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(La0.97RE0.01Yb0.02)2O2S Nanophosphors Converted from Layered Hydroxyl Sulfate and Investigation of Upconversion Photoluminescence (RE=Ho, Er)
Nanoscale Research Lettersvolume 12, Article number: 508 (2017)
Phase-pure (La0.97RE0.01Yb0.02)2O2S upconversion (UC) nanophosphors (average crystallite size ~ 45 nm; RE=Ho, Er) were annealed from their hydrothermally crystallized layered hydroxyl sulfate precursors in flowing hydrogen at 1200 °C for 1 h, with water vapor as the only exhaust. Under 978-nm laser excitation (up to 2.0 W), the Ho3+-doped phosphor exhibited green (medium), red (weak), and near-infrared (strong) emissions at ~ 546 (5F4 → 5I8), 658 (5F7 → 5I8), and 763 nm (5F4 → 5I7), respectively, and has the stable chromaticity coordinates of about (0.30, 0.66) in the visible-light region (400–700 nm). The Er3+-doped UC phosphor, on the other hand, showed weak green (~ 527/549 nm, 2H11/2,4S3/2 → 4I15/2), weak red (~668/672 nm, 4F9/2 → 4I15/2), and strong near-infrared (~ 807/58 nm, 4I9/2 → 4I15/2) luminescence, whose emission color in the visible region drifted from yellowish-green [(0.36, 0.61)] to green [(0.32, 0.64)] with increasing excitation power. Analysis of the power-dependent UC luminescence found three- and two-photon processes for RE=Ho and Er, respectively, and the possible UC mechanisms were proposed.
Upconversion (UC) phosphor is drawing considerable attention due to its unique ability to convert longer wavelength radiation into shorter wavelength fluorescence [1, 2] and is finding wide applications in the fields of solid-state lasers , multi-color displays , drug delivery , fluorescent biological labels , wavelength converters for solar cells , and so forth. A UC phosphor is commonly formed by doping a host lattice with a sensitizer/activator pair, where the sensitizer is usually Yb3+ and the activator is frequently being Ho3+, Er3+, or Tm3+. This is because Yb3+ can efficiently absorb 980-nm near-infrared laser excitation and the three types of activators have the ladder-like energy levels that are beneficial to sequential photon absorption and energy transfer . The fundamentals of UC luminescence and the energy transfer in lanthanide upconversion can be found in the review articles by Auzel  and Dong et al. , respectively. Gai et al.  recently compiled the recent progress achieved in rare-earth micro/nanocrystals for downconversion (DC) and UC purposes, including soft chemical synthesis, luminescent properties, and biomedical applications. Wang et al. , on the other hand, extensively summarized in their review article the application of rare-earth ion-doped UC and DC phosphors in optical thermometry. The property of a UC phosphor is significantly affected by the type of host lattice, sensitizer/activator combination, dopant concentration, particle/crystallite morphology, crystallinity, excitation power, and the actual lattice site where the dopant ion resides [8,9,10,11,12,13]. For example, two non-equivalent Gd-activated crystallographic sites were identified in an Er3+-doped hexagonal Na1.5Gd1.5F6 phosphor, and through time-resolved spectroscopy, it was proved that the two green emissions from the 4S3/2 level of Er3+ separately originate from the Gd1 (540 nm) and Na2/Gd2 (550–555 nm) crystallographic sites while the 657-nm red emission from the 4F9/2 level only originates from the Na2/Gd2 site . A recent study of novel Er3+-doped transparent Sr0.69La0.31F2.31 glass ceramics, on the other hand, illustrated that the spectrum split, thermal quenching ratio, population stability, and temperature sensitivity from the three thermally coupled energy levels (TCL) of 2H11/2/4S3/2, 4F9/2(1)/4F9/2(2), and 4I9/2(1)/4I9/2(2) are dependent on the pump power of 980 nm laser, and a new fitting method was developed to establish the relation between fluorescence intensity ratios and temperature . Rare-earth (RE) halides (such as NaYF4:Yb/Er) are currently the most efficient UC phosphors owing to their low phonon energies (ℏω < 400 cm−1) [8, 10, 11, 14], though the toxic raw materials involved in the synthesis and the air sensitivity of many halides restrain their application and production. Another type of widely investigated UC phosphors is RE2O3 (such as Y2O3:Yb/Er), whose relatively high phonon energy (ℏω ~ 600 cm−1; ~ 591 cm−1 for Y2O3 and 612 cm−1 for Lu2O3) , however, lowers the efficiency of UC luminescence due to photon-phonon coupling. From the view point of biocompatibility, Li et al.  synthesized Yb3+- and Ho3+-codoped fluorapatite crystals (nanorods of 16 by 286 nm) via hydrothermal reaction, and UC luminescence of Ho3+ at 543 and 654 nm was attained through a two-photon process under 980-nm laser excitation. The crystals also exhibited clear fluorescent cell imaging after the surfaces were grafted with hydrophilic dextran .
RE2O2S oxysulfide is an important family of compounds in the phosphor field and can be advantageous over oxide for luminescent applications. For example, the occurrence of S2− → Eu3+ charge transfer transition in Eu3+-activated RE2O2S significantly extends the effective excitation wavelength to ~ 400 nm [17,18,19], which makes the phosphor useful as the red component in near-UV (365–410 nm)-excited white LEDs as reviewed by Ye et al. . The most matured technique to synthesize RE2O2S is solid-state reaction, which has the advantages of high yield and convenience, but high reaction temperature, uncontrollable product morphology, and especially the employment of environmentally harmful sulfur sources are apparent shortcomings [21,22,23]. Sulfurization of RE2O3 by H2S or CS2 gas at an elevated temperature [24,25,26] is another frequently used strategy to produce RE2O2S. Since the methodology for controlled synthesis of RE2O3 is rich and well developed, RE2O2S with various particle morphologies has thus been produced through the sulfurization route, though the complicated procedures are less feasible for industrial production. Other techniques for RE2O2S synthesis may include precipitation , hydrothermal reaction , two-step solution gel polymer thermolysis , gelatin-templated synthesis , gel thermolysis , solvothermal pressure-relief synthesis , and combustion . The involvement of harmful sulfur sources or by-products (such as C2S, H2S, and thiourea) is, however, still hard to avoid. The appearance of sulfate-type layered rare-earth hydroxide (RE2(OH)4SO4∙2H2O, SO4 2−-LREH) in 2010  provided a unique chance to solve the aforementioned issues, since this group of compounds has exactly the same RE/S molar ratio of RE2O2S. Homogenous hydrolysis of RE2(SO4)3·8H2O in the presence of Na2SO4 and hexamethylenetetramine (C6H12N4) is the classic technique to produce SO4 2−-LREH but is limited to RE=Pr–Tb in the lanthanide family . We extended the group of compounds to RE=La–Dy via reacting aqueous solutions of RE(NO3)3·nH2O and (NH4)2SO4 under hydrothermal conditions [17,18,19] and subsequently manifested that RE2O2S can be facilely produced through thermolysis of SO4 2−-LREH in a reducing atmosphere [17,18,19]. RE2O2S was recently identified to have relatively low phonon energy (ℏω ~ 500 cm−1) , good chemical stability, and particularly high UC efficiency comparable to halides [35, 36], but the study on this type of promising UC phosphors is yet far from sufficiency [8, 10, 11, 37, 38]. La3+ does not have unoccupied 4f sub-orbital and is optically inert, and thus, its compounds are suitable host lattices for luminescence. We thus synthesized in this work La2O2S:Yb/RE UC phosphors (RE=Ho, Er) via annealing the hydrothermally crystallized SO4 2−-LREH in flowing H2, and the luminescent properties and UC processes were elaborated in detail.
The starting materials of RE(NO3)3·6H2O (RE=La, Ho, Er, and Yb; > 99.99% pure), (NH4)2SO4 (> 99.5% pure), and NH3·H2O solution (28%, ultrahigh purity) were purchased from Kanto Chemical Co., Inc. (Tokyo, Japan), and were used as received. Yb3+/Ho3+- and Yb3+/Er3+-doped La2(OH)4SO4·2H2O was separately synthesized via hydrothermal reaction. The dopant contents are 2 at.% for Yb3+ and 1 at.% for both Ho3+ and Er3+ according to the literature . In a typical synthesis , 6 mmol of (NH4)2SO4 was dissolved in 60 ml of an aqueous solution of the rare earths (0.1 mol/L for total RE3+), followed by dropwise addition of NH3·H2O until pH = 9. After continuous stirring for 15 min, the resultant suspension was transferred into a Teflon-lined autoclave of 100-ml capacity for 24 h of hydrothermal crystallization in an electric oven preheated to 100 °C. The resultant product was collected via centrifugation, washed with filtered water three times and ethanol once, and finally dried in air at 70 °C for 24 h. The La2O2S:Yb/RE UC phosphors were then annealed from their SO4 2−-LREH precursors in flowing H2 (200 mL/min) at 1200 °C for 1 h, with a heating rate of 5 °C/min in the ramp stage.
Phase identification was performed via X-ray diffractometry (XRD; Model RINT2200, Rigaku, Tokyo, Japan) under 40 kV/40 mA, using nickel-filtered Cu-Kα radiation (λ = 0.15406 nm) and a scanning speed of 1°/min. Structure parameters of the products were derived from the XRD data using the TOPAS software . Particle morphology was observed by field emission scanning electron microscopy (FE-SEM; Model S-5000, Hitachi, Tokyo) under an acceleration voltage of 10 kV. UC luminescence spectra were obtained at room temperature using an FP-6500 fluorospectrophotometer (JASCO, Tokyo) under 978-nm near-infrared laser excitation of the phosphors with a continuous wavelength (CW) laser diode (Model KS3–12322-105, BWT Beijing Ltd., Beijing, China). The signal/noise ratio (S/N) of the spectrometer is of ≥ 200, and the sensitivity was set low due to the strong UC luminescence of the phosphors. The experimental setup can be found in Additional file 1: Figure S1.
Results and Discussion
Figure 1 shows XRD patterns of the hydrothermal products, where it is seen that in each case, all the diffraction peaks can be well indexed with the layered compound of La2(OH)4SO4·2H2O [17, 18]. In an aqueous solution containing SO4 2−, the rare-earth (RE) cations would undergo hydration and partial hydrolysis to form the complex ions of [RE(OH) x (H2O) y (SO4) z ]3-x-2z [17,18,19]. Either a higher temperature or solution pH will promote RE3+ hydrolysis, leading to more OH− while less SO4 2− (smaller SO4 2−/OH− molar ratio) in the complex ion. Under the optimized hydrothermal conditions of 100 °C and pH = 9 [17,18,19], the complex ion may have a proper SO4 2−/OH− molar ratio, and thus, the aimed SO4 2−-LREH compound can be crystallized via condensation reactions. The structure parameters of the hydrothermal products are summarized in Table 1. It is clear that (La0.97Ho0.01Yb0.02)2(OH)4SO4·2H2O has larger lattice constants (a, b, c) and cell volume (V) than (La0.97Er0.01Yb0.02)2(OH)4SO4·2H2O. This is understandable in view that Ho3+ (1.072 Å for CN = 9) is larger than Er3+ (1.062 Å for CN = 9). Both the products have smaller cell constants and cell volume than the un-doped La2(OH)4SO4·2H2O (SO4 2−-LLaH), in accordance with the fact that La3+ is the largest (1.216 Å for CN = 9) among the four types of RE ions. The differing cell parameters provided direct evidence of solid-solution formation.
Figure 2 shows XRD patterns of the products annealed from their SO4 2−-LREH precursors at 1200 °C for 1 h in flowing H2. The diffraction peaks can be fully indexed with the hexagonal structured La2O2S in each case (space group: P-3m1; JCPDS card no. 00-075-1930). SO4 2−-LLaH would decompose to La2O2SO4 up to 1200 °C in air through the reactions of La2(OH)4SO4·2H2O → La2(OH)4SO4 + 2H2O (dehydration) and La2(OH)4SO4 → La2O2SO4 + 2H2O (dehydroxylation) . In a H2 atmosphere, the S6+ in SO4 2− would be reduced to S2− following the reaction of La2O2SO4 + 4H2 → La2O2S + 4H2O, and thus, La2O2S can be resulted with water vapor as the only by-product . The lattice parameters and cell volume of (La0.97RE0.01Yb0.02)2O2S are shown in Table 2 together with those of La2O2S . The decreasing cell dimension towards a smaller RE3+ indicates the successful formation of solid solution.
Figure 3 shows the particle morphology of the layered precursors and the resultant UC phosphors. It is seen that the SO4 2−-LREH crystallized as nanoplates of ~ 150–550 nm in lateral size and ~ 20–30 nm in thickness. The nanoplates underwent significant disintegration upon calcination at 1200 °C to yield rounded particles. The average crystallite size was assayed with the Scherrer equation to be ~ 45 nm for the UC phosphors.
Figure 4a shows UC luminescence spectra of the (La0.97Ho0.01Yb0.02)2O2S phosphor under 978-nm laser excitation. The emissions at ~ 546, 658, and 763 nm are attributed to the 5F4 → 5I8, 5F7 → 5I8, and 5F4 → 5I7 transitions of Ho3+, respectively , with the 763-nm NIR emission being predominant. The sensitizing effect of Yb3+ is significant, and ~ 15 and 20 times stronger green (546 nm) and NIR (763 nm, not sensitive to human eyes) emissions, respectively, were produced by codoping of 2 at.% Yb3+ (Additional file 1: Figure S2a). Under 50-mW laser pumping, vivid and strong green emission was observed for the (La0.97Ho0.01Yb0.02)2O2S phosphor with naked eyes, as shown in the insert in Fig. 4a. In spite of the excitation power, the CIE color coordinates calculated from the emission spectra in the visible light region (400–700 nm) are stable at about (0.30, 0.66), typical of a vivid green color (Additional file 1: Table S1 and Figure S3).
In general, the number of photons required to populate the upper emitting state under unsaturated condition can be obtained from the relation I em∝P n , where I is the luminescence intensity, P the pumping power, and n the number of laser photons. Figure 4b shows the log(I em)-log(P) plot of the above relation, from which the n value was determined from the slope of the linear fitting to be ~3.02, 3.14, and 2.92 (approximately 3) for the UC emissions peaked at ~546, 658, and 763 nm, respectively. The results thus suggest that a three-photon process was involved to generate the observed UC luminescence.
In principle, three basic population mechanisms, namely excited state absorption (ESA), energy transfer (ET), and photon avalanche, may be involved in an UC process [8,9,10]. Since no power threshold was observed in the range of this study, the photon avalanche mechanism can be neglected. The energy diagram of Yb3+/Ho3+ in La2O2S has rarely been reported and is not available to us for comparison. Nonetheless, UC luminescence involving three phonons was seen from the previous work on Yb3+/Ho3+-codoped other material systems [42, 43]. Therefore, the energy diagram and UC process of (La0.97Ho0.01Yb0.02)2O2S were constructed in Fig. 5 by referring to these previous studies and are detailed below: (1) excitation of Yb3+ by laser photons [ESA; 2F7/2(Yb3+) + hν (978 nm) → 2F5/2(Yb3+)]; (2) population of the 5I6 energy level of Ho3+ after Yb3+ absorbing the first laser photon and transferring energy to Ho3+ [ET1; 2F5/2(Yb3+) + 5I8(Ho3+) → 2F7/2(Yb3+) + 5I6(Ho3+)]; (3) non-radiative (NR) relaxation to the 5I7 level of Ho3+ [NR; 5I6(Ho3+) ~ 5I7(Ho3+)]; (4) excitation of Ho3+ from the 5I7 to 5F5 level after Yb3+ absorbing the second laser photon and transferring energy to Ho3+ [ET2; 2F5/2(Yb3+) + 5I7(Ho3+) → 2F7/2(Yb3+) + 5F5(Ho3+)]; (5) NR relaxation to the 5I5 level of Ho3+ [5F5(Ho3+) ~ 5I5(Ho3+)]; (6) excitation of Ho3+ from the 5I5 to 5F4/5S2 level after Yb3+ absorbing the third laser photon and transferring energy to Ho3+ [ET3; 2F5/2(Yb3+) + 5I5(Ho3+) → 2F7/2(Yb3+) + 5F4/5S2(Ho3+)]; and (7) back-jumping of the excited electrons from the populated 5F4/5S2 level to the 5I8 ground state to produce the green emission (~546 nm; 5F4,5S2 → 5I8). The electrons can also relax to the 5F5 and 5I4 levels via NR processes, from which the red (~658 nm; 5F5 → 5I8) and near-infrared (~763 nm; 5I4 → 5I8) emissions were resulted. The strong near-infrared UC emission at ~763 nm may imply that NR relaxation to the 5I8 energy level is significant.
The 2F5/2 → 2F7/2 emission transition of Yb3+ and the 4I15/2 → 4I11/2 excitation transition of Er3+ have well-matching energies, which makes Yb3+/Er3+ the most widely investigated activator/sensitizer pair for UC luminescence in various types of host lattices [8,9,10,11,12,13]. Similar to Ho3+, the UC emission of Er3+ was also dramatically enhanced by Yb3+ codoping (Additional file 1: Figure S2b). Taking the 527-nm green emission for example, 2 at.% of Yb3+ improved the Er3+ luminescence by a factor of ~ 14. Under 978-nm laser excitation, the (La0.97Er0.01Yb0.02)2O2S UC phosphor exhibits emission bands in the green (~ 527 and 549 nm), red (~ 668 and 672 nm), and near-infrared (~ 807 and 858 nm) regions (Fig. 6a), which are arising from the 2H11/2/4S3/2 → 4I15/2, 4F9/2 → 4I15/2, and 4I9/2 → 4I15/2 transitions of Er3+, respectively . The color coordinates determined for the UC luminescence in the visible light region (400–700 nm) drifted from the yellowish-green [(0.36, 0.61)] to green [(0.32, 0.64)] region in the CIE chromaticity diagram along with increasing excitation power from 0.7 to 2.0 W (Additional file 1: Figure S3b and Table S2). The color change also agrees well with the gradually larger intensity ratio of the green to red emissions (I 549/I 668 and I 527/I 668, Additional file 1: Table S3) under a higher excitation power. Excitation-power-dependent emission-color tuning of the Yb3+/Er3+ pair was previously observed in Y2O2S . The number of pumping photons required to populate the emitting states was derived from the slope of the log(I em)-log(P) plot (Fig. 6b), and the three groups of emissions were found to have the similar n values of ~ 2. This indicates that a two-phonon process is largely responsible for the observed UC luminescence.
The energy diagram and photon process leading to the UC luminescence of (La0.97Er0.01Yb0.02)2O2S are schematically shown in Fig. 7. Excited-state absorption and Yb3+ → Er3+ energy transfer excitation are mainly involved in the UC mechanism, with the latter being dominant [8,9,10,11,12,13,14, 39, 43]. Upon excitation with 978-nm laser, the 2F7/2 ground state electrons of Yb3+ are pumped to the 2F5/2 excited state (ESA). Since the 2F5/2 level of Yb3+ and the 4I11/2 level of Er3+ are matching well with each other, energy transfer from Yb3+ to Er3+ readily takes place. The Er3+ electrons can thus be excited from the 4I15/2 ground state to the 4I11/2 level with the energy transferred from Yb3+ (one photon, ET1). The absorption cross section of Er3+ is smaller than that of Yb3+ at ~ 980 nm [42, 45], so energy transfer (ET) dominates the real excitation of Er3+. The excitation energy at the 4I11/2 level may non-radiatively (NR) relax to the 4I13/2 level, from which the electrons can be excited to the 4F7/2 state by ET of a second laser photon (ET2). After NR processes, the three groups of emissions (Fig. 6a) can then be produced via the electronic transitions shown in Fig. 7. The photon reactions of the whole UC process can be presented as follows: (1) 2F7/2(Yb3+) + hν (978 nm) → 2F5/2(Yb3+) and 4I15/2(Er3+) + hν (978 nm) → 4I11/2(Er3+); (2) 2F5/2(Yb3+) + 4I15/2(Er3+) → 2F7/2(Yb3+) + 4I11/2(Er3+); (3) 4I11/2(Er3+) ~ 4I13/2(Er3+); (4) 2F5/2(Yb3+) + 4I13/2(Er3+) → 2F7/2(Yb3+) + 4F7/2(Er3+); (5) 4F7/2(Er3+) ~ 2H11/2/4S3/2(Er3+), 4F9/2(Er3+), and 4I9/2(Er3+); and (6) 2H11/2/4S3/2(Er3+) → 4I15/2(Er3+) + hν (~527 and 549 nm), 4F9/2(Er3+) → 4I15/2(Er3+) + hν (~668 and 672 nm), and 4I9/2(Er3+) → 4I15/2(Er3+) + hν (~807 and 858 nm). The aforementioned emission color change may suggest a more efficient population of the 4F7/2(Er3+) energy level under a higher excitation power, and the 4F7/2(Er3+) ~ 2H11/2/4S3/2(Er3+) NR process becomes successively stronger than 4F7/2(Er3+) ~ 4F9/2(Er3+).
(La0.97RE0.01Yb0.02)2O2S upconversion (UC) nanophosphors (RE=Ho, Er) were successfully produced via thermal decomposition of their layered hydroxyl sulfate precursors in flowing H2 at 1200 °C, with water vapor as the only exhaust. The precursors crystallized as nanoplates with the lateral sizes of ~ 150–550 nm and thicknesses of ~ 20–30 nm, which disintegrated into rounded nanoparticles (average crystallite size: ~ 45 nm) upon thermal decomposition. The oxysulfide phosphors exhibit strong UC luminescence under 978-nm laser excitation, through a three-photon process for Ho3+ and a two-photon process for Er3+. For the UC luminescence in the visible region (400–700 nm), the chromaticity coordinates of (La0.97Ho0.01Yb0.02)2O2S are stable at around (0.30, 0.66), while those of (La0.97Er0.01Yb0.02)2O2S changed from about (0.36, 0.61) to (0.32, 0.64) along with the excitation power increasing from 0.7 to 2 W.
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This work is supported in part by the National Natural Science Foundation of China (Grant Nos. 51672039, 51702020, and U1302272) and the Fundamental Research Fund for the Central Universities (Grant No. N160204008).
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Configuration of laser pumping and UC measurement in JASCO FP-6500 sample chamber. Figure S2. UC luminescence comparison of (La,RE,Yb)2O2S and (La,RE)2O2S under the excitation of 978-nm laser. Table S1. CIE chromaticity coordinates of the (La0.97Ho0.01Yb0.02)2O2S UC phosphor under different excitation power. Figure S3. CIE chromaticity diagram for the UC emissions of (La0.97Ho0.01Yb0.02)2O2S (a) and (La0.97Er0.01Yb0.02)2O2S (b). Table S2. CIE chromaticity coordinates of the (La0.97Er0.01Yb0.02)2O2S UC phosphor under different excitation power. Table S3. Excitation power dependence of the I549/I668 and I527/I668 intensity ratios for the (La0.97Er0.01Yb0.02)2O2S UC phosphor. (DOC 2407 kb)