Broadening the Photoluminescence Excitation Spectral Bandwidth of YVO4:Eu3+ Nanoparticles via a Novel Core-Shell and Hybridization Approach

For many optoelectronic applications, it is desirable for the lanthanide-doped phosphors to have broad excitation spectrum. The excitation mechanism of the lanthanide-doped YVO4, a high quantum efficient lasing material, primarily originates from the energy transfer process from the host VO43− complexes to the lanthanide ions, which has an excitation spectral bandwidth range of 230–330 nm. For applications in silicon solar cells, such phosphors can convert ultraviolet light to visible light for more efficient power generation, but this spectral range is still not broad enough to cover the entire ultraviolet spectrum of solar light. In this work, a novel core-shell and inorganic–organic hybridization strategy has been employed to fabricate Eu3+-doped YVO4 nanoparticles to broaden their photoluminescence excitation spectral bandwidth to the range of 230–415 nm, covering the entire ultraviolet spectrum of solar light and enabling their potential applications in silicon solar cells.

The strong photoluminescence of YVO 4 :Ln 3+ relies on the efficient energy transfer process from the host VO 4 3complexes to the Ln 3+ [11], in which the VO 4 3is well excited by the UV lights. Such an energy transfer process effectively overcomes the drawback of low absorption cross sections of parity forbidden 4f-4f transitions of Ln 3+ ions. The excitation spectrum of VO 4 3− peaks at about 325 nm, and has a spectral range from 260 nm to 330 nm at the half maximum and a band edge at 350 nm [12]. This, however, is too narrow to cover the UV spectrum (250-400 nm) of the solar lights on earth surface for applications in solar cells. To extend its excitation spectrum, Bi 3+ ions have been co-doped into YVO 4 :Ln 3+ [5,6,8,13,14]. It was found that the peaking wavelength of its excitation spectrum is shifted to the longer wavelength at 342 nm and the band edge is extended to about 360 nm [8]. Up to now, however, the excitation bandwidth of YVO 4 :Ln 3+ is still not wide enough to cover the UV spectrum of the solar irradiance. Previous work on LaF 3 :Eu 3+ [15] has shown that its excitation spectral bandwidth can be effectively broadened by using a mixed photoluminescence sensitization from different types of organic ligands. In this work, a novel strategy was employed to remarkably broaden the excitation spectrum of YVO 4 :Eu 3+ nanoparticles through a dual-channel excitation approach, and the entire UV spectrum of the solar irradiance can be effectively covered. This was implemented by combining the two strong excitation channels, the efficient VO 4 3− →Eu 3+ energy transfer after annealing and the photoluminescence sensitization by organic ligand after forming an inorganic-organic hybrid nanoparticle [16,17].

Experimental
The experimental procedure is illustrated in Figure 1, and it is detailed in the following paragraphs.

Synthesis and Annealing of YVO 4 :Eu 3+ Nanoparticles
The YVO 4 :Eu 3+ nanoparticles were synthesized using the solvothermal method [18]. First, 0.75 mmol Na 3 VO 4 ·12H 2 O and 0.05 g PVP were dissolved in 8 mL ethylene glycol and 1 mL deionized water. Then, 9 mL of ethylene glycol solution containing 1 mmol Y(NO 3 ) 3 ·6H 2 O and 0.05 mmol Eu(NO 3 ) 3 ·6H 2 O were added to the above solution with vigorous magnetic stirring for 5 min. Then, 17 mL of de-ionized water was added to the above mixture solution, with stirring for another 5 min. The mixture solution was poured into a 50 mL Teflon-lined stainless steel autoclave and heated at 180 • C for 6 h to obtain YVO 4 :Eu 3+ nanoparticles, and then naturally cooled to room temperature. The YVO 4 :Eu 3+ nanoparticles were collected by centrifugation, washed with anhydrous ethanol for 3 times, and dried at 80 • C in air.
As shown later in this paper, the as-synthesized YVO 4 :Eu 3+ nanoparticles possess only weak luminescence, which can be greatly enhanced by annealing. In this work, an annealing process at 800 • C for 2 h in air was conducted for the as-synthesized YVO 4 :Eu 3+ nanoparticles for strong photoluminescence (Figure 1). It was found that their particle size only increased to some extent after the annealing, but they remained to be in nanometer scale. However, if the annealing temperature is over 800 • C, the YVO 4 :Eu 3+ nanoparticles quickly grow to a much larger size. Similar behavior has been reported by others [19,20].

Synthesis of YVO 4 :Eu 3+ @YVO 4 Core-Shell Nanoparticles
The annealed YVO 4 :Eu 3+ nanoparticles (Section 2.2) were used as a core to grow a pristine YVO 4 shell for forming the YVO 4 :Eu 3+ @YVO 4 core-shell nanoparticles ( Figure 1). The pristine YVO 4 shell, which is homo-nanoparticled to the core, is expected to passivate the surface non-radiative traps or defects of the annealed YVO 4 :Eu 3+ nanoparticles [21][22][23]. First, 0.75 mmol Na 3 VO 4 ·12H 2 O and 0.05 g PVP were dissolved in 8 mL ethylene glycol and 1 mL deionized water. Second, 1 mmol of the annealed YVO 4 :Eu 3+ nanoparticles were re-dispersed into 9 mL ethylene glycol under continuous ultrasonication for 30 min. The above two solutions were mixed, and appropriate amount of Y(NO 3 ) 3 ·6H 2 O was added to the mixture solution with vigorous magnetic stirring for 5 min. Third, 17 mL of de-ionized water was added to the above mixture solution under magnetic stirring for another 5 min. The mixture solution was poured into a 50 mL Teflon-lined stainless steel autoclave and heated at 200 • C for 2 h, and then naturally cooled down to room temperature. The resultant YVO 4 :Eu 3+ @YVO 4 core-shell nanoparticles were collected by centrifugation, washed with anhydrous ethanol for 3 times, and dried at 80 • C in air. The molar ratios of the core to the shell (core/shell ratio) were 1:0.2, 1:0.25, 1:0.5, and 1:1. In this work, it was found when the core/shell ratio was 1:0.25, the YVO 4 :Eu 3+ @YVO 4 core-shell nanoparticles had the highest photoluminescence intensity. Thus, the core/shell ratio of 1:0.25 was chosen for the following experimental steps. another 5 min. The mixture solution was poured into a 50 mL Teflon-lined stainless steel autoclave and heated at 200 °C for 2 h, and then naturally cooled down to room temperature. The resultant YVO4:Eu 3+ @YVO4 core-shell nanoparticles were collected by centrifugation, washed with anhydrous ethanol for 3 times, and dried at 80 °C in air. The molar ratios of the core to the shell (core/shell ratio) were 1:0.2, 1:0.25, 1:0.5, and 1:1. In this work, it was found when the core/shell ratio was 1:0.25, the YVO4:Eu 3+ @YVO4 core-shell nanoparticles had the highest photoluminescence intensity. Thus, the core/shell ratio of 1:0.25 was chosen for the following experimental steps.

Synthesis of YVO4:Eu 3+ @ YVO4:Eu 3+ Nanoparticles
As shown in Figure 1, an ion exchange method was used to transform the as-synthesized YVO4:Eu 3+ @YVO4 core-shell nanoparticles (Section 2.3) to YVO4:Eu 3+ @YVO4:Eu 3+ core-shell nanoparticles by following the procedures described previously [18]. Appropriate amount of Eu(NO3)3·6H2O was dissolved in 15 ml of anhydrous ethanol. The YVO4:Eu 3+ @YVO4 nanoparticles were added to above solution with ultrasonic processing for 30 min and then vigorous magnetic stirring for 1 h, the mixture solution was let to stand for 24 h. The resultant product was collected by centrifugation, washed with anhydrous ethanol for 3 times, and finally dried in air at 80 °C.

Synthesis of YVO 4 :Eu 3+ @ YVO 4 :Eu 3+ Nanoparticles
As shown in Figure 1, an ion exchange method was used to transform the as-synthesized YVO 4 :Eu 3+ @YVO 4 core-shell nanoparticles (Section 2.3) to YVO 4 :Eu 3+ @YVO 4 :Eu 3+ core-shell nanoparticles by following the procedures described previously [18]. Appropriate amount of Eu(NO 3 ) 3 ·6H 2 O was dissolved in 15 mL of anhydrous ethanol. The YVO 4 :Eu 3+ @YVO 4 nanoparticles were added to above solution with ultrasonic processing for 30 min and then vigorous magnetic stirring for 1 h, the mixture solution was let to stand for 24 h. The resultant product was collected by centrifugation, washed with anhydrous ethanol for 3 times, and finally dried in air at 80 • C.

Synthesis of YVO 4 :Eu 3+ @YVO 4 :Eu 3+ -TTA Nanoparticles
At last, the YVO 4 :Eu 3+ @YVO 4 :Eu 3+ -TTA inorganic-organic hybrid nanoparticles were synthesized by the previously described procedures [17,18]. It was found in this work that both the as-synthesized YVO 4 :Eu 3+ and the annealed YVO 4 :Eu 3+ cannot form YVO 4 :Eu 3+ -TTA inorganic-organic hybrid structures. Only the ion exchanged YVO 4 :Eu 3+ (Section 2.4) can readily chelate with TTA to form YVO 4 :Eu 3+ -TTA inorganic-organic hybrid structures [18]. The YVO 4 :Eu 3+ @YVO 4 :Eu 3+ nanoparticles (Section 2.4) were added to 10 mL anhydrous ethanol with ultrasonic processing for 30 min. An appropriate amount of TTA was dissolved in 10 mL of anhydrous ethanol with ultrasonic processing for 15 min and then slowly dripped into the YVO 4 :Eu 3+ @YVO 4 :Eu 3+ ethanol solution. The mixture solution was stirred for 1 h and then was left at rest for 12 h. The resultant product was collected by centrifugation, washed with anhydrous ethanol for 3 times and finally dried in air at 80 • C to obtain the YVO 4 :Eu 3+ @YVO 4 :Eu 3+ -TTA inorganic-organic hybrid nanoparticles. The amounts of TTA used in this step were 0.05, 0.1, 0.15, and 0.2 mmol.

Characterization
X-ray diffraction (XRD, Bruker D8 Focus, Karlsruhe, Germany) and transmission electron microscopy (TEM, JEM 2010, Japan) were used to investigate the crystal structures and microstructures of the nanoparticles. Their luminescence excitation and emission spectra were recorded with a Hitachi F-4600 fluorescence spectrophotometer, and their chemical bonding information was characterized by Fourier transform infrared (FTIR) spectroscopy (Nicolet 380, Thermo Fisher, USA). Their UV-Vis absorption spectra were monitored with a TU-1950 UV-Vis spectrophotometer. All measurements were conducted in air at room temperature. Figure 2 shows the X-ray diffraction (XRD, Bruker D8 Focus) patterns of the as-synthesized YVO 4 :Eu 3+ , the annealed YVO 4 :Eu 3+ and the YVO 4 :Eu 3+ @YVO 4 :Eu 3+ -TTA hybrid nanoparticles. All of the diffraction peaks of samples are identical to the JCPDS 17-0341 of the tetragonal YVO 4 , and no impurity phases were present. The strong and narrow XRD peaks of the annealed YVO 4 :Eu 3+ nanoparticles in Figure 2 suggested that the annealing process greatly enhanced their crystallization. It is also noted in Figure 2 that, after the core-shell process and hybridization with TTA, the YVO 4 :Eu 3+ nanoparticles still had the same XRD patterns. inorganic-organic hybrid nanoparticles. The amounts of TTA used in this step were 0.05, 0.1, 0.15, and 0.2 mmol.

Characterization
X-ray diffraction (XRD, Bruker D8 Focus, Karlsruhe, Germany) and transmission electron microscopy (TEM, JEM 2010, Japan) were used to investigate the crystal structures and microstructures of the nanoparticles. Their luminescence excitation and emission spectra were recorded with a Hitachi F-4600 fluorescence spectrophotometer, and their chemical bonding information was characterized by Fourier transform infrared (FTIR) spectroscopy (Nicolet 380, Thermo Fisher, USA). Their UV-Vis absorption spectra were monitored with a TU-1950 UV-Vis spectrophotometer. All measurements were conducted in air at room temperature. Figure 2 shows the X-ray diffraction (XRD, Bruker D8 Focus) patterns of the as-synthesized YVO4:Eu 3+ , the annealed YVO4:Eu 3+ and the YVO4:Eu 3+ @YVO4:Eu 3+ -TTA hybrid nanoparticles. All of the diffraction peaks of samples are identical to the JCPDS 17-0341 of the tetragonal YVO4, and no impurity phases were present. The strong and narrow XRD peaks of the annealed YVO4:Eu 3+ nanoparticles in Figure 2 suggested that the annealing process greatly enhanced their crystallization. It is also noted in Figure 2 that, after the core-shell process and hybridization with TTA, the YVO4:Eu 3+ nanoparticles still had the same XRD patterns.

UV-Vis Absorption Spectra
The UV-Vis (TU-1950) absorption spectra of the annealed YVO 4 :Eu 3+ nanoparticles, TTA, and YVO 4 :Eu 3+ @YVO 4 :Eu 3+ -TTA hybrid nanoparticles are shown in Figure 5. The broad absorption band (230-330 nm) of the annealed YVO 4 :Eu 3+ nanoparticles was due to the VO 4 3− groups in the host lattice of YVO 4 nanoparticle [30]. It also can be seen in Figure 5 that TTA had a broad absorption band of 240-420 nm. Consequently, YVO 4 :Eu 3+ @YVO 4 :Eu 3+ -TTA nanoparticles possessed two broad absorption bands in the ultraviolet range, which is due to the presence of VO 4 3− groups and TTA ligands. The UV-Vis (TU-1950) absorption spectra of the annealed YVO4:Eu 3+ nanoparticles, TTA, and YVO4:Eu 3+ @YVO4:Eu 3+ -TTA hybrid nanoparticles are shown in Figure 5. The broad absorption band (230-330 nm) of the annealed YVO4:Eu 3+ nanoparticles was due to the VO4 3-groups in the host lattice of YVO4 nanoparticle [30]. It also can be seen in Figure 5 that TTA had a broad absorption band of 240-420 nm. Consequently, YVO4:Eu 3+ @YVO4:Eu 3+ -TTA nanoparticles possessed two broad absorption bands in the ultraviolet range, which is due to the presence of VO4 3− groups and TTA ligands.
After annealing, a remarkable enhancement in their photoluminescence was resulted. This is consistent with the previously reported results [19,21,34,35]. An increase of about 22 times in the emission of the YVO4:Eu 3+ nanoparticles at 623 nm was observed after the annealing process. As shown in Section 3.1, the average particle size of the YVO4:Eu 3+ nanoparticles increased from about 15 nm to 28 nm after annealing, but this size difference may not sufficiently explain such a remarkable increase of their photoluminescence. The XRD patterns in Figure 2 indicates that the crystallinity of the as-synthesized YVO4:Eu 3+ nanoparticles was greatly improved by the annealing process. Furthermore, the number of crystal defects in the YVO4:Eu 3+ nanoparticles as well as the non-radiative traps should also simultaneously decrease. These factors are mainly responsible for the notable enhancement of photoluminescence of the YVO4:Eu 3+ nanoparticles by annealing. Figure 7 shows the photoluminescence excitation and emission spectra of the YVO4:Eu 3+ @YVO4 core-shell nanoparticles having different core/shell ratios. Such a homo-nanoparticled shell is believed to be able to effectively passivate the non-radiative traps or defects on the surface of the core [19,20]. The photoluminescence spectra in Figure 7 are typical for the Eu 3+ -doped YVO4 phosphors. After the core-shell process, the YVO4:Eu 3+ @YVO4 nanoparticles had stronger photoluminescence than the unshelled YVO4:Eu 3+ nanoparticles (core/shell ratio = 1:0 in Figure 7),

Photoluminescence Properties
The photoluminescence excitation and emission spectra (Hitachi F-4600) of the as-synthesized (unannealed) YVO 4 :Eu 3+ nanoparticles and the annealed YVO 4 :Eu 3+ nanoparticles are shown in Figure 6. In Figure 6a, the strongest excitation band peaking at about 316 nm was a result of the charge transfer absorption of VO 4 3groups [31]. The minor hump-like peak at about 260 nm was due to the charge transfer band of Eu-O. The other weak excitation peaks at 397 nm ( 7 F 0,1 → 5 L 6 ) and 469 nm ( 7 F 0,1 → 5 D 2 ) were from Eu 3+ ions [32]. In Figure 6b, all of the photoluminescence emissions were from the f-f transitions of Eu 3+ , including 542 nm ( 5 D 1 → 7 F 1 ), 598 nm ( 5 D 0 → 7 F 1 ), 623 nm ( 5 D 0 → 7 F 2 ), 655 nm ( 5 D 0 → 7 F 3 ), and 705 nm ( 5 D 0 → 7 F 4 ) [33]. After annealing, a remarkable enhancement in their photoluminescence was resulted. This is consistent with the previously reported results [19,21,34,35]. An increase of about 22 times in the emission of the YVO 4 :Eu 3+ nanoparticles at 623 nm was observed after the annealing process. As shown in Section 3.1, the average particle size of the YVO 4 :Eu 3+ nanoparticles increased from about 15 nm to 28 nm after annealing, but this size difference may not sufficiently explain such a remarkable increase of their photoluminescence. The XRD patterns in Figure 2 indicates that the crystallinity of the as-synthesized YVO 4 :Eu 3+ nanoparticles was greatly improved by the annealing process. Furthermore, the number of crystal defects in the YVO 4 :Eu 3+ nanoparticles as well as the non-radiative traps should also simultaneously decrease. These factors are mainly responsible for the notable enhancement of photoluminescence of the YVO 4 :Eu 3+ nanoparticles by annealing. Figure 7 shows the photoluminescence excitation and emission spectra of the YVO 4 :Eu 3+ @YVO 4 core-shell nanoparticles having different core/shell ratios. Such a homo-nanoparticled shell is believed to be able to effectively passivate the non-radiative traps or defects on the surface of the core [19,20].

Conclusions
In summary, YVO 4 shells were grown on annealed YVO 4 :Eu 3+ nanoparticle cores, and they were ion exchanged with Eu 3+ ions, followed by hybridizing with TTA to successfully form the YVO 4 :Eu 3+ @YVO 4 :Eu 3+ -TTA inorganic-organic hybrid nanoparticles. It has been found that annealing process considerably enhances the photoluminescence of the as-synthesized YVO 4 :Eu 3+ nanoparticles, and the presence of YVO 4 shells further improves the photoluminescence of the annealed YVO 4 :Eu 3+ nanoparticles. When the core/shell ratio was 1:0.25, the photoluminescence of the annealed YVO 4 :Eu 3+ nanoparticles was the strongest. The inorganic-organic hybridization process greatly broadens the photoluminescence excitation spectral bandwidth range of the YVO 4 :Eu 3+ nanoparticles. After hybridization with TTA, the YVO 4 :Eu 3+ @YVO 4 :Eu 3+ -TTA inorganic-organic hybrid nanoparticles broadened the excitation spectral bandwidth range from 230-330 nm to 230-415 nm, which covers the entire ultraviolet spectrum of the solar light and enables their potential applications in silicon solar cells.