higher symmetry in Eu-S1-700 than in Eu-S1. Nevertheless, as shown in Figure 7a (right) and Figure S6, the emission intensities of Eu-S1-700 is much weaker than those of Eu-S1. For the similar textural parameters, e.g. Rshell, Din, Dout, and number of shells, this could be attributed to the variation of nanostructure of building blocks, i.e., elliptic nanoparticles. As shown in Figure 3, after annealing, most nanoparticles become hollow-structured, and the inner diameter of hollow nanoparticles is increased while the shell thickness of hollow nanoparticles is decreased. Therefore, it is concluded that the nanostructure of building blocks of LuVO4:Eu3+ hollow spheres also has an effect on the luminescent properties of the resulted materials. For all four samples, no emission from VO43− groups was observed, demonstrating that the energy transfer from VO43− groups to Eu3+ ions is very efficient.28 The quantum efficiencies (QE) of Eu-S1, Eu-S2, Eu-S3, and Eu-S1-700 are 22%, 41%, 32%, and 30%, respectively (see Figure 7b (red dashed line)). A large number of hydroxyl groups that covered the surfaces of nanoparticles during the synthesis process could quench the emission from Eu3+ ions and decrease QE.31 Since the asobtained LuVO4:Eu3+ hollow spheres are all assembled from nanoparticles, the quench effect would also occur on the asobtained samples. The exterior shell of the double-shelled EuS2 could protect the interior shell from quencher, leading to a higher QE than that of the single-shelled Eu-S1. Annealing process could remove the hydroxyl groups, decrease the quench effect, and also develop the crystallinity,31 and thus Eu-S1-700 has a higher QE than Eu-S1. On the other hand, annealing process also changed nanostructures of the building blocks of Eu-S1, resulting in a lower QE of Eu-S1-700 than that of the double-shelled Eu-S2 without annealing treatment. The reason for the higher QE of Eu-S3 than Eu-S1 and Eu-S1-700 is possibly attributed to the weak absorption during the excitation process (see Figure 7a (left)). Therefore, it is concluded that the textural parameters of the as-obtained LuVO4:Eu3+ hollow spheres could also affect their QE, and the double-shelled LuVO4:Eu3+ hollow spheres has the highest QE. G

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Figure 8. (a) HeLa cells viabilities measured by MTT assay after incubation with different concentrations of sample S1 for 24 h. (b−d) Photos of the stained HeLa cells after incubation for 4 h with sample S1 (b: 0 μg mL−1; c: 125 μg mL−1; d: 200 μg mL−1).

The decay curves of the four LuVO4:Eu3+ hollow samples (Eu-S1, Eu-S2, Eu-S3, and Eu-S1-700) were also investigated (see Figure S7). The decay curves for emissions of all four samples can be well-fitted into a biexponential function such as32,33 I(t) = I0 + A1e−t/τ1 + A2e−t/τ2, where τ1 and τ2 are the decay times for the exponential components and A1 and A2 are the fitting parameters, respectively. The corresponding average lifetime (τav) can be calculated by the following equation τav = (A1τ12 + A2τ22)/(A1τ1 + A2τ2). As shown in Figure S7, the average lifetimes of Eu-S1, Eu-S2, Eu-S3, and Eu-S1-700 were calculated to be 73.4, 521, 92.1, and 99.8 μs, respectively. The latter three samples have longer lifetime than that of Eu-S1. It is known that the kinetic behaviors of decay are closely related to the number of luminescent centers, defects, and surface effect of the host.32,33 From the above analyses, Eu3+ ions occupy the similar sites of Lu3+ ions with an inversion center to show the same luminescent centers for Eu-S1, Eu-S2, and Eu-S3. Both annealing treatment and increasing the number of shells of LuVO4:Eu3+ hollow samples could decrease defects and surface effect, resulting in an increase of lifetime. Hence, Eu-S1-700 and Eu-S2 have a longer lifetime than Eu-S1. On the other hand, for hollow spheres, the bigger the diameter is, the larger the surface willbe and thus a greater surface effect. However, the lifetime of Eu-S3 with a bigger diameter and a thinner shell is longer than that of Eu-S1. It is known that the more luminescent centers exist, the more likely the emission be quenched. For the samples with the similar diameter, the thinner the shell is the fewer luminescent centers it would have. Therefore, the longer lifetime of Eu-S3 with a thinner shell could possibly be attributed to the relatively small amount of luminescent centers. On the basis of the above analyses, it can

be suggested that the lifetime can also be tuned through modulating the textural parameters of the as-obtained LuVO4:Eu3+ hollow samples and increasing the number of shells is the most efficient way to prolong the lifetime. Figure S8 shows the Commission Internationale d’Eclairage (CIE) chromaticity diagram of four LuVO4:Eu3+ samples. The CIE coordinates of LuVO4:Eu3+ samples Eu-S1, Eu-S2, Eu-S3, and Eu-S1-700 are (a) x = 0.643, y = 0.356, (b) x = 0.633, y = 0.367, (c) x = 0.627, y = 0.373, and (d) x = 0.635, y = 0.364, respectively, being all located in the saturated red color region. 3.6. Biocompatibility Study. As inspired by the excellent luminescent properties and in particular the unique structure of the resulted hollow spheres, it is expected that these materials may find potential applications in biological fields.18,23 The stability of sample in water is an important evaluation for biological applications. As shown in Figure S9, the zeta potential of sample S1 (aqueous suspension) was −14.3 mV, demonstrating that S1 could form a relatively stable aqueous suspension. In addition, the photophysical properties of Eu−S1 aqueous suspension were also considered (see Figure S10), and it was found that the photophysical properties could be well preserved in suspension. To test the potential applications of the as-obtained LuVO4 hollow spheres in biological fields, cytotoxic measurements by MTT assays on HeLa cells were carried out by taking sample S1 as an example. Figure 8a shows the effect of sample S1 of various concentrations on HeLa cell viabilities. The viabilities of untreated cells were set at 100%. The assay result demonstrates that sample S1 shows almost no toxicity upon incubation with HeLa cells. The morphologies of the stained HeLa cells show almost no change after incubation with sample S1 (125 and 200 H

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μg mL−1) for 4 h (Figure 8b−d), indicating the considerable biocompatibility of S1. Hence, the obtained LuVO4 hollow spheres could be applied in biological fields such as drug delivery and medical imaging, etc.

LnMo0.15V0.85O4 (Ln = Ce, Pr and Nd). Appl. Catal., A 2008, 351, 45−53. (2) Martínez-Huerta, M. V.; Coronado, J. M.; Fernández-García, M.; Iglesias-Juez, A.; Deo, G.; Fierro, J. L. G.; Bañares, M. A. Nature of the vanadia−ceria interface in V5+/CeO2 catalysts and its relevance for the solid-state reaction toward CeVO4 and catalytic properties. J. Catal. 2004, 225, 240−248. (3) Terada, Y.; Shimamura, K.; Kochurikhin, V. V.; Barashov, L. V.; Ivanov, M. A.; Fukuda, T. Growth and optical properties of ErVO4 and LuVO4 single crystals. J. Cryst. Growth 1996, 167, 369−372. (4) Lisiecki, R.; Solarz, P.; Dominiak-Dzik, G.; Ryba-Romanowski, W.; Lukasiewicz, T. Effect of temperature on spectroscopic features relevant to laser performance of YVO4:Tm3+, GdVO4:Tm3+, and LuVO4:Tm3+ crystals. Opt. Lett. 2010, 35, 3940−3942. (5) Deng, H.; Yang, S.; Xiao, S.; Gong, H. M.; Wang, Q. Q. Controlled synthesis and upconverted avalanche luminescence of Cerium(III) and Neodymium(III) orthovanadate nanocrystals with high uniformity of size and shape. J. Am. Chem. Soc. 2008, 130, 2032− 2040. (6) Khatkar, S. P.; Han, S. D.; Taxak, V. B.; Kumar, R.; Kumar, D. Eu3+ activated LnVO4 (Ln = Y and Gd) phosphors: a facile combustion synthesis and optical properties. Bull. Electrochem. 2006, 22, 97−101. (7) Liang, X.; Kuang, S.; Li, Y. Solvothermal synthesis and luminescence of nearly monodisperse LnVO4 nanoparticles. J. Mater. Res. 2011, 26, 1168−1173. (8) Xu, Z.; Li, C.; Hou, Z.; Peng, C.; Lin, J. Morphological control and luminescence properties of lanthanide orthovanadate LnVO4 (Ln = La to Lu) nano-/microcrystals via hydrothermal process. CrystEngComm 2011, 13, 474−482. (9) Qian, L.; Zhu, J.; Chen, Z.; Gui, Y.; Gong, Q.; Yuan, Y.; Zai, J.; Qian, X. Self-assembled heavy lanthanide orthovanadate architecture with controlled dimensionality and morphology. Chem.Eur. J. 2009, 15, 1233−1240. (10) Liu, J.; Li, Y. General synthesis of colloidal rare earth orthovanadate nanocrystals. J. Mater. Chem. 2007, 17, 1797−1803. (11) Deng, H.; Liu, C.; Yang, S.; Xiao, S.; Zhou, Z. K.; Wang, Q. Q. Additive-mediated splitting of lanthanide orthovanadate nanocrystals in water: morphological evolution from rods to sheaves and to spherulites. Cryst. Growth Des. 2008, 8, 4432−4439. (12) Fan, W.; Zhao, W.; You, L.; Song, X.; Zhang, W.; Yu, H.; Sun, S. simple method to synthesize single-crystalline lanthanide orthovanadate nanorods. J. Solid State Chem. 2004, 177, 4399−4403. (13) Yu, C.; Yu, M.; Li, C.; Zhang, C.; Yang, P.; Lin, J. Spindle-like lanthanide orthovanadate nanoparticles: facile synthesis by ultrasonic irradiation, characterization, and luminescent properties. Cryst. Growth Des. 2009, 9, 783−791. (14) Li, X.; Tang, C.; Ai, M.; Dong, L.; Xu, Z. Controllable synthesis of pure-phase rare-earth orthoferrites hollow spheres with a porous shell and their catalytic performance for the CO + NO reaction. Chem. Mater. 2010, 22, 4879−4889. (15) Guo, S.; Dong, S.; Wang, E. Raspberry-like hierarchical Au/Pt nanoparticle assembling hollow spheres with nanochannels: an advanced nanoelectrocatalyst for the oxygen reduction reaction. J. Phys. Chem. C 2009, 113, 5485−5492. (16) Wang, B.; Chen, J. S.; Wu, H. B.; Wang, Z.; Lou, X. W. Quasiemulsion-templated formation of α-Fe2O3 hollow spheres with enhanced lithium storage properties. J. Am. Chem. Soc. 2011, 133, 17146−17148. (17) Cao, S. W.; Zhu, Y. J. Hierarchically nanostructured α-Fe2O3 hollow spheres: preparation, growth mechanism, photocatalytic property, and application in water treatment. J. Phys. Chem. C 2008, 112, 6253−6257. (18) Kang, X.; Yang, D.; Dai, Y.; Shang, M.; Cheng, Z.; Zhang, X.; Lian, H.; Ma, P.; Lin, J. Poly(acrylic acid) modified lanthanide-doped GdVO4 hollow spheres for up-conversion cell imaging, MRI and pHdependent drug release. Nanoscale 2013, 5, 253−261. (19) Wei, W.; Ma, G. H.; Hu, G.; Yu, D.; McLeish, T.; Su, Z. G.; Shen, Z. Y. Preparation of hierarchical hollow CaCO3 particles and the

4. CONCLUSIONS Here in this paper, the generalization of this method for the successful preparation of series heavy lanthanide orthovanadates (LnVO4, Ln = Tb, Dy, Er, Tm, Yb, Lu) hierarchical hollow spheres and Ho(OH)CO3@HoVO4 yolk−shell structure was realized for the first time by adopting an appropriate amount of Ln(OH)CO3 colloid spheres as sacrificial templates and employing NH4VO3 as the vanadium source. In particular, the building blocks of the as-obtained LuVO4 hollow spheres were constituted of numerous hollow-structured elliptic nanoparticles. It was found that the textural parameters such as the inner and outer diameters, shell thicknesses, and number of shells could be finely tuned through introducing different amounts of NH4VO3 and employing Lu(OH)CO3 templates with different sizes. In addition, the detailed microstructures of the obtained samples were revealed, and the corresponding formation mechanisms were proposed. Under ultraviolet excitation, the obtained LuVO4:Eu3+ hollow spheres showed a strong red emission located in the saturated red color region. The modulation of the PL emission intensity and color purity for the tetragonal LuVO4:Eu3+ materials could be realized through tuning the textural parameters of as-obtained hollow spheres. Besides, the nanostructure of the building blocks of LuVO4:Eu3+ hollow spheres also has an effect on the luminescent properties of the as-obtained materials. It is also found that the textural parameters of LuVO4:Eu3+ hollow spheres could influence their quantum efficiency. Particularly, the double-shelled LuVO4:Eu3+ hollow sample has the highest quantum efficiency. The as-obtained LuVO4 hollow spheres showed great biocompatibility, which favors their potential applications in biological fields such as drug delivery, disease therapy, medical imaging, cell biology, and diagnosis, etc.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, Tables S1 and S2, and Figures S1−S10. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (W.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the National Natural Science Foundation of China (Grants 21073084 and 20773065), Natural Science Foundation of Jiangsu Province (Grant BK2011438), National Basic Research (973) Program of China (Grant 2009CB623504), National Science Fund for Talent Training in Basic Science (No. J1103310), and Modern Analysis Center of Nanjing University.

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REFERENCES

(1) Mahapatra, S.; Vinu, R.; Row, T. N. G.; Madras, G. Kinetics of photoconversion of cyclohexane and benzene by LnVO4 and I

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application as anticancer drug carrier. J. Am. Chem. Soc. 2008, 130, 15808−15810. (20) Zhou, L.; Li, Z.; Ju, E.; Liu, Z.; Ren, J.; Qu, X. Aptamer-directed synthesis of multifunctional lanthanide-doped porous nanoprobes for targeted imaging and drug delivery. Small 2013, DOI: 10.1002/ smll.201301239. (21) Liu, Z.; Liu, X.; Yuan, Q.; Dong, K.; Jiang, L.; Li, Z.; Ren, J.; Qu, X. Hybrid mesoporous gadolinium oxide nanorods: a platform for multimodal imaging and enhanced insoluble anticancer drug delivery with low systemic toxicity. J. Mater. Chem. 2012, 22, 14982−14990. (22) Zhang, L.; You, H.; Yang, M. The principle of rare-earth luminescent material and the applications. J. Phys. Chem. Solids 2012, 73, 368−373. (23) Zhang, J.; Wang, Y.; Xu, Z.; Zhang, H.; Dong, P.; Guo, L.; Li, F.; Xin, S.; Zeng, W. Preparation and drug-delivery properties of hollow YVO4:Ln3+ and mesoporous YVO4:Ln3+@nSiO2@mSiO2 (Ln = Eu, Yb, Er, and Ho). J. Mater. Chem. B 2013, 1, 330−338. (24) Yang, X.; Zhang, Y.; Xu, L.; Zhai, Z.; Li, M.; Li, M.; Liu, X.; Hou, W. Surfactant-free sacrificial template synthesis of submicrometer-sized YVO4:Eu3+ hierarchical hollow spheres with tunable textual parameters and luminescent properties. Dalton Trans. 2013, 42, 3986−3993. (25) Li, J.; Li, X.; Sun, X.; Ishigaki, T. Monodispersed colloidal spheres for uniform Y2O3:Eu3+ red-phosphor particles and greatly enhanced luminescence by simultaneous Gd3+ doping. J. Phys. Chem. C 2008, 112, 11707−11716. (26) Nanda, J.; Sapra, S.; Sarma, D. D.; Chandrasekharan, N.; Hodes, G. Size-selected zinc sulfide nanocrystallites: synthesis, structure, and optical studies. Chem. Mater. 2000, 12, 1018−1024. (27) Yang, X.; Zhai, Z.; Xu, L.; Li, M.; Zhang, Y.; Hou, W. LaCO3OH microstructures with tunable morphologies: EDTA-assisted hydrothermal synthesis, formation mechanism and adsorption properties. RSC Adv. 2013, 3, 3907−3916. (28) Xu, Z.; Li, C.; Hou, Z.; Peng, C.; Lin, J. Morphological control and luminescence properties of lanthanide orthovanadate LnVO4 (Ln = La to Lu) nano-/microcrystals via hydrothermal process. CrystEngComm 2011, 13, 474−482. (29) Zhang, Y.; He, H.; Zhu, W.; Zheng, A. LuVO4:Ln3+ (Ln = Sm, Eu, Dy, Er and Tm) with high uniform size and morphology: Controlled synthesis, growth mechanism and optical properties. CrystEngComm 2011, 13, 6471−6480. (30) Yao, Y.; Yao, J.; Narasimhan, V. K.; Ruan, Z.; Xie, C.; Fan, S.; Cui, Y. Broadband light management using low-Q whispering gallery modes in spherical nanoshells. Nat. Commun. 2012, 3, 664. (31) Huignard, A.; Buissette, V.; Franville, A. C.; Gacoin, T.; Boilot, J. P. Emission processes in YVO4:Eu nanoparticles. J. Phys. Chem. B 2003, 107, 6754−6759. (32) Cai, R.; Liu, H.; Zhang, W.; Tan, H.; Yang, D.; Huang, Y.; Hng, H. H.; Lim, T. M.; Yan, Q. Controlled synthesis of double-wall aFePO4 nanotubes and their lib cathode properties. Small 2013, 9, 1036−1041. (33) Yang, L.; Li, G.; Zhao, M.; Zheng, J.; Guan, X.; Li, L. Preparation and morphology-sensitive luminescence properties of Eu 3+ -doped YVO 4 : a defect chemistry viewpoint of study. CrystEngComm 2012, 14, 3227−3235.

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