Purification of NaYF4-Based Upconversion Phosphors - Chemistry of

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Purification of NaYF4‑Based Upconversion Phosphors Alexander Stepuk,† Gioele Casola,† Christoph M. Schumacher,† Karl W. Kram ̈ er,‡ and Wendelin J. Stark*,† †

Department of Chemistry and Applied Biosciences, Institute for Chemical and Bioengineering, ETH Zurich 8093 Zurich, Switzerland ‡ Department of Chemistry and Biochemistry, University of Bern, 3012 Bern, Switzerland S Supporting Information *

ABSTRACT: Applications of upconversion phosphors have grown extensively in number during the past decade. Hexagonal sodium yttrium fluoride (β-NaYF4) is known to be one of the best host lattices for upconversion materials. We developed a novel technique for transforming cubic sodium yttrium fluoride (α-NaYF4) phosphors into the hexagonal modification and remove oxygen impurities that hinder the upconversion luminescence. We transformed cubic α-NaYF4 nanoparticles from flame-spray synthesis with a particle size less than 50 nm into more efficient β-NaYF4 phosphors. The application of SnF2 and ZnF2 as oxygen scavengers allowed the formation of the pure hexagonal phase and improved the upconversion luminescence intensity. The developed process utilizes no free HF gas in the production and does not contaminate the upconversion phosphors with scavenger material. The treatment increases the particle size to between approximately 500 nm and 1 μm. Upconversion luminescence spectra revealed the characteristic blue Tm3+ and green Er3+ emissions of β-NaYF4: Yb,Tm and Yb,Er, respectively.

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determined by Thoma et al.24 Various pathways for the preparation of β-NaYF4 have been reported: solid state,25 hydrothermal syntheses,26 thermolysis,27 and precipitation.28 Rare earth doped upconversion nanoparticles could also be synthesized by flame pyrolysis.29 At present, the upconversion efficiency is size dependent.13,30 The highest quantum yields were reported for microparticles prepared by solid-state synthesis.31 Depending on the thermal treatment of NaYF4 powders in solid-state synthesis, both cubic and hexagonal modifications could be obtained.25,29 Flame pyrolysis seems to be a suitable technique for preparing NaYF4 upconversion phosphors due to the scalability, efficient control of the particle size, and morphology. It was previously reported that flame spray synthesis leads to the formation of pure α-NaYF4.29 To move the equilibrium toward β-NaYF4, an excess of NaF should be added to the mixture.25 The established solid-state synthesis requires sintering in an HF/Ar atmosphere to remove oxygen impurities, which disturb the upconversion efficiency due to multiphonon relaxation.25 Such processes are obviously limited due to complicated preparation procedures and require HF gas at elevated temperatures. There are several other HF-free methods for the synthesis of rare earth fluorides. Oxygen can be removed by fluorination with NH4F·HF or NH4F, which is not sufficient for the complete removal of oxygen impurities,32 by sublimation,

INTRODUCTION The development of new strategies in optical electronics,1 biomedical markers,2 and solar cells3 has dramatically expanded the field of upconversion materials.4 Upconversion phosphors (UCP) have seen use in many fields of research, dating back to the 1970s, particularly for solid-state lasers,5 and later for LED display technologies.6 In the field of renewable energy technologies, UCPs are promising candidates for enhancing the efficiency of photovoltaic cells.7 UCPs are also widely used in bioimaging, as the autoluminescence of biological tissues does not disturb the upconversion luminescence.8−10 More recently, UCPs have been introduced as photosensitizing agents in photodynamic therapy for cancer treatment.11,12 UCPs are also being considered for optical encoding13 and printing technologies.14 Various host lattices doped with rare earth ions show efficient upconversion emission. Choice of material and particle size is dependent on the application. For instance, nanoparticles of NaYF4: Yb3+, Tm3+ and Yb3+, Er3+, or NaLuF4: Yb3+, Tm3+, and GdF3: Yb3+, Er3+ are used in biosensing.10,15 TiO2: Yb3+, Er3+ finds applications in dye-sensitized solar cells.16 Nd3+-sensitized upconversion nanophosphors have been broadly investigated and show high upconversion efficiency upon laser excitation at 800 nm.17,18 So far, β-NaYF4: Er3+ shows the highest efficiency for solar cells and photovoltaic applications.19−21 For these applications, the size varies from nano- to micrometers. In particular, the hexagonal modification of sodium yttrium fluoride (β-NaYF4) is known to be one of the best host lattices for efficient upconversion of infrared (IR) to visible (vis) light.22,23 The corresponding NaF−YF3 phase diagram was © 2014 American Chemical Society

Received: October 21, 2013 Revised: February 19, 2014 Published: February 20, 2014 2015

dx.doi.org/10.1021/cm403459v | Chem. Mater. 2014, 26, 2015−2020

Chemistry of Materials

Article

Figure 1. Photo (a) and scheme (b) of the setup. Upconversion phosphor nanoparticles and the scavenger material are both inserted into an Inconel 600 tube and attached to the gas handling system. The removal of oxygen occurs under constant Ar flow without direct contact between the materials.

though restricted due to low volatility of rare earth fluorides, or by melt purification.33 Various fluorides, such as PbF2, ZnF2, SnF2, and CdF2, are known to show high fluorination activity;33,34 however, in the melt, these scavengers contaminate the rare earth fluorides with heavy metal cations.35,36 The toxicity of Pb and Cd also raises additional concerns. In the present study, an HF-free synthesis is used to prepare oxygen-free β-NaYF4 particles. SnF2 and ZnF2 were studied as oxygen scavengers to purify NaYF4:Yb3+, Tm3+ and Yb3+, Er3+ codoped materials from flame spray pyrolysis. We demonstrate oxygen removal from the UCP through a gas exchange mechanism without a contamination by the scavenger material. The prepared upconversion phosphors were characterized by their upconversion luminescence, particle size, and morphology

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Table 1. Compositions of Upconversion Phosphors and Scavengers Sintered at 590 °C composition NaY0.747Yb0.25Tm0.003F4

NaY0.8Yb0.18Er0.02F4

scavenger type

UCP/ scavenger mol ratio

phase composition after purification

HF/Ar − ZnF2 SnF2 SnF2 SnF2 ZnF2

− − 1:1 1:1 1:2 1:10 1:1

β-NaYF420 α-/β-NaYF4 β-NaYF4 β-NaYF4 β-NaYF4 β-NaYF4 β-NaYF4

The composition of the upconversion phosphors was investigated by powder X-ray diffraction (PANalytical X’Pert PRO-MPD, Cu Kα radiation, X’Celerator linear detector system, 10° ≤ 2θ ≤ 70°, step size of 0.033° 2θ, ambient conditions). The chemical composition of the samples was analyzed with an energy dispersive X-ray spectrometer (EDX) (EDAX TEAM) (30 kV, measurement time of 30 s) and by inductively coupled plasma optical emission spectroscopy (ICP OES, Horiba Ultra 2). The relative upconversion luminescence was measured using a 980 nm IR diode laser coupled to an 1 mm diameter glass fiber. Briefly, the samples of the UCP were placed in glass tubes of 1 mm inner and 1.5 mm outer diameter. The nonfocused laser beam excited the sample in a spot of about 1 mm2 through a Y-fiber. The fiber end was at a distance of 2 mm from the glass surface. The fiber also collects the upconversion luminescence, which is analyzed by an Ocean Optics SD1000 spectrometer with a resolution of 0.45 nm. The spectrometer is equipped with an IR glass filter to block the excitation wavelength. The upconversion luminescence spectra are corrected for the spectral response of the detection system. The laser emission is controlled by a stabilized power supply. The sample morphologies were analyzed by a scanning electron microscope (SEM, Nova NanoSEM 450, FEI).

EXPERIMENTAL PROCEDURES

Nanoparticles of α-NaYF4: Yb,Tm and Yb,Er were prepared by flamespray pyrolysis.24 Briefly, the rare earth acetates, Y(CH3CO2)3·H2O, Yb(CH3CO2)3·H2O, and Tm(CH3CO2)3·4H2O (all ABCR Chemicals, 99.9%) or Er(CH3CO2)3·H2O (Sigma Aldrich, 99.9%), were converted to ethylhexanoates by distillation with 2-ethylhexanoic acid (EHA) (Sigma Aldrich). The precursor solution for the pyrolysis consists of a stoichiometric mixture of rare earth ethylhexanoates, sodium 2-ethylhexanoate (ABCR Chemicals, 97%), and fluorobenzene (ABCR Chemicals, 97%). It was further diluted with xylene (Fluka, 99.9%) or tetrahydrofuran (Sigma Aldrich, 99.9%). The solution was combusted in a flame with a flow rate of 5 mL/minute and an oxygen/ methane (PanGas, technical grade) flow rate of 5 L/min. The nanoparticles were collected from the exhaust gas on a glass fiber filter. The nanoparticles are further processed and purified in a tube furnace; see Figure 1. The furnace contains an Inconel 600 tube consisting of 72 wt % Ni, 14−17 wt % Cr, and 6−10 wt % Fe (Bibus Metals AG), where two boats are placed. One boat contains the UCP nanoparticles and the other the scavenger material SnF2 (Sigma Aldrich, 98%) or ZnF2 (Fluka, 97%); see Table 1. There is no direct contact between the scavenger material and the upconversion phosphor. The reactor is evacuated and preheated to 120 °C for 1 h with a heating rate of 10 °C/min. Afterward, it is filled with argon (PanGas, 5.0) and finally heated to 590 °C for 24 h. Optionally, a second sintering is applied for a further 24 or 48 h. During the purification, the reactor was fed with a constant argon flow. To ensure that no toxic gas traces were released from the apparatus, two gas washing bottles were placed in the outlet gas stream, the second bottle filled with 1 M CaCl2 solution.

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RESULTS AND DISCUSSION Two upconversion sample compositions were prepared by flame-spray pyrolysis: Na 0. 8 9 Y 0 . 77 Yb 0 . 26 Tm 0 . 03 F 4 and NaY0.747Yb0.25Tm0.003F4 + NaF; see Table 1. The amount of NaF was 25 mol % for NaY0.747Yb0.25Tm0.003F4 + NaF and 35 mol % for Na0.89Y0.77Yb0.26Tm0.03F4; see Figure S1, Supporting Information. The diffraction patterns of the α-NaYF4 phase show broad diffraction lines due to the nanoscale particle size, 2016



Article

whereas the NaF has smaller line widths due to larger crystallites; see Figure 2. So far, the XRD spectra have contained no yttrium oxide or oxyfluoride.

Figure 2. Phase composition of the upconversion phosphors after scavenging with SnF2. Flame spray-prepared nanoparticles revealed αNaYF4. Sintering without scavenger leads to formation of cubic and hexagonal modifications of NaYF4.

The thermal treatment of the UCP nanoparticles in the presence of a scavenger promotes the phase transition from the high temperature α-phase to the low temperature β-phase. The β-phase is thermodynamically stable below 665 °C, but the pure α-phase is metastable and does not transform to β-NaYF4 in the absence of a catalyst. The NaY0.747Yb0.25Tm0.003F4 + NaF sample transformed to β-NaYF4, but for Na0.89Y0.77Yb0.26Tm0.03F4, a mixture of cubic and hexagonal phases was obtained. NaF was present in both products. The UC luminescence spectra show the characteristic blue Tm3+ or green Er3+ emissions, respectively; see Figure 3. The luminescence intensity of upconversion nanophosphors is low, due to the fact that smaller particles have a higher surface-tovolume ratio. The lattice defects and killer traps are localized preferentially on the surface and therefore quench most of the nanoparticle excitations. Both compositions showed UC luminescence; however, the Na0.89Y0.77Yb0.26Tm0.03F4 sample had an admixture of the α-phase, which has lower upconversion efficiency. Therefore, we further investigated only the NaY0.747Yb0.25Tm0.003F4 + NaF sample. SnF2 has a melting point of 210−215 °C.37 At 300−400 °C, there is a partial pressure of SnF2 with later decomposition to Sn and F2.38 So far, no studies of scavenger processes via the gas phase have been reported. The evaporation of SnF2 was confirmed by gravimetrical analysis, resulting in 90% weight loss. It recrystallized in the cold part of the Inconel tube. The XRD patterns of the remaining content of the crucible originally containing the scavenger compound showed SnO2 after the sintering of the UCP with the SnF2 scavenger. Moreover, due to the high volatility of the scavenger, a varying amount of the SnF2 in the process had no influence on the end product; see Table 1. Thus, only the oxygen species contained in the initial powder of UCP nanoparticles are involved in the

Figure 3. Upconversion luminescence of Er3+(a) and Tm3+(b) doped phosphors after scavenger treatment. The intensity of the corresponding emissions strongly increases after sintering with a scavenger. HFtreated samples derived from K. Krämer’s group (University of Bern, Switzerland) were used as control.

scavenging process. Also, the crystal morphology of the UCP did not change for different amounts of SnF2. Samples treated with SnF 2 showed a stronger UC luminescence than those sintered without scavenger. Absence of scavenger atmosphere during sintering leads to the formation of both hexagonal and cubic phases of NaYF4. This correlates well with the fact that the cubic modification of sodium yttrium fluoride has a lower upconversion efficiency.25,39 Similarly to SnF2, ZnF2 can also be used for in situ fluorination during crystal growth.34 Due to the higher melting point (Tm = 872 °C), ZnF2 has a concentration lower than that of SnF2 in the gas phase during sintering. The gravimetrical analysis showed a mass loss of 8 wt % after sintering for 24 h. According to the XRD analysis of the final product (see Figure S2, Supporting Information), this amount corresponds to the zinc oxide located in the crucible. The XRD pattern of the upconversion particles after sintering shows the hexagonal βNaYF4 phase and NaF. Thus, processing with ZnF2 and SnF2 permits the cubic to hexagonal phase transition. The sintered upconversion particles from the flame synthesis initially contained both NaF and α-NaYF4 phases; thus, it is not 2017



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Figure 4. SEM images of flame spray-derived upconversion nanoparticles (a) and after sintering with SnF2 (b). Further sintering for 48 h leads to an increase in particle size (c). Sintered particles without oxygen purification resulted in a mixture of cubic and hexagonal phases of NaYF4 (d).

ZnF2 + H 2O → ZnO + 2HF

possible to estimate the oxygen amount in the UCP. Therefore, we suggested the following mechanism for oxygen purification utilizing a scavenger: −

2−

2OH → O

+ H 2O

SnF2 + H 2O → SnO + 2HF

HF + OH− → F− + H 2O

(I)

(III)

HF + OH− → F− + H 2O

(IV)

Oxygen-containing impurities form radicals and water in the gaseous phase upon heating of the upconversion phosphors. The water that is formed reacts with gaseous tin fluoride (II) scavenger, producing tin oxide (II). The latter further oxidizes to tin oxide (IV), which was confirmed by X-ray data of the scavenger after purification. Hydrofluoric acid as a byproduct again reacts with hydroxyl radicals in the gaseous phase. Such reactions run repeatedly until there has been complete removal of oxygen species from the upconversion phosphors. Decrease of luminescence after sintering for 48 h proves the hypothesis, as abandonment of the reaction can lead to reincorporation of impurities. The scavenging mechanism with ZnF2 could be described similarly: 2OH− → O2 − + H 2O

(III)

Despite Y2O3 not being detected in the XRD pattern of the nanoparticles from the flame synthesis, it evolves in the mixture after sintering in air above 800 °C. Finally, pure yttrium oxide is formed after prolonged heating; see Figure S3, Supporting Information. The gravimetrical analysis of the scavenger after the purification process showed no significant weight loss; the boat contained 10% of ZnO, according to Rietveld analysis of the scavenger. While the reduction of oxygen in the NaYF4 is driven not only by thermodynamic, but also by kinetic mechanisms, a precise estimation of the amount of oxygen impurities in the upconversion nanoparticles was not possible. The surplus of NaF present in the UCP could be separated easily by washing the powders with water. The weight loss of the product after washing was 30%. The corresponding XRD pattern of UCP after washing showed only β-NaYF4; see Figure 2. The removal of NaF from the upconversion phosphor improves the upconversion efficiency, because NaF is an inactive diluent. The samples were further sintered for 24 or 48 h after purification with the scavenger. However, such prolonged heat treatment did not improve the upconversion luminescence. The samples increased in size up to 2−5 μm (see

(II)

SnO + 1 2 O2 → SnO2

(II)

(I) 2018



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Figure 4) similar to the phosphors produced by K. Krämer et al.25 The morphologies of the UCP after the second sintering showed larger agglomerates of hexagonal crystals with embedded nanoparticles; see Figure 4. We found that sintering of the washed upconversion phosphors does not improve the luminescence, and further sintering even decreases the upconversion emission intensity. To validate that the latter one does not originate from impurities from the scavenger material, we performed EDX and ICP-OES analyses. The EDX spectra and ICP-OES results revealed no significant amounts of Sn or Zn in the purified samples; see Figures S4 and S5 and Table S1, Supporting Information. The scavenging treatment was performed both for NaYF4: Yb, Tm, and NaYF4: Yb, Er. In the case of erbium-doped green upconversion phosphors, the use of SnF2 as a scavenger similarly increased the luminescence and produced the hexagonal phase. The corresponding luminescence spectrum reveals the characteristic 2H11/2+4S3/2 → 4I15/2 emissions in the green spectral range around 550 nm and a weak 4F9/2 → 4I15/2 emission in the red spectrum; see Figure 3a. The Yb, Tm codoped UCP purified with ZnF2 and sintered for 48 h shows an intense luminescence; see Figure 3b. The SEM images showed agglomerates of hexagonal NaYF4 with particle sizes up to 5 μm. Sintering the particles without purification by a scavenger results in a mixture of the cubic and hexagonal NaYF4 phases as confirmed by XRD patterns (see Figure 2) and morphology micrographs (see Figure 4).

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Article

ASSOCIATED CONTENT

S Supporting Information *

EDX spectra, X-ray diffraction patterns of samples prepared with different compositions and treated with ZnF2, sintered at 800 °C, and ICP-OES analysis data (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +41 44 632 0980. Fax: +41 44 633 1571. Author Contributions

The manuscript was written through contributions from all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Urs Krebs, ETH Zurich, for assistance in the setup design and fabrication. REFERENCES

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CONCLUSION

In summary, we developed a novel technique for the purification of NaYF4-based upconversion phosphors. Utilizing SnF2 and ZnF2 without a direct contact with the upconversion particles, we performed a scavenging process through the gas phase. Such oxygen scavengers could be a valid substitution for HF gas treatment of the UCP. The scavenging process results in the formation of pure hexagonal sodium yttrium fluoride possessing strong upconversion luminescence. The technique has a potential for large-scale applications, because the starting materials produced from industrial flame spray pyrolysis could be produced on a tonnage scale and the scavenger material could be reused. The flame-spray pyrolysis produces cubic αNaYF4 nanoparticles below 50 nm in size. The use of ZnF2 or SnF2 and sintering at 590 °C results in the successful phase transition to hexagonal β-NaYF4 with submicrometer size particles. Both NaYF4: Yb3+, Er3+ and NaYF4: Yb3+, Tm3+ particles show green and blue UC luminescence, respectively, upon near-infrared laser excitation at 980 nm. The exact amount of oxygen impurities still needs to be estimated, as thermodynamic and also kinetic parameters are involved in the scavenging process. The phase composition and luminescence properties are independent of the amount of scavenger in the system as long as all oxygen is removed from the UCP. The upconversion luminescence intensity of purified upconversion phosphors drastically improved in comparison to UCP sintered without scavenger. Further sintering of the particles after purification leads to particle growth and did not improve luminescence properties. This advantageous purification technique might be utilized for fluoride-based materials in other fields of application. 2019



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NOTE ADDED AFTER ASAP PUBLICATION Page numbers were missing in the references in the version published ASAP 03/03/14; the corrected version was published ASAP 03/11/14.

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Purification of NaYF4-Based Upconversion Phosphors - Chemistry of

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