Emission Enhancement and Color Tuning for GdVO4:Ln3+ (Ln = Dy

Dec 19, 2016 - Key Laboratory of Applied Chemistry and Nanotechnology at Universities of Jilin Province, Changchun University of Science and Technolog...
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Emission Enhancement and Color Tuning for GdVO4:Ln3+ (Ln = Dy, Eu) by Surface Modification at Single Wavelength Excitation Yan Song,†,‡ Baiqi Shao,† Yang Feng,† Wei Lü,† Jiansheng Huo,† Shuang Zhao,† Man Liu,† Guixia Liu,*,‡ and Hongpeng You*,† †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ Key Laboratory of Applied Chemistry and Nanotechnology at Universities of Jilin Province, Changchun University of Science and Technology, Changchun 130022, P. R. China S Supporting Information *

ABSTRACT: The surface modification can realize systematically the emission enhancement of GdVO4:Ln3+ (Ln = Dy, Eu) microstructures and multicolor emission at single component. The structure, morphology, composition, and the surface ligands modification of as-prepared samples were studied in detail. It is found that the surface-modified ligands can act as sensitizer to improve the emission of the Eu3+ and Dy3+ ions via the energy transfer besides the VO43−− Eu3+/Dy3+ process. More importantly, under a single wavelength excitation, the emission color can be effectively tuned by manipulating the doping ratio of the Eu3+ ions in the internal crystal lattice and the Tb3+ ions in the external surface ligands, simultaneously. And further, multicolor emissions are obtained under single wavelength excitation due to the high overlapping between the VO43− absorption and the π−π* electron transition of the ligands. These findings may open new avenues to design and develop new highly efficient luminescent materials.

1. INTRODUCTION

considerable attention in terms of luminescent applications, specifically, as ideal candidates for light phosphors.16−20 The nano- and microstructures arouse novel physical properties and potential applications for luminescent hosts. Smaller size also results in much more defects and stronger surface quenching effects due to their high surface-to-volume ratio.21,22 Higher surface-to-volume ratio of nanomaterials affects the distribution of phonon energies and increases quenching sites, which will result in nonradiative transitions and luminescence quenching.23,24 In addition, a decrease of particle size leads to an increase in the concentration of dopant on the outermost surface during the lanthanide ions doping processes.25−27 Consequently, there is a higher probability of the excitation energy loss for the surface dopant ions; for example, it can cause excitation trap and phonon couple with solvent molecules, passivating ligands, or surface impurities, leading to nonradiative relaxation and emission quenching.21,28,29 Therefore, we believe that surface activator sites are important to improve the overall luminescent performance. Recently, several good strategies have been developed to promote luminescence intensity of nanoparticles. The construct of core−shell structure is a convenient and effective strategy through chemically modifying sensitizing agent on the surface of luminescence particles or epitaxial growth of a shell layer to

Lanthanide-doped nanocrystalline materials exhibit fascinating luminescence properties due to the intra 4fN electronic transition, which offer sharp emission bands, long-lived excited electronic states, higher photostability, long emission lifetimes, etc.1,2 The luminescence materials have attracted recent interest in applications of lighting, information display technologies, solar cells, and especially in biological labeling and biomedical imaging technologies.3−6 Particularly, lanthanide-doped nanocrystalline materials can create a variety of colors by proper selection of lanthanide dopants, which cover almost the entire visible spectrum, and multicolor emission could be realized in a single-phase host matrix,7−10 which can be accurately regulated by proper control of the lanthanide dopants based on variant characteristic emission bands of different lanthanide ions.11,12 Nowadays, stringent demands for lighting technology require highly efficient luminescent materials with small size and various morphologies driven by the nanoscience and nanotechnology. Till now, researchers have successfully developed a good deal of efficient nanophosphors. Among them, rare-earth vanadate is a highly efficient luminescent material due to the strong absorption of ultraviolet radiation of VO43− groups and the efficient energy transfer from VO43− to activator ions, and they are widely used in phosphor lamps, polarizers, display devices, laser hosts, and catalysts.13−15 Especially, lanthanide activator doped vanadates of Y, La, Gd, and Lu have attracted © XXXX American Chemical Society

Received: September 6, 2016

A

DOI: 10.1021/acs.inorgchem.6b02125 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry passivate the surface.30−32 In addition, some reports have focused on protecting the dopants against surface quenchers by employing the surface modification, such as host crystalline shell, amorphous shell, and sensitizer-encapsulated shells.33−35 According to a previous report, ∼25−30% of the dopant ions were expected to occupy lattice sites on the surface for a 5 nm particle.25 To fully utilize the lanthanide-doped ions on the surface of the nanoparticles and reduce luminescence quenching, surface modification is a direct and efficacious strategy to enhance the excitation efficiency for activator. It is well-established that the organic ligands show strong energy absorption in the ultraviolet region due to their rich π electrons, which contribute to efficient charge transport and fast energy transfer between organic ligands and lanthanide ions.36−39 Thus, it is expected that organic ligands are ideal modifications to efficiently sensitize the surface lanthanide ions and decrease surface quenching sites.40−43 Furthermore, modifying agent can act as a platform to induce other lanthanide activators to realize multicolor emission in a single crystal lattice. Herein, we developed an efficient strategy of surface modification with organic ligands to simultaneously achieve enhancement of the emission intensity and multicolor emission. The monodisperse and uniform REVO4 (RE = La, Y, Gd, Lu) sub-microflowers were synthesized via a hydrothermal method. REVO4 microstructures were further modified with the benzoate (BA) and 1,10-phenanthroline (Phen) for effectively sensitizing the dopant ions. Subsequently, another kind of doping ions is introduced and coordinated with the surfacemodified ligands. Interestingly, under a single wavelength excitation at near-ultraviolet region, we achieved an obvious emission enhancement for the Eu3+ and Dy3+ ions in GdVO4 hosts. In addition, the emission color can be finely adjusted by rational selection of the lanthanide ions in the building unit. More importantly, this strategy can be extended to other rareearth ion-doped lanthanide vanadates (Y, La, Lu). Such novel surface modification strategy may provide a new way to improve the efficiency for luminescence nanocrystals and extend their applications.

were collected by centrifugation at 7000 rpm for 5 min and washed at least twice with deionized water and ethanol. The precipitate was redispersed in ethanol for further use. 2.3. Synthesis of GdVO4:Eu3+/Tb(BA)3Phen. In a typical synthesis process, the above-synthesized GdVO4:Eu3+ (0.5 mmol) ethanol solution was diluted with ethanol to 20 mL, and their pH was adjusted to 6.5. Then, sodium benzoate (1 mmol) and 1,10phenanthroline (3 mmol) was gradually added to the previous solution under vigorous magnetic stirring. The different concentrations of Tb(NO3)3 solution were dropwise added to previous solution to form a homogeneous solution. The reaction mixture was heated at 60 °C for 3 h. Subsequently, the solution was cooled to room temperature and kept stirring for 12 h. Afterward, the resulting products were washed with deionized water and ethanol in turn several times. 2.4. Characterizations. X-ray powder diffraction (XRD) measurements of as-synthesized samples were performed with a Bruker D8 FOCUS diffractometer operated at a voltage of 40 kV and a current of 40 mA with Cu Kα radiation (λ = 0.154 18 nm) in the 2θ range from 10° to 70°. Scanning electron microscopy (SEM) images and energydispersive spectroscopy (EDS) were performed using a Hitachi S-4800 field emission scanning electron microscope (FE-SEM) equipped with an energy-dispersive X-ray spectrometer operating at working voltage of 10 or 20 kV, and the samples were prepared by dispersing the samples in ethanol and drop-casting a drop of the suspension on a smooth silicon wafer. Transmission electron microscope (TEM) images, high-resolution transmission electron microscope (HRTEM) images, and selected area electron diffraction (SAED) patterns were recorded on a JEOL-2010 electron microscope operated at an accelerating voltage of 200 kV. Samples for TEM were prepared by dropping from ethanol suspensions onto TEM copper grids covered with a thin holey carbon film. The elemental mapping analysis of the samples was performed during TEM measurements to determine the element distributions of the samples. Fourier transform infrared spectra (FT-IR) measurements were performed using a Shimadzu model 8400s FT-IR spectro-photometer in the range of 4000−400 cm−1 with KBr pellet technique (sample/KBr = 1:100). Thermogravimetric analyses (TGA) were performed on a SDT 2960 thermal analysis instrument with temperature increased linearly by a heating rate of 10 °C min−1 in an air flow of 100 mL min−1 from room temperature to 800 °C. The photoluminescence excitation and emission spectra were recorded at room temperature with Hitachi F7000 spectro-photometer equipped with a 150 W xenon lamp as the excitation source with photomultiplier tube voltage of 700 V and the instrument parameters of 2.5 nm for excitation emission slits. The photoluminescence quantum efficiency was measured using an absolute photoluminescence quantum yield measurement system (C9920−02, Hamamatsu Photonics K. K., Japan).

2. EXPERIMENTAL SECTION 2.1. Materials. Gadolinium oxide (Gd2O3, 99.99%), yttrium oxide (Y2O3, 99.99%), lanthanum oxide (La2O3, 99.99%), lutetium oxide (Lu2O3, 99.99%), europium oxide (Eu2O3, 99.99%), terbium oxide (Tb4O7, 99.99%), and dysprosium oxide (Dy2O3, 99.99%) were purchased from Shanghai Yuelong Non-Ferrous Metals Limited. Ammonium metavanadate (NH4VO3), sodium citrate, BA, and Phen were purchased from Sinopharm Chemical Reagent Co., Ltd. All the chemicals in this investigation were of analytical grade and used without further purification. Rare-earth nitrate (RE(NO3)3) stock solutions were obtained by dissolving the corresponding rare-earth oxides in dilute HNO3 under heating with agitation followed by evaporating the solvent. 2.2. Synthesis of GdVO4:Ln3+ (Ln = Eu, Dy). The GdVO4:Ln3+ (Ln = Eu, Dy) sub-microflowers were synthesized via a hydrothermal route. In the typical procedures of preparing GdVO4:Ln3+ submicroflowers, the stoichiometric amounts of the 1.0 mmol RE(NO3)3 solutions (molar ratio Gd3+:Ln3+ = 95:5) was added into 30 mL of deionized water at room temperature under vigorous magnetic stirring to form a homogeneous solution. Subsequently, 1.0 mmol of sodium citrate was introduced, and the solution was constantly vigorously stirred for 30 min. After addition of 1.0 mmol of ammonium metavanadate (NH4VO3), the resulting solution was stirred and transferred into a 50 mL Teflon bottle held in a stainless steel autoclave. The autoclave was maintained at 180 °C for 10 h. As the autoclave was cooled to room temperature naturally, the final products

3. RESULTS AND DISCUSSION 3.1. Structure, Morphology, and Composition of the Samples. The crystal structure and phase purity of the asprepared samples were first examined by XRD. The corresponding XRD patterns of the as-prepared samples and standard card data of GdVO4 crystal were provided in Figure 1. All of the diffraction peaks can be readily indexed to pure tetragonal phase of GdVO4, and the locations and relative intensities of the diffraction peaks coincide well with the standard card (PDF No. 17−0260) without any impurity phase. For the surface-modified samples, an additional amorphous phase can be observed on the baseline at the low-angle region, which may be caused by the BA and Phen and formation of amorphous complex on the surface of GdVO4 crystals. The results indicate that the surface-modified ligands have no obvious effect on the crystal phase structure of samples. The XRD profiles of as-synthesized GdVO4:Dy3+ nanocrystals and surface-modified GdVO4:Dy3+ sub-microflowers are shown in Figure S1. The patterns of as-synthesized samples match well B

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dimensions. Figure 2d,e shows the morphology for the GdVO4:Eu3+ with modification of Tb(BA)3Phen complex. Compared with the morphology of GdVO4:Eu3+, the surfaces of the modified GdVO4:Eu3+ sub-microflowers are coated with amorphous complex, corresponding to the Tb(BA)3Phen. Furthermore, Figure 2c,f represents the EDX spectra of GdVO4:Eu3+ and GdVO4:Eu3+/Tb(BA)3Phen samples. In the EDX spectrum of GdVO4:Eu3+ (Figure 2c), the peaks of Gd, V, O, and Eu elements can be detected. After modified with BA and Phen ligands, the EDX spectrum (Figure 2f) shows the presence of C, N, and Tb besides the elements of Gd, V, O, and Eu elements, suggesting the successful modification. Figure 3 shows the TEM images, SAED pattern, and EDX elemental mappings of the GdVO4:Eu3+ and GdVO4:Eu3+/ Tb(BA) 3Phen samples. The TEM image confirms the monodispersity and relatively uniform size and morphology. High-magnification TEM image taken in the Figure 3b shows that the microflower consists of many layers of nano sheetlike structure and that it has a well-defined crystalline structure with a lattice plane spacing of 0.36 nm, which is consistent with the (200) facet of GdVO4. The corresponding SAED pattern displays discontinuous rings and the lattice plane spacing that match well with the d-spacing values of the (200), (211), (220), and (103) planes of GdVO4, revealing that the samples consist of polycrystals with an oriented crystallographic axis. To verify the successful modification of complex on the surface of GdVO 4 :Eu 3+ sample, the distribution of elements of GdVO4:Eu3+/Tb(BA)3Phen samples was studied with EDX elemental mappings (Figure 3e). The elemental mappings show that C, N, Tb, Gd, V, O, and Eu element are detected and C, N, Tb distributed throughout particle, suggesting that Tb(BA) 3Phen is successfully modified on the surface of GdVO4:Eu3+ samples. The YVO4:Eu3+, LaVO4:Eu3+, and LuVO4:Eu3+ sub-micrometer structures with surface-modified BA and Phen were also characterized under the same condition by SEM. The results show that they have a homologous shape with an average size of ∼400 nm and that the BA and Phen ligands are modified successfully, as displayed in Figures S3−S5 (Supporting Information).

Figure 1. XRD patterns of GdVO4:Eu3+ sub-microflowers (a), GdVO4:Eu3+ modified with BA and Phen (b), and GdVO4:Eu3+/ Tb(BA)3Phen (c) samples. The standard card of GdVO4 (PDF No. 17−0260) was given as reference.

with the tetragonal phase of GdVO4 crystal, and no extra diffraction peak is observed after Eu(BA)3Phen modification on the GdVO4:Dy3+ crystals. In addition, the crystal phases of assynthesized LnVO4:Eu3+ (Ln = Y, La, Lu) sub-microflowers with and without surface modification also were detected by XRD. As shown in Figure S2, all of the diffraction peaks can be readily indexed to a tetragonal structure YVO4, LaVO4, and LuVO4 according to the International Centre for Diffraction Data (ICDD) powder diffraction file PDF cards 72−0274, 70− 5226, and 17−0880, respectively. The morphologies, size, and structure of the as-synthesized GdVO4:Eu3+ samples with or without modification of BA and Phen ligands were characterized by SEM, TEM, and EDX measurements, as displayed in Figures 2 and 3. The assynthesized GdVO4:Eu3+ (Figure 2a) consists of highly monodisperse and uniform sub-microflowers with size in the range of 400−500 nm. The high-magnification SEM image (Figure 2b) shows that the sub-microflower is composed of compact nanoplatelets packed aligned regularly in three

Figure 2. SEM images of GdVO4:Eu3+ (a, b) and GdVO4:Eu3+/Tb(BA)3Phen (d, e). EDX spectrum of GdVO4:Eu3+ (c) and GdVO4:Eu3+/ Tb(BA)3Phen (f) sub-microflowers. C

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Figure 3. TEM image, HRTEM image, and SAED pattern of GdVO4:Eu3+ (a−c), TEM image (d), and EDX elemental mappings (e) of GdVO4:Eu3+/Tb(BA)3Phen sub-microflowers.

FT-IR spectroscopy was performed to demonstrate the successful modification of BA and Phen ligands on the surface of GdVO4:Eu3+ samples. Figure 4 shows the typical FT-IR

is ascribed to the V−O bond stretching vibration from the VO43− group. Moreover, the two bands centered at ∼1410 and 1521 cm−1 correspond to the symmetrical (vs) and asymmetrical (vas) vibration modes of carboxyl (−COO−) groups, respectively. We could also observe a clear change at 1072, 850.3, and 718.9 cm−1, which corresponds to stretching frequency of the carbon−nitrogen double bond (CN) in the Phen ligand and the N−CH3 scissoring mode. In addition, the broad band centered at 3400 cm−1 can be attributed to the characteristic hydroxyl group (O−H) stretching vibration of H2O molecule. To further determine the successful modification and investigate the thermal stability after modification, TGA is performed from room temperature to 800 °C with a heating rate of 10 °C min−1 in air. Figure 5 illustrates the TGA curves

Figure 4. FT-IR spectral profiles for the GdVO4:Eu3+ (a) and GdVO4:Eu3+/Tb(BA)3Phen (b).

spectra of as-formed GdVO4:Eu3+ samples and GdVO4:Eu3+/ Tb(BA)3Phen samples, respectively. The FT-IR spectrum of the GdVO4:Eu3+ sample (Figure 4a) displays a strong absorption band at 820 cm−1 and a weak peak at 450 cm−1, which are attributed to the vibration of the V−O bond and the Gd−O (Eu−O) bond from the Eu3+ ion-doped GdVO4 crystal structure, respectively. In addition, the broad absorption band around 3400 cm−1 and the peak centered at ∼1600 cm−1 are ascribed to the stretching and bending vibrations of the hydroxyl groups (O−H) from surface absorbent H2O molecule. In the FT-IR spectrum of Tb(BA)3Phen (Figure S6, Supporting Information), the characteristic vibration modes, corresponding to N−CH3, − COO−, CC, and C−H group stretching vibrations from benzene ring, originate from the complex. After the Tb(BA)3Phen are modified on the surface of GdVO4:Eu3+, the FT-IR spectrum of GdVO4:Eu3+/Tb(BA)3Phen samples exhibit that the strong absorption band centered at ∼820 cm−1

Figure 5. TGA data of GdVO4:Eu3+, Tb(BA)3Phen, and GdVO4:Eu3+/ Tb(BA)3Phen.

of the pure GdVO4:Eu 3+, Tb(BA)3Phen complex, and GdVO4:Eu3+/Tb(BA)3Phen samples. There is no obvious weight loss from the room temperature to 800 °C except for the volatilization of water in the TGA curve for the GdVO4:Eu3+ samples. However, three distinct stages of weight loss are observed for the Tb(BA)3Phen complex. The first weight loss is shown in the temperature range of 30−110 °C D

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wavelength, which will achieve tunable emitting colors through controlling the concentration of activator. The photoluminescence excitation and emission spectra of the GdVO4:Dy3+ submicroflowers and Eu(BA)3Phen complex are given in Figure 7.

due to the loss of absorbed water. In addition, the second weight loss starting at ∼250 °C can be attributed to almost complete decomposition of Phen ligands in the complex. The subsequent process is observed at 560 °C, which can be assigned to the pyrolysis of the Tb(BA)3 complex and the loss of benzoate ligands. After introduction of complex in the GdVO4, the GdVO4:Eu3+/Tb(BA)3Phen composites have the high thermal stability without loss of weight below 300 °C. When the temperature is headed up to 300 °C and above 500 °C, the obvious weight losses are around 6.9% and 14.3%, which are assigned to the pyrolysis of Phen ligands and benzoate ligands, respectively. As a result, it further indicates that the complexes are successfully modified on the surface of GdVO4:Eu3+ samples. 3.2. Photoluminescence Properties of the Samples. Figure 6 gives the photoluminescence excitation and emission

Figure 7. Excitation and emission spectra of the Eu(BA)3Phen complex (a) and GdVO4:Dy3+ (b).

The excitation spectrum (Figure 7b) consists of a strong broad absorption from 200 to 350 nm. This broad absorption is ascribed to the charge transfer from the oxygen ligands to the central vanadium ions inside the VO43− groups. In the Figure 7a, the broad band from π−π* electron transition of the Eu(BA)3Phen complex is located in close proximity with that of VO43− groups. As is expected, after modified with Eu(BA)3Phen complex, the composites will display color emission of the Dy3+ and Eu3+ ions excited under a single wavelength. Figure 8a shows the photoluminescence excitation spectra of the GdVO4:Eu3+ samples with or without modified BA and Phen ligands at the room temperature. By contrast, a significant enhancement was observed for the surface modification samples. Moreover, monitored the characteristic emission of the Eu3+ ions at 619 nm, the excitation spectrum of the GdVO4:Eu3+/(BA)3Phen samples contains a broader excitation band ranging from 200 to 375 nm. The phenomena indicate that the energy of activator (Eu3+) derives from the GdVO4 matrix and the surface modified BA and Phen ligands. The respective emission spectra were obtained under the excitation of 290 nm, and they present identical profiles except for the intensity. Thus, the introducing of ligands can effectively improve the emission of GdVO4:Eu3+ samples. Figure 8b shows the photoluminescence excitation and emission spectra of the GdVO4:Dy3+ and GdVO4:Dy3+/(BA)3Phen samples. Monitored the emission of the Dy3+ ions at 574 nm, a broader excitation band is also observed, associated with the VO43− groups absorption and the π−π* electron transition of the ligands. Meanwhile, the emission intensity of the Dy3+ ions obviously enhances due to sensitization effect of the surface modification ligands. In addition, the same situation can be observed for doped YVO4, LaVO4, and LuVO4 samples, as shown in Figures S7−S9. Similar results reveal that modification ligands can effectively improve the emission intensity. On the basis of the above research, we also expect that the combination of the GdVO4:Eu3+ and Tb(BA)3Phen will generate tunable color emission. Figure 9 shows the variation of photoluminescence emission spectra of and emission intensity of the Tb3+ ions in the GdVO4:Eu3+/Tb(BA)3Phen

Figure 6. Excitation and emission spectra of the Tb(BA)3Phen complex (a) and GdVO4:Eu3+ (b).

spectra of GdVO4:Eu3+ sub-microflower and Tb(BA)3Phen complex recorded at room temperature. One can notice that the Tb(BA)3Phen complex displays the characteristic green emission from the Tb3+ ions and that the GdVO4:Eu3+ displays the sharp red emission of the Eu3+ ions. Monitored at the respective characteristic emission peaks of the Tb3+ and Eu3+ ions, the excitation spectra of Tb(BA)3Phen complex and GdVO4:Eu3+ materials were achieved, respectively. For Tb(BA)3Phen complex, the excitation spectrum exhibits a broad band from 200 to 400 nm, which is attribution to the π−π* electron transition of greater π bond conjugated system from the benzene ring and the n−π* electron transitions of the N atom in the azomethine group from the ligand of Phen. The excitation spectrum of GdVO4:Eu3+ consists of a strong absorption band ranging from 200 to 350 nm with a maximum peak at 290 nm due to the charge transfer from the oxygen ligands to the central vanadium ions inside the VO43− groups. By comparative investigation of the excitation spectra for both samples, there is a distinct overlap between the VO43− absorption band and the π−π* electron transition of the complex. Therefore, the samples GdVO4:Eu3+ with surface modified with Tb(BA)3Phen complex can achieve excitation energy through double ways upon excitation at 290 nm due to the absorption of VO43− groups and the ligands, which may effectively enhance the emission of active lanthanide ion (Eu3+ ions) from the GdVO4 matrix. More importantly, we expect that the composite exhibits the characteristic emission of the Eu3+ and Tb3+ ions simultaneously at the excitation of single E

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D1→7F 1, 5D1→7F2, 5D0→7F1, 5D0→7F2, 5D0→7F3, and D0→7F4 radiation transitions of the Eu3+ ions, respectively. After introduction of Tb3+ ions by Tb(BA)3Phen on the surface, the sharp characteristic emission peaks are displayed and centered at 492, 547, and 589 nm, which are attributed to the 5D4→7FJ (J = 6, 5, 4) radiation transitions of the Tb3+ ions, respectively. When the samples contain Tb3+ and Eu3+ ions, the emission spectra of all samples involve the characteristic emission peaks of the Tb3+ and Eu3+ ions. The emission spectra of all samples have similar emission wavelengths but different relative emission intensities depending on the relative content of Tb 3+ and Eu 3+ ions. With an increase in the Tb3+ concentration, the emission intensity of the Tb3+ ions increases systematically, while the emission intensity of the Eu3+ ions shows no relative change. The results indicate that the tunable emission colors can be realized by adjusting different amounts of the green color emission from the Tb3+ and red color emission from the Eu3+ in the GdVO4:Eu3+/Tb(BA)3Phen samples. As an important parameter for phosphors, the quantum efficiency was also measured. The measured quantum efficiencies of GdVO4:Eu3+/Tb(BA)3Phen samples with the increase of Tb3+ content are determined to be 10.6%, 17.3%, 28.5%, 35.9%, 42.5%, 50.7%, and 62.9% at 290 nm excitation. To elucidate the energy transfer mechanism of the Eu3+ and Tb3+ photoluminescence emission, the excitation spectra of the GdVO4:Eu3+/Tb(BA)3Phen sample (A6 sample) were studied, as displayed in Figure 10. The excitation spectrum of the 5 5

Figure 8. Excitation and emission spectra of the GdVO4:Eu3+ (a) and GdVO4:Dy3+ (b) with or without surface modification of BA and Phen.

Figure 10. Excitation spectra of the GdVO4:Eu3+/Tb(BA)3Phen by monitoring the wavelength at 619 and 545 nm.

GdVO 4 :Eu 3+ /Tb(BA) 3 Phen samples monitored at the 5 D0→7F2 transition (at 619 nm) consists of strong and broad excitation band ranging from 200 to 375 nm, which contains the VO43− absorption and the π−π* electron transition of the ligands. This result suggests that the Eu3+ ions are excited through the energy transfer from the GdVO4 host and the surface-modified ligands. While monitored at the 5D4→7F5 transition of the Tb3+ ions (at 545 nm), the excitation spectrum exhibits a broad band ranging from 200 to 400 nm with centers at 292 nm. The excitation band is similar to the photoluminescence excitation spectrum of the Tb(BA)3Phen complex (Figure 6a), which is attributed to the π−π* electron transition of the benzene ring and the n−π* electron transitions of the Phen ligands. These results clearly indicate that the Tb3+ ions are excited just through the energy transfer from the surface-modified ligands.

Figure 9. Emission spectra of the GdVO4:Eu3+/Tb(BA)3Phen with various Tb3+ content under 290 nm excitation.

samples with the variation of Tb3+ content (0, 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0 mmol), respectively. Upon excitation at 290 nm, the characteristic emission peaks of the Eu3+ ions at 538, 559, 592, 619, 652, and 700 nm are detectable, corresponding to the F

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Inorganic Chemistry To achieve a gradual color change from blue-green toward yellow, we regulated the Eu3+ ion content (0, 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0 mmol) in the fabrication process of the GdVO4:Dy3+/Eu(BA)3Phen sample. Figure 11 provides the

Figure 12. Photoluminescence excitation spectra of the GdVO4:Dy3+/ Eu(BA)3Phen by monitoring the wavelength at 619 and 574 nm.

illustrated in Figure S10. Upon excitation at 290 nm, the electrons are excited to the excited-state 1T2 level and then relaxed to the 1T1 level. After that, the energy transfer occurs from the 1T1 level of VO43− to the 5HJ excited level of the Eu3+ ion. For the GdVO4:Dy3+/Eu(BA)3Phen, the energy transfer occurs from the1T1 level of VO43− to the 4I9/2 excited level of the Dy3+ ion, while the S0−S1 transition first occurred in the outer complex and then populated to T1 by intersystem crossing. Subsequently, the energy transfers from the T1 level of the ligands to the 5D4 excited level of the Tb3+ ions and 5D2 excited level of the Eu3+ ions, respectively. Finally, the electrons fall back to lower energy levels and cause different emissions. According to the photoluminescence emission spectra of the samples, the color tuning can be realized through simply changing the active lanthanide ion ratios in the single particle. In the following, we investigate the chromaticity coordinates and optical color of all samples. Figure 13 shows the emission colors optical photographs and the chromaticity coordinates in

Figure 11. Emission spectra of the GdVO4:Dy3+/Eu(BA)3Phen with various Eu3+ content under 290 nm excitation.

variation of photoluminescence spectra and emission intensity of Eu3+ in the GdVO4:Dy3+/Eu(BA)3Phen sample with the increase of Eu3+ content from 0 to 1.0 mmol. Obviously, the sharp characteristic peaks located at ∼482 and 574 nm are ascribed to the magnetic dipole transition (4F9/2→6H15/2) and the hypersensitive electric dipole transition (4F9/2→6H13/2) for the Dy3+ ions. The sharp peaks centered at 592 and 619 nm originate from the 5D0→7F1 and 5D0→7F2 transitions of the Eu3+ ions, respectively. With increasing Eu3+ content, the emission intensity of the Eu3+ raises monotonically leading to the gradual change in emission color. In addition, upon excitation at 290 nm, the quantum efficiencies of GdVO4:Dy3+/ Eu(BA)3Phen samples with the increase of Eu3+ content were determined to be 8.2%, 12.7%, 19.5%, 28.5%, 35.6%, 42.7%, and 49.2%, respectively. Similarly, we investigate the energy transfer between activator and sensitizer in the GdVO4:Dy3+/Eu(BA)3Phen samples. Figure 12 shows the photoluminescence excitation spectra though monitoring the Dy3+ and Eu3+ characteristic emission at 574 and 619 nm, respectively. The excitation spectrum of GdVO4:Dy3+/Eu(BA)3Phen samples monitored at 574 nm consists of strong and broad excitation band ranging from 200 to 350 nm, which are attributed to the VO43− absorption and the π−π* electron transition of the ligands. These results suggest that the excited energy can be transferred from the GdVO4 host and the surface-modified ligands to the Dy3+ ions. When monitored at 619 nm of the Eu3+ ions, the excitation spectra of the GdVO4:Dy3+/Eu(BA)3Phen shows a broad band around 300 nm and a sharp peak at 394 nm, which are related to the electron transition of the ligands and the f−f transition (7F0→5L6) of the Eu3+ ions, respectively. It indicates that the Eu3+ ions are essentially excited through the energy transfer from the surface-modified ligands. On the basis of the excitation and emission spectra, the schematic of the energy transfers in GdVO4:Eu3+/Tb(BA)3Phen and GdVO4:Dy3+/Eu(BA)3Phen samples are

Figure 13. CIE chromaticity coordinates and digital camera image for luminescence of a series of samples of CIE chromaticity diagram. G

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4. CONCLUSIONS In summary, we have developed a novel strategy to efficiently enhance the emission intensity and tune the emission color for a series of lanthanide ion (Eu3+, Dy3+)-doped vanadate (REVO4, RE = Y, La, Gd and Lu) sub-microflowers via a surface modification process. The BA and Phen ligands are modified to the surface of vanadate sub-microflowers, and the different doping ions then are bonded with the surface ligands. The surface-modified ligands could act as sensitizer to enhance the emission of the Eu3+ and Dy3+ ions via the energy transfer besides the VO43−−Eu3+/Dy3+ process. Moreover, the emission color can be adjusted from blue and bluish-green to green and yellow region by manipulating the doping ratio at single wavelength excitation. The tunable photoluminescent properties of samples show their great potential application in multicolor luminescence devices.

the 1931 Commission Internationale de L’Eclairage (CIE) diagram. It can be seen that the emitting color of the phosphors can be easily modulated from red to yellow by simply varying the content of Tb3+ from 0.1 to 1.0 mmol. As expected, the emitting color of the samples can be easily modulated from orange-red region to yellow-green region with the increase of Tb3+ ions in the GdVO4:Eu3+/Tb(BA)3Phen system. Moreover, similar results about color tuning can be accomplished when the Eu3+ and Dy3+ ions serve as activator. The CIE chromaticity coordinates of all GdVO4:Dy3+/Eu(BA)3Phen samples show a distinct shift of color position from bluish-green region to yellow region with the increase of Eu3+ content. The CIE chromaticity coordinates of all samples are listed in Table 1. Table 1. CIE Chromaticity Coordinates of Samples sample A1 A2 A3 A4 A5 A6 A7

CIE coordinates (x, y) (0.599, (0.560, (0.532, (0.483, (0.454, (0.423, (0.396,

sample

0.336) 0.365) 0.395) 0.424) 0.457) 0.472) 0.487)

B1 B2 B3 B4 B5 B6 B7

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CIE coordinates (x, y) (0.355, (0.324, (0.329, (0.371, (0.395, (0.437, (0.468,

0.385) 0.367) 0.362) 0.368) 0.375) 0.370) 0.368)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02125. Listings of XRD patterns of GdVO4:Dy3+ with or without modification complex; FT-IR spectrum of pure Tb(BA)3Phen; XRD patterns, SEM images, excitation and emission spectra of the REVO4:Eu3+ and REVO4:Dy3+ (RE = Y, La, Lu) with or without modification (BA)3Phen complex; schematic of energy transfer mechanism in GdVO 4 :Eu 3 + /Tb(BA) 3 Phen and GdVO4:Dy3+/Eu(BA)3Phen samples (PDF)

The related tunable colors are visibly demonstrated by the digital camera images of all samples under ultraviolet light exposure, which vividly reflect the color modulation. To verify the photoluminescence stability, the dependence of emission intensity on irradiation time was studied for the A7 and B7 samples. Figure 14 displays the dependence of the normalized

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hongpeng You: 0000-0003-2683-6896 Notes

The authors declare no competing financial interest.

■ Figure 14. Dependence of the normalized emission intensity of the 5 D0→7F2 (Eu3+) and 5D4→7F5 (Tb3+) transition on irradiation time for GdVO4:Eu3+/Tb(BA)3Phen and GdVO4:Eu3+/Tb(BA)3Phen samples.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 21271167 and 51472236) and the Fund for Creative Research Groups (Grant No. 21521092), the National Basic Research Program of China (973 Program, Grant No. 2014CB643803), and Key Program of the Frontier Science of the Chinese Academy of Sciences (Grant No. YZDY-SSW-JSC018).

emission intensity of the D4→ F5 (Tb ) and D0→ F2 (Eu ) transitions on irradiation time for GdVO4:Eu3+/Tb(BA)3Phen and GdVO4:Eu3+/Tb(BA)3Phen samples, respectively. It is interesting to observe that the emission intensity gradually increases in the incipient stage of exposure time and then maintains constant. The intensity enhancement with exposure time can be attributed to optical modification of the surface defects. These results show that the samples have good photostability, which is a favorable performance for application in display devices.

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DOI: 10.1021/acs.inorgchem.6b02125 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b02125 Inorg. Chem. XXXX, XXX, XXX−XXX