Effects of Annealing on the Photoluminescence Properties of Citrate

Article pubs.acs.org/JPCC

Effects of Annealing on the Photoluminescence Properties of CitrateCapped YVO4:Bi3+,Eu3+ Nanophosphor Yoshiki Iso, Satoru Takeshita,* and Tetsuhiko Isobe* Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan S Supporting Information *

ABSTRACT: We investigated the effects of annealing on the photoluminescence (PL) properties of a citrate-capped YVO4:Bi3+,Eu3+ nanophosphor. The 300 °C annealed nanophosphor exhibited the highest PL intensity at 619.5 nm corresponding to an f−f transition of Eu3+ under near-UV excitation. Fourier transform infrared spectroscopy indicated a decrease in the amounts of adsorbed water and citrate on the nanophosphor surface with annealing. Dehydration and the increase in near-UV absorption contributed to the improved PL intensity, since water is a Eu3+ luminescence quencher. The thermal decomposition of citrate, which photoreduces vanadate under near-UV irradiation, caused the improved photostability. Annealing at ≥400 °C decreased the PL intensity. A color change from white to yellow may be attributed to absorption by byproducts, possibly formed by the segregation of YVO4:Bi3+,Eu3+. A shorter PL lifetime and lower activation energy of thermal quenching at higher annealing temperature were confirmed from PL decay curves and the temperature dependence of PL intensity, respectively. The formation of surface defects resulted from the thermal decomposition of adsorbed citrate. The decrease in PL intensity was caused by the formation of byproducts and surface defects.

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followed by energy transfer to Eu3+. Bi3+ contributes to narrowing the band gap, so YVO4:Bi3+,Eu3+ emits red under excitation at longer wavelengths than YVO 4 :Eu 3 + . YVO4:Bi3+,Eu3+ nanoparticles have potential in near-UV white LEDs,29,30 spectral down-shifters for solar cells,31−33 and bioimaging.34,35 This is because of their near-UV to red wavelength conversion, high transparency from their low visible light scattering,36 and higher photostability than organic dyes and metal complexes.37 YVO4:Bi3+,Eu3+ nanoparticles have been synthesized by various wet processes including coprecipitation,38 hydrothermal,39,40 and sol−gel31 methods. We previously prepared the nanophosphor by aqueous wet synthesis from a citrate precursor.41,42 The nanophosphor was well-dispersed, because of electrostatic repulsion from negatively charged adsorbed citrate. However, an improvement in its PL quantum efficiency was required for its application as a spectral down-shifter for a monocrystalline silicon solar cell module.32 Adsorbed citrate acts as a reducer,43,44 and therefore should be eliminated to improve the photostability of nanophosphors. Adsorbed citrate reduces YVO4:Bi3+,Eu3+ nanoparticles from V5+ to V4+ under UV irradiation, which is accompanied by the formation of charge compensating oxygen vacancies. This in turn decreases the PL intensity.45 We attempted to eliminate adsorbed citrate by repeated centrifugation and aqueous redispersion and

INTRODUCTION Inorganic phosphors are generally very durable. Inorganic semiconductor nanophosphors such as quantum dots have been applied as bioconjugates for labeling, imaging, and sensing, because of their high photostabilities compared with organic dyes and metal complexes.1−5 The photoluminescence (PL) properties of these nanophosphors with high specific surface areas are significantly influenced by their surface states. A decrease in the PL intensity under continuous excitation has been observed for II−VI semiconductor nanophosphors.6−9 This is attributed to photobleaching through photooxidation of the nanophosphor surface by capping agents, dispersants, or singlet oxygen. Photobleaching has also been observed for oxide nanophosphors such as Ce3+-doped Y3Al5O12 and Eu3+doped YVO4.10−13 Bulk Y3Al5O12:Ce3+ is used in blue-excited white LEDs because of its excellent photostability, so photobleaching is characteristic of nanophosphors with high specific surface areas. The suppression of photobleaching and understanding its mechanism are important challenges for the application of nanophosphors. Nanophosphors doped with 3d- and 4f-metal ions have attracted recent interest.14−19 YVO4 nanophosphors exhibit PL properties which depend on their doped trivalent rare-earth ions.20−26 Bi3+-doped YVO4 has a valence band largely consisting of bonding O2− 2p and Bi3+ 6s orbitals and a conduction band largely consisting of V5+ 3d, antibonding O2− 2p, and Bi3+ 6p orbitals, according to theoretical studies.27,28 YVO4:Bi3+,Eu3+ emits red under near-UV excitation through transition between the valence band and the conduction band, © 2014 American Chemical Society

Received: February 20, 2014 Revised: April 28, 2014 Published: May 1, 2014 11006

dx.doi.org/10.1021/jp501799c | J. Phys. Chem. C 2014, 118, 11006−11013

The Journal of Physical Chemistry C

Article

hydrothermal treatment.46,47 However, some citrate remained after such post-treatments. A more efficient approach to eliminate citrate is required. In the present study, we investigate decomposition of adsorbed citrate on YVO4:Bi3+,Eu3+ nanoparticles by annealing to improve PL intensity. The PL intensity of Eu3+ decreases through energy transfer from Eu3+ to OH groups.48,49 Citrate contains OH groups, and adsorbed H2O also exists on the nanophosphor surface. Their elimination by annealing should improve the PL intensity.50 A further improvement in photostability may also be caused by the thermal decomposition of adsorbed citrate. Here, we characterize the effects of annealing on the PL properties and photostability of citratecapped YVO4:Bi3+,Eu3+ nanoparticles.

A365 = 1 −

EXPERIMENTAL SECTION Preparation and Annealing of YVO4:Bi3+,Eu3+ Nanoparticle Powder. The detailed preparation procedure for the aqueous paste of YVO4:Bi3+,Eu3+ nanoparticles (37.6 wt %) is described in the Supporting Information. An aqueous solution of Y(CH3COO)3 and Eu(CH3COO)3 and an ethylene glycol solution of Bi(NO3)3 were mixed with an aqueous solution of sodium citrate. The nominal Y:Bi:Eu composition was 57.0:7.0:36.0. A basic aqueous solution of Na3VO4 was added to the above prepared white suspension. The pH was adjusted to 9.0 with aqueous NaOH, and the suspension was aged at 85 °C for 1 h. After cooling to room temperature, a paste was collected by centrifugation and diluted with deionized water. The diluted paste was hydrothermally treated at 130 °C for 6 h in an autoclave. The resulting colloidal solution was washed with deionized water and centrifuged to obtain an aqueous paste of YVO4:Bi3+,Eu3+ nanoparticles. A YVO4:Bi3+,Eu3+ nanoparticle powder was obtained by grinding the nanophosphor paste dried at 30 °C for 24 h with an agate mortar and pestle. The crude nanophosphor powder in an alumina boat was annealed in an electric tube furnace. Annealing was performed at 200−600 °C for 2 h, under an air flow of 300 mL min−1. The temperature was raised from room temperature to the desired temperature, at a heating rate of 10 °C min−1. Characterization. PL and PL excitation (PLE) spectra were measured at room temperature using a fluorescence spectrometer (FP-6500, JASCO). The spectral response was calibrated using an ethylene glycol solution of Rhodamine B (5.5 g L−1) and a standard light source (ESC-333, JASCO). Changes in the PL intensity during continuous near-UV excitation were measured with the same apparatus. An integrating sphere (ISF-513, JASCO) was attached to measure the PL quantum efficiency of the nanophosphor powder, QE, which is defined as Iem Iex − Iref

(2)

and was measured simultaneously. The thermal quenching behavior was evaluated using the same fluorescence spectrometer with a heating attachment (HPC-503, JASCO). PL decay curves were measured at room temperature with another fluorescence instrument (FluoroCube, Horiba), under 365 nm excitation by a flash Xe lamp. Elemental compositions were determined by the fundamental parameter method, on an X-ray fluorescence (XRF) analyzer (ZSX mini II, Rigaku). X-ray diffraction (XRD) profiles were measured on an X-ray diffractometer (Rint-2200, Rigaku), with a Cu Kα radiation source and a monochromator. Raman spectra were measured on a Raman microscope with a 785 nm laser (inVia StreamLine PlusB, Renishaw). Particle morphologies were observed with a scanning electron microscope (SEM, JSM-7600F, JEOL) and a transmission electron microscope (TEM, Tecnai F20, FEI). The samples for SEM and TEM observation were prepared by drying a drop of colloidal solution on a silicon substrate and a copper microgrid, respectively, at 30 °C for 1 day. The nanophosphors on silicon substrates were precoated with osmium before SEM observation to prevent charging. Nitrogen adsorption isotherms at 77 K were measured with an automatic surface area analyzer (Tristar II 3200, Micromeritics), to determine Brunauer− Emmett−Teller (BET) specific surface areas. Nanophosphor powders were degassed by heating at 100 °C for 1 h under reduced pressure prior to measurement. Absorption spectra were measured on a UV/visible/near-IR optical absorption spectrometer (V-570, JASCO). Electron spin resonance (ESR) spectra were measured at room temperature using a spectrometer (Bruker, ELEXSYS E500), at the X-band (9.86 GHz) with a modulation frequency of 100 kHz. Fourier transform infrared (FT-IR) absorption spectra of pressed KBr disks containing nanophosphor powders were measured on a spectrometer (FT/IR-4200, JASCO).

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QE =

Iref Iex

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RESULTS AND DISCUSSION Photoluminescence Properties of Annealed YVO4:Bi3+,Eu3+ Nanoparticles. Figure 1 shows room temperature PL and PLE spectra of crude and annealed YVO4:Bi3+,Eu3+ nanophosphor powders. A broad intense excitation peak caused by the interband transition of YVO4:Bi3+,Eu3+ followed by energy transfer to Eu3+ is observed in the near-UV region of the PLE spectra, measured at the 619.5 nm emission of Eu3+. Sharp weak peaks corresponding to the direct excitation of Eu3+ are observed in the visible region.51 Sharp emission peaks corresponding to the 5D0 → 7FJ (J = 1, 2, 3, 4) transitions of Eu3+ are observed in the PL spectra measured under 365.0 nm excitation.51 Emission by the host YVO4 and the 6s6p → 6s2 transition of doped Bi3+ are not observed in these spectra. The maximum PL intensity is achieved following annealing at 300 °C, and the PL intensity decreases after annealing at temperatures ≥400 °C. Figure 2 shows the PL decay curves and PL lifetimes, τ, of the samples. τ was calculated from

(1)

where Iem, Iex, and Iref are the integrated intensities of emission from the sample, incident excitation light, and excitation light reflected by the sample, respectively. The excitation wavelength was 365.0 nm. A reflectance standard (Spectralon SRS-99, Labsphere) was used to measure the value of Iex. Iex − Iref indicates the integrated intensity of the excitation light absorbed by the sample. The absorption efficiency of the excitation light by the sample, A365, is defined by

⎛ t⎞ I = I0 exp⎜ − ⎟ ⎝ τ⎠

(3)

where I is the PL intensity at time t and I0 is the initial PL intensity. τ decreases with increasing annealing temperature, especially at ≥400 °C. 11007



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In other words, the nominal and actual Y:Bi:Eu compositions are 57.0:7.0:36.0 and 56.9:7.2:35.9, respectively. The actual composition does not change after annealing at 200−600 °C within experimental error (see Figure S1 in the Supporting Information). Figure 3 shows the XRD profiles of crude and annealed nanophosphor powders. In Figure 3A, all peaks of the crude

Figure 1. PL and PLE spectra of crude and annealed nanophosphor powders. λex = 365.0 nm, λem = 619.5 nm.

Figure 3. XRD profiles of crude and annealed nanophosphor powders: (A) whole profiles; (B) normalized (200) peaks. International center for diffraction data (ICDD) data of tetragonal YVO4 (No. 17-341) is also shown.

and 200−500 °C annealed samples are assigned to tetragonal YVO4. An extra peak appears at ∼29° for the sample annealed at 600 °C. This indicates the existence of byproducts such as other vanadate compounds and cubic Y2O3 (see Figure S2 in the Supporting Information). Raman spectra also indicate possible byproducts (see Figure S3 in the Supporting Information). The XRD peaks corresponding to the (200) plane of tetragonal YVO4 are normalized in Figure 3B. Crystallite sizes, dXRD, calculated from the (200) peak by the Scherrer equation are shown in Table 1. The crystallite size of the crude sample is 7.6 nm. Growth of crystallites is not observed at annealing temperatures ≤400 °C. Figure 2. PL decay curves of crude and annealed nanophosphor powders. Black circles, crude; blue squares, 200 °C; green diamonds, 300 °C; orange triangles, 400 °C; red inverted triangles, 500 °C. The inset shows the change in calculated τ with annealing temperature. λex = 365 nm, λem = 620 nm.

Table 1. Calculated Crystallite Sizes (dXRD) and Primary Particle Sizes (dBET) for Crude and Annealed Nanophosphor Powders dXRD (nm) dBET (nm)

The reasons for the maximum PL intensity after annealing at 300 °C and the shorter PL lifetime after annealing at higher temperatures are discussed in the following sections. Composition, Particle, and Structural Properties of Annealed YVO4:Bi3+,Eu3+ Nanoparticles. The nominal Y:Bi:Eu:V com po sition of the nanophosphor is 30.0:3.7:18.9:47.4. The actual Y:Bi:Eu:V composition of the crude nanophosphor powder as determined by XRF is 31.5:4.0:19.9:44.5, which is close to the nominal composition.

crude

200 °C

300 °C

400 °C

500 °C

600 °C

7.6 7.6

7.7 7.1

7.5 8.9

7.7 12

9.0 21

13 39

Primary particle sizes (dBET) of the samples were estimated from specific surface areas, because primary particles could not be clearly identified from SEM and TEM observations (see Figures S4 and S5 in the Supporting Information). Specific surface areas of the nanophosphors estimated from the BET method decrease with increasing annealing temperature (see 11008



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formed through segregation of YVO4:Bi3+,Eu3+ during annealing. This would decrease the PL intensity when annealing at ≥400 °C. Changes in States of Adsorbed Molecules on YVO4:Bi3+,Eu3+ Nanoparticles by Annealing. Figure 5A

Figure S6 and Table S1 in the Supporting Information). dBET values calculated from specific surface areas assuming spherical shapes are shown in Table 1. The value of dBET for the crude sample is 7.6 nm, which is equal to dXRD. The difference between dBET and dXRD increases with increasing annealing temperature. This is attributed to the promotion of coalescence between the monocrystalline nanoparticles by annealing. Absorption Properties of Annealed YVO4:Bi3+,Eu3+ Nanoparticles. The absorption efficiency of the nanophosphor powder for excitation at 365.0 nm increases with increasing annealing temperature (see Figure S7 in the Supporting Information), although the reason for this has not been clarified. This contributes to the increase in PL intensity. The sample color changed from white to yellow after annealing. Figure 4 shows normalized absorption spectra of crude and

Figure 5. FT-IR spectra of crude and annealed nanophosphor powders: (A) whole spectra; (B) expansion of the adsorbed citrate spectral region.

shows FT-IR absorption spectra of crude and annealed nanophosphor powders. The observed absorption peaks are assigned in Table 2. The spectra are normalized to the absorption peak no. 7 at ∼800 cm−1, which is assigned to the V−O stretching vibration of VO 4 3− in YVO 4 :Bi 3+ ,Eu 3+ (ν(VO)).58 Expanded spectra of the adsorbed citrate region at 1800−1200 cm−1 are shown in Figure 5B. For the crude sample, the peak nos. 3 and 4 at 1571 and 1499 cm−1, respectively, are assigned to the asymmetric stretching mode of citrate carboxyl groups. The peak no. 5 at 1380 cm−1 is assigned to the symmetric stretching mode of citrate carboxyl groups (νs(COO−)). The coordination of carboxyl groups to metal cations can be classified as monodentate or bidentate.57,59 When the difference between the peak positions of the asymmetric and symmetric stretching modes is >300 cm−1, citrate coordination is monodentate.60 When the difference is ∼190 °C.61 Annealing at higher temperature promotes the thermal decomposition of citrate. Figure 7 also shows the change in the ratio of A3 (assigned to νas1(COO−)) to A4 (assigned to νas2(COO−)) with increasing annealing temperature. A3 preferentially decreases with increasing

⎛ I ⎞ E ln⎜ 0 − 1⎟ = − a + C kBT ⎝ I (T ) ⎠ 11010

(4)



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where I(T) is the PL intensity measured at temperature T (K), kB the Boltzmann constant, and C a constant.62−64 Table 3

annealed nanophosphor was 67.3% of its initial intensity, after continuous irradiation at 365.0 nm for 3 h. This value was higher than the 53.7% for the crude nanophosphor. Photostability was also improved by the thermal decomposition of citrate, which photoreduces vanadate under near-UV irradiation. Annealing at ≥400 °C decreased the PL intensity. The color of the nanophosphor changed from white to yellow upon annealing. This may be attributed to the absorption of blue light by byproducts, which were possibly formed by the segregation of YVO4:Bi3+,Eu3+. PL decay curves revealed a shortening of the PL lifetime upon annealing. The lifetimes of the crude and 500 °C nanophosphors were 707 and 262 μs, respectively. The temperature dependence of PL intensity indicated a lower activation energy of thermal quenching at higher annealing temperature. Activation energies were 0.19 and 0.23 eV for the 500 and 200 °C annealed nanophosphors, respectively. These results indicated that the formation of surface defects, which might be dangling bonds of trivalent metal cations at the surface, resulted from the thermal decomposition of adsorbed citrate, especially that with bidentate bridging coordination. The decrease in PL intensity upon annealing at ≥400 °C was caused by the formation of byproducts and surface defects.

Table 3. Estimated Ea of Thermal Quenching for Nanophosphor Powders Annealed at 200−500 °C annealing temperature (°C) Ea (eV)

200

300

400

500

0.23

0.24

0.19

0.19

shows Ea estimated from the Arrhenius plots shown in Figure 8B. Ea decreases upon annealing at ≥400 °C. This may be attributed to the formation of surface defects that increase the probability of nonradiative relaxation. This would also be one of the reasons for the decrease in PL intensity at room temperature after annealing at ≥400 °C. Photostabilities of Crude and 300 °C Annealed YVO4:Bi3+,Eu3+ Nanoparticles. The change in PL intensity at 619.5 nm corresponding to the 5D0 → 7F2 transition of Eu3+ during continuous irradiation at 365.0 nm for 3 h was measured to evaluate photostability. Figure 9 shows photobleaching

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ASSOCIATED CONTENT

S Supporting Information *

Details of the preparation procedure of the YVO4:Bi3+,Eu3+ nanoparticles; elemental compositions of crude and annealed YVO4:Bi3+,Eu3+ nanoparticle powders (Figure S1); expanded XRD profile of the 600 °C annealed YVO4:Bi3+,Eu3+ nanoparticle powder (Figure S2); Raman spectra of crude and annealed YVO4:Bi3+,Eu3+ nanoparticle powders (Figure S3); SEM images of crude and annealed YVO4:Bi3+,Eu3+ nanoparticle powders (Figure S4); TEM images of crude and annealed YVO4:Bi3+,Eu3+ nanoparticle powders. (Figure S5); changes in the amount of adsorbed N2 with relative pressure and BET plots for crude and annealed YVO4:Bi3+,Eu3+ nanoparticle powders (Figure S6); change in the absorption efficiency at 365.0 nm with annealing temperature (Figure S7); ESR spectra of crude and annealed YVO4:Bi3+,Eu3+ nanoparticle powders (Figure S8); and specific surface areas of crude and annealed YVO4:Bi3+,Eu3+ nanoparticle powders measured by the BET method (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 9. Change in the PL intensity of crude and 300 °C annealed nanophosphor powders, during near-UV light excitation for 3 h. λex = 365.0 nm, λem = 619.5 nm.

curves normalized to the initial PL intensity. The PL intensity of the crude nanophosphor decreases to 53.7% of the initial intensity. The PL intensity of the nanophosphor annealed at 300 °C decreases to 67.3%. This improvement in photostability would be caused by the thermal decomposition of adsorbed citrate. The QE of the 300 °C annealed sample before and after photobleaching was 22.8 and 17.5%, respectively. These values were higher than those of 21.4 and 14.6% for the crude sample, respectively.

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CONCLUSIONS The citrate-capped YVO4:Bi3+,Eu3+ nanophosphor was annealed at 200−500 °C, to investigate the effects of annealing on its PL properties. The 300 °C annealed nanophosphor exhibited a maximum PL intensity at 619.5 nm, corresponding to the f−f transition of Eu3+ under excitation at 365.0 nm. The absorption efficiency of the nanophosphor at 365.0 nm was increased by annealing. Decomposition of Fourier transform infrared spectra revealed that absorption peaks of water and bidentate coordinated citrate decreased with annealing. This indicated the elimination of adsorbed water and citrate from the nanophosphor surface. Dehydration and the increase in the near-UV absorption efficiency contributed to the improvement in PL intensity upon annealing at 300 °C, since water is a Eu3+ luminescence quencher. The PL intensity of the 300 °C

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +81 45 566 1531. Fax: +81 45 566 1551. *E-mail: [email protected]. Phone: +81 45 566 1554. Fax: +81 45 566 1551. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mr. Yusuke Noguchi and Mr. Kenji Akisada at Keio University for preliminary experiments. We also thank SINLOIHI Co., Ltd. for preparation of the nanophosphor paste and HORIBA, Ltd. for measurement of photoluminescence lifetimes. 11011



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Effects of Annealing on the Photoluminescence Properties of Citrate

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