Reversible Modulated Upconversion Luminescence of MoO3:Yb3+

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Reversible Modulated Upconversion Luminescence of MoO3:Yb3+,Er3+ Thermochromic Phosphor for Switching Devices Mingjun Li, Zhengwen Yang,* Youtao Ren, Jiufeng Ruan, Jianbei Qiu, and Zhiguo Song College of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China

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S Supporting Information *

ABSTRACT: Reversible modulation of upconversion luminescence induced by the external field stimuli exhibits potential applications in various fields, such as photoswitches, optical sensing, and optical memory devices. Herein, a new MoO3:Yb3+,Er3+ thermochromic phosphor was synthesized via a high-temperature solid-state method, and the reversible color modification of the MoO3:Yb3+,Er3+ phosphor was obtained by alternating the sintering conditions either in a reducing atmosphere or in air. The color of the MoO3:Yb3+,Er3+ phosphor changed from light-yellow to blue under sintering in the reducing atmosphere and returned back after sintering again in air. The influence of reversible thermochromism on the upconversion luminescence of MoO3:Yb3+,Er3+ phosphor was investigated. The MoO3:Yb3+,Er3+ phosphor prepared in air exhibited visible upconversion luminescence, while the MoO3:Yb3+,Er3+ phosphor has no upconversion luminescence after sintering in the reducing atmosphere. This up-conversion luminescence modulation shows excellent reproducibility after several cycles. The thermochromic MoO3:Yb3+,Er3+ phosphor with reversible modulated upconversion luminescence shows great potential for practical applications in optical switches and optoelectronic multifunctional devices.

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INTRODUCTION The modulation of upconversion luminescence (UCL) of rare earth ion doped phosphors has attracted considerable attention due to the potential applications of these phosphors in threedimensional displays, solid state lasers, biological imaging, and photovoltaic devices.1−5 At present, the common ways of regulating the UCL are changing the concentration of rare earth ions, controlling the laser power, and regulating the photonic band gap.6−9 However, it is difficult for these approaches to achieve reversible modulation of UCL. Reversible modulation of UCL paves the way for practical applications to data storage, photoswitches, optical sensing, and optical memory devices.10 Therefore, it is of importance to develop new methods or design new phosphors to obtain the reversible modulation of UCL. Recently, some groups have demonstrated that external field stimuli can induce reversible modulation of UCL, including electric fields, light irradiation, magnetic fields, mechanical stress, and thermal stimulation.10−14 Compared to other approaches, a larger degree of reversible modulation can be obtained by thermal stimulation.14 In © XXXX American Chemical Society

principle, thermal stimulation can induce reversible transformation of host material between two kinds of forms with different colors (thermochromism), leading to reversible modulation of UCL. Among thermochromic materials, transition metal oxides are good candidates in thermometers and temperature sensors due to their fast response time, longlasting stability, and large degree of thermochromism.15−22 However, there are few reports of developing new thermochromic phosphors that are capable of reversible modulation of UCL under a thermal stimulus. In this work, a new MoO3:Yb3+,Er3+ phosphor was prepared by a high-temperature solid-phase method. The thermochromism of MoO3:Yb3+,Er3+ phosphor was obtained by alternating the sintering conditions either in a reducing atmosphere or in air. In addition, the relationship of thermochromism of MoO3:Yb3+,Er3+ phosphor to its UCL properties was also investigated. The MoO3:Yb3+,Er3+ phosphors prepared in air exhibited visible UCL, while the MoO3:Yb3+,Er3+ phosphor Received: February 22, 2019

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DOI: 10.1021/acs.inorgchem.9b00526 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. XRD patterns of M, M-200-2-H1, M-300-2-H1, M-400-2-H1, M-450-2-H1, and M-500-2-H1 (a), crystal structure of MoO3 (b), Rietveld structure refinement of M (c), EDS pattern of M (d), SEM image of M (e), and TEM image of M (f). Inset of panel f is the selected area electron diffraction of M. MoO3:Yb3+,Er3+ phosphor was induced by sintering the samples at different temperatures (200, 300, 400, 450, and 500 °C) either in the reducing atmosphere or in air. Materials Characterization. The crystallinity and phase purity of MoO3:Yb3+,Er3+ phosphors were characterized by X-ray powder diffraction (TD-3500, Tongda, China). Transmission electron microscopy (JEM-2100, JEOL, Japan) and scanning electron microscope (Quanta-200, FEI, USA) images were used to characterize the morphology of the MoO3:Yb3+,Er3+ phosphors. Elemental analysis was conducted with an energy dispersive spectrometer (Genesis 2000, EDAX, USA). The UCL spectra of samples were measured using a fluorescence spectrophotometer (F-7000, Hitachi, Japan) upon 980 nm excitation. The absorbance spectra were used to characterize the absorbance property of samples before and after thermochromic reaction (U-4100 spectrophotometer, Hitachi, Japan). For the characterization of chemical states of MoO3, 900 mg samples

samples had no UCL after sintering in the reducing atmosphere. In addition, this new thermochromic phosphor shows excellent reproducibility of reversible modulation after several cycles, indicating its potential application for the switching devices.

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EXPERIMENTAL SECTION

Experimental Procedures. All chemical reagents (analytical grade), including the ammonium molybdate ((NH4)6Mo7O24·4H2O), ytterbium oxide (Yb2O3), and erbium oxide (Er2O3), were used without further purification. The MoO3−1 mol % Yb3+−1 mol % Er3+ phosphor was prepared by a high-temperature solid-state method. Stoichiometric (NH4)6Mo7O24·4H2O, Yb2O3, and Er2O3 were mixed in an agate mortar. The MoO3:Yb3+,Er3+ phosphor was prepared by sintering the mixture at 500 °C for 2 h in air. The thermochromism of B

DOI: 10.1021/acs.inorgchem.9b00526 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Absorbance spectra (a) of M, M-200-2-H1, M-300-2-H1, M-400-2-H1, M-450-2-H1, and M-500-2-H1 (inset shows optical images of samples, from left to right), EPR spectra of M and M-500-2-H1 (b), and XPS spectra of M (c), M-200-2-H1 (d), M-300-2-H1 (e), M-400-2-H1 (f), M-450-2-H1 (g), and M-500-2-H1 (h).

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were weighed. The XPS spectra were characterized by X-ray photoelectron spectroscopy (PHI5000 Versaprobe-II, Ulvac-Phi, Japan) with Al Kα radiation under vacuum conditions and at room temperature. For the measurement of oxygen vacancies, the 80 mg samples were weighed, and their electron-spin resonance spectra (EPR A300-6/1/S-LC, Bruker, France) were measured with a Bruker X-band A300-6/1 paramagnetic resonance spectrometer at a frequency of 9.2 GHz at room temperature.

RESULTS AND DISCUSSIONS Phase, Structure, Chemical Composition, and Morphology. The raw MoO3:Yb3+,Er3+ phosphor was prepared at 500 °C for 2 h and was denoted as M. The phases of the MoO3:Yb3+,Er3+ phosphors were characterized by the XRD, as shown in Figure 1a. It can be seen that all the XRD peaks of the M sample are in good agreement with those of the orthorhombic α-MoO3 phase (JCPDS card No. 35-0609). No C

DOI: 10.1021/acs.inorgchem.9b00526 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. UCL spectra (a) of M, M-200-2-H1, M-300-2-H1, M-400-2-H1, M-450-2-H1, and M-500-2-H1, ΔRn at 529 nm (b) and 553 nm (c) UCL intensity as a function of the second sintering temperature in the reducing atmosphere, and (d) UCL intensity of M sample as a function of excitation light power.

other phases were detected in the final product, demonstrating the orthorhombic phase of the MoO3:Yb3+,Er3+ phosphor. Figure 1b presents the crystallographic structure of MoO3. The Mo6+ cation sites have six coordinated oxygen atoms in the MoO3 host. The main unit of MoO3 is MoO6 octahedrons. The crystal structure of MoO 3 is composed of MoO6 octahedrons sharing their vertices. The Yb3+ (85.8 Å) and Er3+ (88.1 Å) ions in the MoO3 host may enter into the interstitial sites because of the significant difference of valence state and ionic radius between the Yb3+ (85.8 Å) or Er3+(88.1 Å) ions and Mo6+ (59.0 Å) cation. Rietveld structure refinement of MoO3:Yb3+,Er3+ phosphor was carried out to get the structure information on the crystal. The parameters of the unit cell were refined to be a = 3.962(8) Å, b = 13.855(0) Å, c = 3.696(4) Å, and cell volume is 202.969 Å3. The obtained structural parameters were consistent with the standard values exhibited in the JCPDS card 35-0609. As presented in Figure 1c, the refinement results demonstrate that the atom positions are well satisfied with the reflection conditions. Figure 1d shows the EDS of the M sample, exhibiting peaks of Mo, O, Yb, and Er elements. The morphology of the M sample is shown in Figure 1e,f, revealing particle size of 0.5−1 μm. The inset of Figure 1f displays the selected area electron diffraction of the M sample. The crystal planes of (101) and (200) were obviously observed, which further indicated that the MoO3 phosphor is orthorhombic phase. The raw MoO3:Yb3+,Er3+ phosphor (M) was sintered again at 200, 300, 400, 450, and 500 °C for 2 h in the H2 and N2 reducing atmosphere, and these samples were labeled as M200-2-H1, M-300-2-H1, M-400-2-H1, M-450-2-H1, and M500-2-H1, respectively. All these samples are the same

orthorhombic phase, as shown in Figure 1a. The structure refinement of M-500-2-H1 is shown in Figure S1 of Supporting Information and revealed that the second sintering in the H2 and N2 reducing atmosphere has no influence on the phase of MoO3:Yb3+,Er3+. The SEM image (Figure S1) shows that there is almost no change in the shape and size of M-500-2-H1 compared with those of M. The Mechanism of Thermochromism. Figure 2a shows the absorption spectra of M, M-200-2-H1, M-300-2-H1, M400-2-H1, M-450-2-H1, and M-500-2-H1 samples. The absorption band located at about 350 nm is from the absorption energy band of the α-MoO3 host, which is in good agreement with the results reported in the literature.23,24 It is worth noting that we also observe another absorption band located at a wavelength longer than 450 nm, which increased with the increase of reducing sintering temperature. Correspondingly, an obvious color change from light yellow to blue was observed after the M sample was sintered in the reducing atmosphere, and the blue color of samples became much deeper as a result of increased sintering temperature in reducing conditions. This longer wavelength absorption was extensively reported for semiconductor hosts, such as WO3, CuS, CdO, and MoO3.25−28 Several mechanisms including the F-center model and localized surface plasmon resonance (LSPR) model were put forward to explain the longer wavelength absorption of the semiconductor.25,28 The LSPR absorption can be observed in a nanosized semiconductor,26,29 resulting from the collective oscillations of excess free carriers related to the constitutional defects of nanocrystals.27,30−33 In the present work, however, the particle size of MoO3:Yb3+,Er3+ is about 0.5−1 μm, meaning that the occurrence of collective D

DOI: 10.1021/acs.inorgchem.9b00526 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Mechanism of upconversion luminescence modulation of MoO3:Yb3+,Er3+ based on the thermochromic reaction.

H1 samples, respectively. ΔRn at 529 and 553 nm UCL are presented in Figure 3b,c as a function of the second sintering temperature in the reducing atmosphere. The value of ΔRn increased with the increase of second sintering temperature from 200 to 500 °C. The ΔRn values at 529 nm UCL for the M-200-2-H1, M-300-2-H1, M-400-2-H1, M-450-2-H1, and M500-2-H1 samples are 31.05%, 37.81%, 71.64%, 75.31%, and 100%, respectively. The ΔRn values at 553 nm UCL for the M200-2-H1, M-300-2-H1, M-400-2-H1, M-450-2-H1, and M500-2-H1 samples are 42.72%, 65.19%, 74.1%, 76.35%, and 100%, respectively. In our work, a larger ΔRn was obtained in the new MoO3:Yb3+,Er3+ UCL material, indicating its great potential application in data storage, optical switches, and optical memory devices. The logarithmic relationship between the laser power and UCL intensity is shown in Figure 3d. The slope (n value) is the absorbed number of infrared photons required for emitting a visible photon. The n values of 2H11/2 →4I15/2 (529 nm), 4S3/2 → 4I15/2 (553 nm), and 4F9/2→ 4I15/2 (662 nm) transitions are about 2.88, 2.56, and 1.44, respectively. The results demonstrated that the UCL at 529 and 553 nm is a threephoton process, and the 662 nm UCL belongs to a two-photon process upon 980 nm excitation. The UCL mechanism of MoO3:Yb3+,Er3+ is shown in Figure 4. The Er3+ ions located at the 4I15/2 energy level were excited to the 4F7/2 and 4F9/2 levels by the three-step and two-step energy transfer from the Yb3+ and Er3+, respectively. The electrons located at the 4F7/2 level relaxed to the 2H11/2 and 4S3/2 levels. The electrons located at the 2H11/2, 4S3/2, and 4F9/2 levels jumped to the ground state, resulting in the 529, 553, and 662 nm UCL, respectively. The UCL modulation mechanism caused by the thermochromic reaction is shown in Figure 4. As discussed previously, oxygen vacancies were generated in the M-200-2-H1, M-300-2-H1, M400-2-H1, M-450-2-H1, and M-500-2-H1 samples, producing the F-center. When the UCL spectra overlap with the absorbance spectra of the F-center in the M-200-2-H1, M300-2-H1, M-400-2-H1, M-450-2-H1, and M-500-2-H1 samples, the phenomenon of energy transfer from the photoluminescence center to the color centers occurs,10,36 resulting in UCL quenching. The absorbance of longer wavelength increased with increase of the second sintering

oscillations of excess free carriers in the nanocrystals is impossible. In order to investigate the mechanism of the observed longer wavelength absorption, we carried out the XPS analysis of M, M-200-2-H1, M-300-2-H1, M-400-2-H1, M-450-2-H1, and M-500-2-H1 samples as shown in Figure 2c−h. The binding energies located at 232.88 eV (3d3/2) and 236.00 eV (3d5/2) correspond to Mo6+, and the peaks located at 233.8 eV (3d3/2) and 236.93 eV (3d5/2) were ascribed to Mo5+.16,34 The amount of Mo5+ can be reflected by the XPS peak intensity of Mo5+. The XPS peak intensity of Mo5+ in the M-200-2-H1, M-300-2-H1, M-400-2-H1, M-450-2-H1, and M500-2-H1 samples increased with the increase of sintering temperature, which suggested that more Mo5+ was formed when the M sample was sintered at a higher temperature in the reducing atmosphere. The formation of Mo5+ is associated with the production of oxygen vacancies in the MoO3 host. To prove the formation of oxygen vacancies, EPR measurement on M and M-500-2-H1 samples was carried out at room temperature in air, displayed in Figure 2b. It is clearly seen that there is no EPR peak observed for the M sample, whereas a signal peak at g = 2.042 was observed in the M-500-2-H1 sample. It is reported that the EPR signal peak located at g = 2.042 is attributed to O− hole centers.35 Therefore, oxygen vacancies were formed in the M-500-2-H1 sample after the sintering of M sample in the reducing atmosphere. The Influence of Thermochromism on Upconversion Luminescence. The UCL spectra of M, M-200-2-H1, M-3002-H1, M-400-2-H1, M-450-2-H1, and M-500-2-H1 samples are shown in Figure 3a. There are three obvious emission peaks located at 529, 553, and 662 nm, which are attributed to the transitions of 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4 I15/2, respectively. It is obvious that UCL modification was obtained by the thermochromic reaction. The UCL intensity decreased with increase of the second sintering temperature in the reducing atmosphere, and the complete quenching of the UCL was observed when the second sintering temperature in the reducing atmosphere was 500 °C. The formula ΔRn = (R0 − Rn)/R0 × 100% can be used to define the modification degree of UCL, where R0 and Rn are the initial UCL intensity of the M sample and the initial UCL intensities of the M-2002-H1, M-300-2-H1, M-400-2-H1, M-450-2-H1, or M-500-2E

DOI: 10.1021/acs.inorgchem.9b00526 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Absorption spectra (a) and UCL spectra (b) of M, M-500-A1-2, M-500-A1-4, M-500-A1-6, M-500-A1-8, and M-500-A1-10, UCL intensity as the function of sintering time (c), and XPS spectra of M-500-A1-2, M-500-A1-4, M-500-A1-6, M-500-A1-8, and M-500-A1-10 (d−h).

4, M-500-A1-6, M-500-A1-8, and M-500-A1-10 samples. The absorption spectra show no significant change for the M-500A1-6, M-500-A1-8, and M-500-A1-10 samples, suggesting the good stability of all the samples. Figure 5b shows the UCL spectra of raw M, M-500-A1-2, M-500-A1-4, M-500-A1-6, M500-A1-8, and M-500-A1-10 samples upon 980 nm excitation. It is notable that UCL was observed again when the M-500-2H1 sample was sintered at 500 °C for 2, 4, 6, 8, and 10 h in air.

temperature in the reducing atmosphere, leading to a larger degree of UCL quenching. Reversible UCL modulation is important for switching devices. The M-500-2-H1 sample without observable UCL was sintered again at 500 °C for 2, 4, 6, 8, and 10 h in air, and the samples were labeled as M-500-A1-2, M-500-A1-4, M-500-A16, M-500-A1-8, and M-500-A1-10, respectively. Figure 5a presents the absorption spectra of M, M-500-A1-2, M-500-A1F

DOI: 10.1021/acs.inorgchem.9b00526 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 6. XRD patterns of M, M-500-A1-2, M-500-A1-4, M-500-A1-6, M-500-A1-8, and M-500-A1-10.

Figure 7. Absorbance spectra and color change of M-500-6-An (a) and M-500-2-Hn (b) and UCL intensity of M-500-6-An (n = 1, 2, 3, 4, 5) and M-500-2-Hn (n = 1, 2, 3, 4, 5) as a function of sintering cycle numbers (c).

depicted in Figure S2. The absorption spectra and photos of M-500-2-Hn (n = 1−5) are shown in Figure 7b. The reversible change of absorption spectra and color between the M-500-6An and the M-500-2-Hn was obtained for each cycle of sintering. Figure S3 shows the UCL spectra of M-500-6-A1 and M-500-2-H1. M-500-6-A1 exhibited visible UCL, while M500-2-H1 has no UCL, and the modification degree (ΔRn) of UCL is about 100%. Figure 7c presents the UCL intensity as a function of sintering cycle number. It can be seen that the UCL intensity of M-500-A1-6 phosphor could be recovered completely, and no deterioration of UCL intensity was observed after 5 cycles. The UCL intensity can be switched on and off, exhibiting the excellent reproducibility after the fifth cycle. Our results demonstrate that the MoO3:Yb3+,Er3+ phosphor with reversible modulated UCL and excellent reproducibility has great potential in the fields of anticounterfeiting and optical switches.

The UCL intensity of M and M-500-A1-2 are almost the same, and the UCL intensity increased as the sintering time was prolonged. Moreover, the UCL intensity showed no obvious change when the sintering time was longer than 6 h, as shown in the Figure 5c. Therefore, the optimal sintering time is set to 6 h to obtain stable UCL. In order to confirm the mechanism, the XPS spectra of M-500-A1-2, M-500-A1-4, M-500-A1-6, M500-A1-8, and M-500-A1-10 are shown in Figure 5d−h. No XPS peaks of Mo5+ ions were observed in the samples, indicating that the Mo5+ in the M-500-2-H1 was oxidized to the Mo6+. Therefore, the UCL enhancement is not attributed to the different valence state of Mo. The XRD patterns of raw M, M-500-A1-2, M-500-A1-4, M500-A1-6, M-500-A1-8, and M-500-A1-10 samples show that the time of sintering in air has no influence on the phase of MoO3:Yb3+,Er3+ (Figure 6). It is clearly seen that some weak XRD peaks were not exhibited in the raw M. However, weak XPD peaks such as 25.68°, 25.88°, 29.68°, 39.65°, and 50.06° were exhibited and sharp for the M-500-A1-6, M-500-A1-8 and M-500-A1-10 samples, which suggested that the crystallinity of samples was increased with the increased second sintering time in air, leading to enhancement of UCL. The M-500-A1-6 sample was then sintered either in a reducing atmosphere or in air. The M-500-A1-6 phosphors sintered at 500 °C in air for 6 h or in a reducing atmosphere for 2 h after 5 cycles were denoted as the M-500-6-An (n = 1− 5) and M-500-2-Hn (n = 1−5), respectively. Figure 7a shows the absorption spectra and photos of M-500-6-An (n = 1−5). The M-500-6-An (n = 1−5) samples exhibited no change in the absorption spectra or color after 5 cycles. However, the Mo5+ ions in the M-500-2-Hn (n = 1−5) were oxidized to the Mo6+ ions for each of the cycles after sintering in air, as

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CONCLUSIONS A novel MoO3:Yb3+,Er3+ inorganic thermochromic material was prepared by a high-temperature solid-state method. The MoO3:Yb3+,Er3+ phosphor shows a pronounced thermochromism phenomenon and obvious color change from light-yellow to blue upon heat stimulus in the reducing atmosphere. The mechanism of color change is attributed to the generation of oxygen vacancies and Mo5+. The up-conversion photoluminescence was modulated due to the energy transfer from the photoluminescence center to the color centers. The upconversion luminescence modulation shows excellent reproducibility after several cycles. The thermochromic MoO3:Yb3+,Er3+ phosphors with reversible modulation of G

DOI: 10.1021/acs.inorgchem.9b00526 Inorg. Chem. XXXX, XXX, XXX−XXX

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upconversion luminescence have great potential for optical switches.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00526. Additional data from Rietveld structure refinement, SEM photos, XPS curves, and upconversion luminescence spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Z.Y. E-mail: [email protected]. ORCID

Zhengwen Yang: 0000-0001-6470-9244 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51762029, 11674137) and the Applied basic research key program of Yunnan Province (2018FA026).

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DOI: 10.1021/acs.inorgchem.9b00526 Inorg. Chem. XXXX, XXX, XXX−XXX