Synthesis and Luminescent Properties of REVO4âREPO4 (RE = Y, Eu

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Synthesis and Luminescent Properties of REVO-REPO (RE=Y,Eu,Gd,Er,Tm or Yb) Heteronanostructures: A Promising Class of Phosphors for Excitation From NIR to VUV Paulo Cesar de Sousa Filho, Thierry Gacoin, Jean-Pierre Boilot, Richard I. Walton, and Osvaldo Antonio Serra J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08249 • Publication Date (Web): 01 Oct 2015 Downloaded from http://pubs.acs.org on October 5, 2015

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The Journal of Physical Chemistry

Synthesis and Luminescent Properties of REVO4-REPO4 (RE=Y,Eu,Gd,Er,Tm or Yb) Heteronanostructures: A Promising Class of Phosphors for Excitation from NIR to VUV

Paulo C. de Sousa Filho, †‡* Thierry Gacoin,‡ Jean-Pierre Boilot,‡ Richard I. Walton,§ Osvaldo A. Serra†

†

Rare Earth Laboratory; Department of Chemistry; Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto; University of São Paulo. Av. Bandeirantes, 3900, 14040-901, Ribeirão Preto, SP, Brazil. ‡

Solid State Chemistry Group; Laboratoire de Physique de la Matière Condensée; Ecole Polytechnique/Université Paris-Saclay. Route de Saclay, 91128 Palaiseau Cedex, France. §

Department of Chemistry; University of Warwick. Coventry, CV4, 7 AL, United Kingdom.

*

Email: [email protected] 1

ACS Paragon Plus Environment


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ARTICLE

Synthesis

and

Luminescent

Properties

of

REVO4-REPO4

(RE=Y,Eu,Gd,Er,Tm or Yb) Heteronanostructures: A Promising Class of Phosphors for Excitation from NIR to VUV

ABSTRACT: Despite presenting very similar structural properties, rare earth (RE) phosphate and vanadate solids display different spectroscopic behaviors, which makes the elaboration of REVO4-REPO4 mixed structures an interesting prospect for the design of luminescent materials with improved activity. This work describes the application of a two-step colloidal precipitation approach for the formation of REVO4-REPO4 heteronanostructures and an investigation of their luminescent properties. The growth of the phosphate phase over REVO4 particles was kinetically evaluated through spectroscopic methodologies comprising the observation of the VO43-→Eu3+ energy transfer and the absorption of vanadium(V) peroxocomplexes in solution. This confirmed an effective coating of the precursor nanoparticles. In order to obtain materials with enhanced properties, a protected annealing methodology in SiO2 was applied, leading to highly crystalline nanorods with low degree of aggregation. The final materials display efficient emissions in the red, green, and blue (RGB) regions under VUV, UV or NIR excitation, according to their composition. The described structures are promising for light generation in several different systems and can be tuned to provide RGB emissions as VUV-excited phosphors and as NIR- or UV colloidal luminescent biomarkers. KEYWORDS: phosphates; vanadates; rare earth; nanoparticles; luminescence. 2


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INTRODUCTION Rare earth elements (RE) display countless applications in several technological fields, and are associated with practically all everyday activities of modern society.1-4 Specifically, the optical properties of RE elements result in an unrivalled importance in the field of luminescent materials due to their unique spectroscopic features, which recently has led this group of elements to be defined as the “vitamins of phosphors”.4-7 This variety of optical properties arise from the partial filling of the 4f orbitals, which typically are only slightly affected by chemical environment and give rise to narrow and intense emission lines.8-10 In turn, this makes lanthanoid ions almost irreplaceable components for the generation of visible light in several systems,11 as well as in devices comprising upconversion12,13 or downconversion14 luminescence. Particularly, RE-doped inorganic nanoparticles15-17 display singular luminescent properties that currently find a number of applications, such as the development of excimer lamps for the elimination of Hg in fluorescent tubes,18-21 plasma displays with improved resolution,22,23 light emitting diode (LED) lamps,24,25 as well as biolabeling materials.26,27. The investigation of different methodologies for the obtainment of RE-based nanoparticles, as well as their structural, morphological and spectroscopic characterization, is therefore a fundamental aspect for the amelioration of these applications.28 The growth of additional layers over the surface of nanostructured luminescent materials is usually necessary to minimize the effects of the dispersing medium by providing a protective layer.29-32 The main synthetic challenges related to such structures are related to the necessity to obtain an adequately passivated surface, at the same time ensuring the initial properties are not affected, and ideally improved. In addition, the shell can provide in an important intermediate for the compatibility between the core particle and the dispersing medium with

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respect to the surface charge, functionality, reactivity or toxicity,31 besides the possibility to combine different properties (optical, magnetic etc.) in a single nanoparticle.32 Regarding to the formation of RE-based luminescent nanostructures, the synthesis of RE vanadates is extensively investigated through liquid phase bottom-up approaches, since these compounds can crystallize at relatively low temperatures.33-38 REVO4 particles are then largely explored for a wide range of applications due to their well-known advantageous luminescent properties.10,11,39,40 For instance, YVO4:Eu3+ is a classical solid state lighting phosphor, since it presents very high luminescence yields, high UV excitability and a practically monochromatic red emission.10,11,39-41 Such characteristics, in combination with their suitable lifetime and large shift between excitation and emission maxima (recently suggested to be named as the “Richardson Shift”),42 also has led to the use of YVO4:Eu3+ nanoparticles in biolabeling systems, notably for the reversible intracellular H2O2 detection.43,44 In addition, other REVO4 structures displaying upconversion luminescence have also been investigated both as markers in biological media and as IR-excitable coatings.45,46 However, REVO4 nanoparticles display some critical drawbacks which limit their broader application as phosphors or biological markers. For instance, the limited chemical stability of vanadates against redox reactions has restricted their use in fluorescent tubes,8-11 whereas their relatively high susceptibility to dissolution in acidic conditions or to the conversion into other more stable solids is still an important issue for their application in vivo.47,48 Moreover, despite their luminescence yields under UV irradiation, RE vanadates are difficult to apply as phosphors with excitation in the vacuum ultraviolet region (VUV, λ280 nm) with the two species in separated phases, its relative intensity is considerably low in comparison to the spectra of the REVO4-REPO4 systems (Figure 4). The occurrence of this band confirms that, even with separated nanoparticles, the interaction between the two phases in suspension is possible and indeed occurs. However, the low intensity associated to this

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energy transfer process shows that the interaction between the two phases is much more effective in the mixed system, which is in agreement with the former observations. Protected annealing. The as-obtained REVO4-REPO4 particles present a very low crystallinity, which can be a limiting factor for their application as phosphors, due to their low luminescence efficiencies. Therefore, a protected annealing methodology (Figure S6) was applied in order to improve the particle crystallinity.45,64,65 The thermal analysis curves (TGA/DTA) of the precursor mixture (solids in silica) indicate the loss of ~50% of the initial weight due to the elimination adsorbed of water and condensation of OH groups, besides the combustion of the residual surfactant molecules (Figure S7). The curves reveal that the sample is thermally stabilized around 650-700 °C, when only an exothermic event takes place, thus being possibly related to the final steps of the sample crystallization. In addition, an isothermal treatment at 900 °C for 2 h hours does not lead to additional weight loss steps or to parallel crystallization events (e.g. silicate formation), since no peaks are observed in the DTA curve. The elimination of the silica matrix through dissolution in HF can be monitored by FTIR spectroscopy, as presented in Figure S8. The initial sample (before HF attack) displays PO43and VO43- signals superposed on the Si-O vibrations at ~1100, 800, and ~450 cm-1. After three hours of treatment, the band initially centered at 1100 cm-1 (P-O and Si-O ν3 stretches) is sharpened and displaced to 1015 cm-1, a characteristic frequency for tetragonal yttrium phosphates,69 thereby evidencing the dissolution of the silica matrix. In addition, with the absence of silica signals at ~450 cm-1 and with no band superposition, an intensity ratio of 2/1 between the PO43- ν3 and ν4 bands is observed in the final sample, which is in agreement with other reported phosphates and phosphovanadates.41,52,53

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The thermal treatment into a mesoporous silica structure resulted in high crystallinity solids with a low degree of particle aggregation. Figure 5 depicts the TEM images of YVO4-YPO4 particles obtained after this procedure, in which elongated particles (> 200 nm) are observed. The high crystallinity of the obtained solids is confirmed by the SAED images (Figure 5(a), inset), in which a clear diffraction pattern is observed in addition to some diffraction rings arising from the polycrystalline nature of the sample. The images show the presence of two distinguishable domains, which are probably related to the YPO4 and YVO4 phases, whereas the high resolution micrographs (Figures 5(c) and (d)) indicates a high degree of epitaxy between them. EDS elemental mappings (Figures S9 and S10) indicate that the vanadate groups are dispersed through the solids, thus suggesting the formation of a solid solution (i.e. Y(P,V)O4) or a concentration gradient at the phase interface.

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(a)

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(b)

200 nm

(c)

(d)

Figure 5. (a-d) TEM micrographs of the annealed YVO4-YPO4 particles obtained after the thermal treatment and elimination of SiO2. (Inset in (a): electron diffraction pattern of a selected particle).

Figure 6 shows the XRD patterns of the YVO4-YPO4 particles after the thermal treatment, in which a detectable increase in the crystallinity is observed, with a marked reduction of the peak width. The diffractograms confirm a high degree of epitaxy between the two phases, since the diffraction pattern of the phosphate phase predominates. Moreover, the diffraction peaks are slightly inhomogeneously broadened to their low 2θ side, which indicates the 18


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occurrence of solid solutions,41,58,59 suggesting the formation of phosphovanadates at the interface. In addition, the comparison of the obtained X-ray diffractogram with the standard pattern of xenotime-type YPO4 reveals a higher intensity than expected for the reflection at 2θ=25.94°, which confirms that the particles are preferentially grown in the direction of the [2 0 0] plane, in agreement with the TEM images. The structure alterations are also pointed by the calculated lattice parameters (inset of Figure 6(a)), since there is a reduction of the unit cell dimensions in the tetragonal structure after the thermal treatment.

(a)

YVO4 YVO4-YPO4

a (Å)

c (Å)

7.149±0.04

6.204±0.01

V (Å3) 317

7.084±0.03

6.231±0.01

312

6.8831±0.001

6.0096±0.002

284

Relative intensity

YVO4-YPO4 (protected annealing)

yttrium orthovanadate JCPDS 00-017-0341 yttrium orthophosphate JCPDS 01-074-2429

10

20

30

40

50

60

70

2θ (deg.) 1.0x10

-2

8.0x10

-3

6.0x10

-3

4.0x10

-3

2.0x10

-3

(b) YVO4 (precursor)

-1

β cosθ / λ (Å )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-3

ε≈ +5x10 ± 4x10

-3

T≈16±5 nm

YVO4-YPO4 (annealed)

0.0 0.00

0.05

0.10

0.15

-3

ε≈ -1.2x10 ±0.5x10

0.20

0.25

0.30

-3

0.35

-1

sinθ / λ (Å )

Figure 6. (a) Powder X-ray diffractograms of the final YVO4-YPO4 particles after the protected annealing in SiO2, and (b) Williamson-Hall plot constructed from the diffraction peaks of the precursor YVO4 particles (Inset in (a): cell parameters calculated for the YVO4, as-obtained YVO4-YPO4, and annealed YVO4-YPO4 tetragonal (I41/amd) solids).

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The results indicate that the unit cells of the precursor YVO4 particles are slightly distorted in comparison to the standard YVO4 lattice (a=7.1192 Å, c=6.2898 Å) and no substantial alterations are introduced after the first coating. On the other hand, the final particles display cell parameters that are practically the same of the YPO4 standard (a=6.881 Å, c=6.017 Å), thus revealing a highly ordered structure. This is further confirmed by Williamson-Hall plots (Figure 6(b)), which compare the initial YVO4 cores and the final YVO4-YPO4 structures. The plots evidence a large increase in the coherence length, as indicated by a lower intercept for the thermally treated material. In addition, the precursor YVO4 presents a low linear correlation coefficient and a positive slope, which is related to a very high degree of structural distortion. On the other hand, the final solid displays a very high linear correlation and a slightly negative slope. These factors are associated with a low degree of strain, thus confirming that the adopted methodology leads to a significant improvement of crystalline ordering. In order to better describe the conversion of the initial particles into elongated structures, the effect of the temperature on the morphology and on the luminescence of the YVO4-YPO4 solids was evaluated. Figures S12 and S13 display the luminescence spectra of YVO4YPO4:Eu3+ and YVO4:Eu3+-YPO4 samples acquired after annealing at different temperatures (after elimination of SiO2, experimental details in the Supporting Information). When compared

to

the

known

luminescence

profiles

of

phosphates,

vanadates,

and

phosphovanadates,41 the present results point to the obtainment of solid solutions at the vanadate/phosphate interface. In the case of the YVO4-YPO4:Eu3+ preparation, the initial spectrum (700 °C) displays a vanadate-type profile under 300 nm excitation. This reveals, in turn, that, in at this intermediate temperature, a partial ion migration occurred at the interface thus leading some europium ions to the more luminescent vanadate phase. At higher 20


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temperatures, the ion migration processes become more effective, thus leading to solid solutions at the borders of the core particle, which is evidenced by the alteration in the ratio between the intensities of the 5D0→7F2 and 5D0→7F1 transitions (Table S1). In addition, there is a hypsochromic shift of the energy transfer band at higher temperatures (Figure S12), which is in agreement with the partial dilution of VO43- groups in the phosphate phase.41 Conversely, if the Eu3+ ions are initially located in the VO43- phase, a similar behavior according to the formation of solid solutions at the interface is obtained (Figure S13). As the annealing temperature controls the degree of mixing between the two phases and, therefore, the emission profile of these samples, the thermal treatment can be used to control the chromaticity with regard to the orange to red ratio, and indicated in Figure S14 and Table S1 (Supporting Information). The evolution of the particle morphology and structure with the protected annealing temperatures was investigated through TEM and temperature dependent XRD. Figures 7 and S15 depicts TEM images of the YVO4-YPO4 particles dispersed in silica at different stages of the protected annealing procedure. In addition, Figures 8 and S16-17 display the evolution with the temperature of the diffraction profiles of YVO4-YPO4 particles dispersed in silica after the drying procedure (90 °C overnight). Initially, the particles are dispersed in the mesoporous silica structure and present a very low degree of crystallinity (Figures 7(a) and 8). At higher temperatures (>300 °C Fig.(8)), the silica phase is progressively dehydrated due to the condensation of silanol groups, which is evidenced by the reduction of background diffraction signals arising from amorphous material, although its non-crystalline nature is still maintained at ~500 °C (Fig.7(b-c). At this stage, the particles display an increased crystallinity and a higher aspect ratio in comparison to the initial solids (Figures 1 and 2), thus indicating the occurrence of preferential growth processes. 21



Temperature (°C)

1000

(b)

800 600 400 200

0

1

2

3

4

5

6

7

Time (h)

(a)

(b)

(c)

(e)

(f)

1000

Temperature (°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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800 600 400 200

0

1

2

3

4

5

6

7

Time (h)

(d)

Figure 7. TEM micrographs of samples collected at different steps of the protected annealing procedure: (a) initial YVO4-YPO4 particles in the presence of the silica matrix, (b) sample collected at 500 °C, (c) after 2 h at 500 °C, (d) at 900 °C, (e) after 1 h at 900 °C, and (f) after 2 h at 900 °C. (Insets: temperature vs. time plot indicating the steps in which the samples were collected).

Above 500 °C the elimination of decomposable species is completed, as indicated by the disappearance of the impurity signals at 32-33° in powder XRD patterns, which are probably associated to carbonates and carboxylates from partial decomposition of PAA and PEO6800. At temperatures above 600 °C, the crystallization of the tetragonal phase is highly improved, as evidenced by the large enhancement of the [2 0 0] and [1 1 2] peak intensity. Moreover, at ~750 °C the silica phase starts to crystallize, as indicated by the diffraction peak that appears at ~21.5°, in agreement with the TGA/DTA results (Figure S7). When the sample reaches the final temperature (900 °C, Fig. 7(d)) the particles already display the high aspect ratio observed in the final solids, while the silica structure losses its porous structure due to pore 22


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occlusion and crystallization. Figures 7(e-f) clearly displays the presence of the silica layer over the final nanorods, which at this point, are highly crystalline.

900

(a)

(b)

(c)

[2 0 0]

[1 1 2]

800

Temperature (°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


700

SiO2

600 500 400 300 200 100 20 22 24 26 28 30 32 34 36 38 40 24

2θ (deg.)

25

26

2θ (deg.)

27 34

35

36

2θ (deg.)

Figure 8 .Dependence of the X-ray diffraction profiles of YVO4-YPO4 samples in SiO2 as a function of the annealing temperature: evolution of the diffraction peaks from 30 to 900 °C (a) in the 2θ=20-40° range and amplified plots in the ranges of the (b) [2 0 0] and (c) [1 1 2] reflections of the tetragonal structure.

Luminescent properties. Having obtained highly crystalline nanostructures after the protected annealing, different spectroscopic characteristics comprising upconversion luminescence and VUV excitation were explored. The (Y0.95Er0.05)VO4 and (Y0.95Er0.05)VO4YbPO4 materials were studied, which present efficient emissions under UV or IR irradiation. The YVO4:Er3+ solid presents an intense green luminescence at 300 nm excitation due to its high excitability in the 250-350 nm range, which is associated to the VO43-→Er3+ energy transfer band (Figure 9(a)).

23



700

(a)

(λem=550 nm)

600 500 400

Relative intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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300 200 100 0 250 1600

300

350

400

Wavelength (nm)

(b)

450

(λem=300 nm)

1400 1200 1000 800 600 400 200 0

400

450

500

550

600

650

700

750

Wavelength (nm) Wavelength (nm)

Figure 9. (a) Excitation (λem=520 nm) and (b) emission (λexc=300 nm) spectra of the prepared YVO4:Er3+ (5% Er3+) particles after the protected annealing in SiO2 and the elimination of the silica matrix.

The emissions associated to the Er3+ 2H11/2→4I15/2 and 4S3/2→4I15/2 transitions are predominant over the transitions arising from the Er3+ lower energy states, thus yielding practically monochromatic green light. On the other hand, the YVO4:Er3+-YbPO4 solid, which is expected to present similar characteristics under UV irradiation, does not display observable luminescence at the same conditions. Such spectral differences between these two cases can be ascribed to an energy transfer process from the core Er3+ ions to the shell Yb3+ ions, thus leading to emissions in the infrared (2F5/2→2F7/2 in Yb3+) at the expense of the Er3+ emissions in the visible. Therefore, the inverse processes, namely the sensitization of the Er3+ in the core ions through the absorption of Yb3+ ions in the shell, is expected to be equally efficient. 24


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Indeed, the upconversion luminescence of the YVO4:Er3+-YbPO4 solid is clearly observable under 980 nm excitation even at low laser power (100 mW), which is due to the sensitizing action of the Yb3+ ions. On the other hand, the upconversion luminescence of YVO4:Er3+ is considerably lower in the same conditions, since it depends on the absorption of two photons by the Er3+ ions only. The upconversion spectra under 980 nm excitation and their dependence upon the laser power are presented in Figure 10. Both spectra reveal the characteristic set of Er3+ emissions, however, the YVO4:Er3+ solid displays a low intensity of red emissions (at ~650 nm, 4F9/2→4I15/2) and high intensities at wavelengths larger than 700 nm. In addition to the higher upconversion intensity, the YVO4:Er3+-YbPO4 sample presents an intensification of the red emissions in comparison to the transitions in the green, which is characteristic of Er3+ emissions in the YbPO4 phosphate host.53,70 This also indicates the presence of solid solutions at the materials interface (i.e. a (Yb,Er)(P,V)O4 composition). The dependence of the emission intensities with regard to the irradiation power is depicted in Figure 10(b), which is in agreement with a two photon excited state absorption (ESA) mechanism for the YVO4:Er3+ sample (slope=1.7 in all cases). In the YVO4:Er3+-YbPO4 structure, the shell→core sensitization and the occurrence of an interfacial solid solution leads to a variation in the slopes between 1.3 and 2.0 for the different Er3+ emissions, which can ascribed to the occurrence of different parallel upconversion/deactivation mechanisms due to the Yb3+→Er3+ energy transfer.53,71

25



(a) 2

4

4

S3/2→ I11/2

3+

YVO4:Er

4

Relative intensity (a.u.)

H11/2→ I11/2 4

4

F9/2→ I11/2

3+

YVO4:Er -YbPO4

500

550

600

650

700

Wavelength (nm) 10

Integrated intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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7

10

6

10

5

3+

YVO4:Er -YbPO4

(b)

(1.3) (1.6) (2.0)

(1.7)

3+

YVO4:Er

2

H11/2

4

S3/2

10

4

4

F9/2

40

60

80

100

120 140 160 180200

Laser Power (mW)

Figure 10. (a) Upconversion spectra of the YVO4:Er3+ and YVO4:Er3+-YbPO4 powders under 980 nm excitation at 200 mW and (b) dependence of the integrated upconversion intensities for the different emissions of the synthesized solids.

In addition to the upconverting materials, different REVO4-REPO4 samples were prepared in order to obtain VUV-excitable phosphors. Although RE vanadates are well known for displaying an intense absorption band in the UV range, their absorptivity in the VUV usually is remarkably low. In contrast, the host lattice absorption band of RE phosphates usually occurs in the VUV range, which can give rise to quite intense absorptions. Nevertheless, contrarily to vanadates, phosphate-based materials commonly do not provide adequate chemical environments to the doping ions with respect to site symmetry and/or ligand polarizability, which can negatively affect the color purity of their emissions.11 Therefore, the 26


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designed REVO4-REPO4 structures can combine the advantages of these two phases in the same material. To test this idea, Eu3+ and Tm3+-doped REVO4-REPO4 structures were prepared and Figures 11-13 present their luminescence spectra under VUV irradiation. The excitation

spectra

of

the

Eu3+

doped

samples

(namely

(Y0.95Eu0.05)VO4,

(Y0.65Gd0.30Eu0.05)VO4, and (Y0.65Gd0.30Eu0.05)VO4-YPO4, Figure 11(a)) indicate that only the mixed system displays a high excitability in the region between 147 and 172 nm, which is the VUV region of highest interest for practical applications (emissions from Xe/Ne plasmas).11,18,41 However, no quantum-cutting processes by Gd3+→Eu3+ energy transfers are seen, since Gd3+ absorptions are not observable in wavelengths lower than 200 nm. This is a direct consequence of the intrinsic vibrational characteristics of REPO4 and REVO4 hosts, in which lattice phonons arising from V-O (ν1, ~900 cm-1) and P-O (ν3, ~1100 cm-1) vibrations dissipate part of the excitation energy within the Gd3+ level structure, thus reducing the probability of generation of two photons in the visible from one excitation photon in the VUV.14,25

27



(λem=612 nm)

(a)

3+

(Y,Gd)VO4:Eu -YPO4

3+

(Y,Gd)VO4:Eu

3+

YVO4:Eu

200

350 3+

5

5

5

7

7

D0→ F4

D0→ F3

7

D0→ F1

300

YVO4:Eu (λexc=250 nm)

7

(b)

250 D0→ F2

150

5

Relative intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3+

(Y,Gd)VO4:Eu (λexc=250 nm)

3+

(Y,Gd)VO4:Eu -YPO4 (λexc=147 nm)

500

550

600

650

700

750

Wavelength (nm) Figure 11. (a) Excitation spectra (λem=612 nm) in the VUV-UV range and (b) emission

spectra under VUV or UV excitation of the YVO4:Eu3+, (Y,Gd)VO4:Eu3+, and (Y,Gd)VO4:Eu3+-YPO4 synthesized through the described methodologies. The spectra display a broadening of the excitation bands to wavelengths lower than 200 nm, as a result of the higher absorptivity of the shell PO43- groups. As a consequence, the pure vanadate samples do not present measurable luminescence under VUV irradiation, while the (Y,Gd)VO4:Eu3+-YPO4 clearly displays the typical 5D0→7FJ Eu3+ emissions when excited at 147 nm (Figure 11(b)). In comparison to the pure vanadate samples, the (Y,Gd)VO4:Eu3+YPO4 solid shows a reduced intensity of the 5D0→7F2 transition in relation to the 5D0→7F1, as a result of the partial incorporation of PO43- ions in the (Y,Gd)VO4 phase at the interface. Consequently, the emission red color purity of this phosphor is slightly reduced (x=0.608, 28


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y=0.337) in comparison to the initial vanadate cores due to the larger contribution of the orange 5D0→7F1 emissions (Figure 12). x

y

GdVO4:Tm3+

0.107

0.098

GdVO4:Tm3+-YPO4

0.149

0.123

YVO4:Eu3+

0.681

0.319

(Y,Gd)VO4:Eu3+

0.675

0.324

(Y,Gd)VO4:Eu3+-YPO4

0.608

0.377

(Y,Gd)VO4:Eu3+-YPO4 (Y,Gd)VO4:Eu3+ YVO4:Eu3+ GdVO4:Tm3+-YPO4

GdVO4:Tm3+

Figure 12. Chromaticity diagram (CIE 1931) and chromaticity coordinates (inset) representing the colors of the emissions of the Eu3+ and Tm3+-doped REVO4 and REVO4REPO4 synthesized structures under UV (pure vanadates) or VUV (mixed solids) excitation.

The same observations are valid for the (Gd0.99Tm0.01)VO4 and (Gd0.99Tm0.01)VO4-YPO4 systems, as depicted in Figure 13. The vanadate sample displays a low absorptivity in the VUV, while the mixed system presents a pronounced absorptivity between 147 and 172 nm due to the presence of the phosphate phase (Figure 13(a)), even though Gd3+→Tm3+ quantum cutting processes are not observable. As a consequence, although the GdVO4:Tm3+ sample presents an intense and practically monochromatic blue emission under UV irradiation, which arises from the 1G4→3H6 Tm3+ emission (x=0.107, y=0.098, Figure 13(b)), this solid do not present any measurable luminescence under VUV excitation. In contrast, the GdVO4:Tm3+YPO4 sample clearly presents the blue Tm3+ emission under 147 nm excitation, which occurs in superposition to an intense VO43- emission band. This band occurs due to the partial 29



dilution of the vanadate groups in the phosphate phase, thus reducing the emission blue color purity of this compound (x=0.149, y=0.120, Figure 12), which is still suitable for the generation of blue light in different systems.11 In addition, the results also show how the prepared structures are able to convert VUV excitation into a wide range of UV-Vis radiation (i.e. from 350 to 550 nm), which is quite promising not only for blue light generation but also for a number of applications.

(λem=474 nm)

(a)

3+

GdVO4:Tm -YPO4

3+

GdVO4:Tm

200

250

300

350

G4→ H6

150

3

(b)

1

Relative intensity

3+

GdVO4:Tm

300

3-

VO4 emission

400

3+

GdVO4:Tm -YPO4

3

D2→ H6

1

3

G4→ H6

(λexc=250 nm)

1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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500

(λexc=147 nm)

600

700

Wavelength (nm)

Figure 13. (a) Excitation spectra (λem=474 nm ) in the VUV-UV range and (b) emission spectra under VUV (GdVO4:Tm3+-YPO4) or UV (GdVO4:Tm3+) excitation of the Tm3+-doped solids obtained after the protected annealing procedure.

30


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For instance, the broad emission spectra of the GdVO4:Tm3+-YPO4 sample under UV or VUV irradiation clearly matches the absorption range of Y3Al5O12:Ce3+ (YAG:Ce3+) solids.11,65 The emissions of GdVO4:Tm3+-YPO4 solids can thus be used to excite YAG:Ce3+ particles in order to generate nearly white light, in an arrangement similar to those applied in LED systems. In order to test this possibility, a simple experiment was performed (Figure 13 and Supporting Information). For this, ethanolic suspensions of the GdVO4:Tm3+-YPO4 sample and of YAG:Ce3+ particles, prepared through the polymeric precursors method,72 were deposited on the opposite sides of a common glass substrate, which acts as a filter absorbing wavelengths lower than 320 nm (inset of Figure 14(b) and Figure S18). When UV radiation reaches the side containing the GdVO4:Tm3+-YPO4 sample, the light collected on the side of the YAG:Ce3+ particles correspond to a mixture of the emission of these two solids. Since the glass substrate avoids the direct excitation of the YAG:Ce3+ sample by the UV radiation, these results indicate that the blue light generated by the mixed vanadate-phosphate phosphor excites the YAG:Ce3+ particles, thus yielding a three band spectrum of a bluish-white emission (CIE coordinates: x=0.22, y=0.37, Figures 14 and S19). Therefore, the emissions of the two phosphors can be successfully combined, so that the appropriated balance between the two compounds can indeed lead to generation of white light under UV or VUV irradiation. This illustrates the promising applicability of the described REVO4-REPO4 phosphors, which can be adapted and combined for the generation of red, green, and blue light for different applications.

31



YAG:Ce3+ excitation (λ λem=525 nm)

(a)

GdVO4:Tm3+-YPO4 emission (λ λexc=147 nm)

Relative intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 43

300

350

(b)

400

450

500

550

(1G4→3H6, Tm3+)

(YAG:Ce3+)

(VO3) 4

400

450

500

550

600

650

700

750

Wavelength (nm)

Figure 14. (a) Superposition of the emission spectrum (λexc=147 nm) of the GdVO4:Tm3+YPO4 particles with the excitation spectrum (λem=525 nm ) of a YAG:Ce3+ solid, and (b) emission spectrum obtained through the combination of GdVO4:Tm3+-YPO4 and YAG:Ce3+ solids under UV excitation (λexc=240 nm ) through the proposed setup (inset of (b)).

CONCLUSIONS The formation of REVO4-REPO4 heteronanostructures is possible through the application of a two-step colloidal precipitation approach. The preparation of these structures can be monitored not only by microscopy images, but also through the application of spectroscopic methodologies comprising the observation of the VO43-→Eu3+ energy transfer and the absorption of vanadium(V) peroxocomplexes in solution, which confirm an effective coating of the precursor nanoparticles. Protected annealing in SiO2 is proved to be efficient for the 32


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production of high crystallinity nanoparticles with low aggregation degree. This step leads to an alteration of the morphology of the solids, thus resulting in nanorods with an epitaxial crystallization of the vanadate and phosphate phases. The final materials can display efficient emissions in the red, green, and blue regions under VUV, UV or NIR excitation, according to their composition. The structures we have described are promising for the tunable generation of light in several different systems, particularly for solid state lighting and biolabeling applications. Moreover, further investigations comprising density functional theory (DFT) studies shall provide highly interesting results regarding the detailed description of VUV excitation mechanisms and energy transfer processes in these mixed systems.

ASSICIATED CONTENT Supporting Information. Additional experimental details, synthesis schemes, microscopy images, X-ray diffractograms, thermal analysis curves, infrared spectra and luminescence spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Solid State Chemistry Group, Laboratoire de Physique de la Matière Condensée; Ecole Polytechnique-Université Paris Saclay. On the leave of absence from Rare Earth Laboratory, Department of Chemistry, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, University of São Paulo. Av. Bandeirantes, 3900, 14015-110, Ribeirão Preto, SP, Brazil. Phone:+55 16 3315 4376.

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Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS The authors thank the Brazilian agencies CAPES, CNPq, FAPESP (Proc. 2013/01669-1 and 2014/22930-2, PCdSF), and CNRS and Ecole Polytechnique (France) for financial support and scholarships. The authors are also grateful to Drs. M. Poggi, E. Larquet and P.-E. Coulon for TEM images, and to Prof. P. Dereń and Dr. A. Watras from INTiBS-PAN (Wrocław, Poland) for the use of the VUV instrumentation.

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72. Cicillini, S. A.; Pires, A. M.; Serra, O. A. Luminescent and Morphological Studies of Tmdoped Lu3Al5O12 and Y3Al5O12 Fine Powders for Scintillator Detector Application. J. Alloys Compd. 2004, 374, 169-172.

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TOC IMAGE Protected Annealing

Colloidal Synthesis

REVO4

REVO4-REPO4

Mesoporous silica host

SiO2 Dissolution

Highly crystalline and well dispersed nanorods

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Synthesis and Luminescent Properties of REVO4âREPO4 (RE = Y, Eu

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