The optical sensitive detection of molybdate ions by layered europium hydroxides

Optical Materials 100 (2020) 109597

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The optical sensitive detection of molybdate ions by layered europium hydroxides Baiyi Shao a, b, Xiaobao Zhang a, b, Xinying Wang a, Fangming Cui b, **, Xiaojing Yang a, * a

Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, 19-Xinjiekouwai Street, Haidian District, Beijing, 100875, China b Qian Xuesen Laboratory of Space Technology, China Academy of Space Technology, 104-Youyi Road, Haidian District, Beijing, 100094, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Layered europium hydroxides Molybdate ions Detection Antenna effect

A mild and effective way working over a wide pH range (from 2 to 12) to detect selectively trace amounts of molybdate ions in aqueous solutions in a short time was realized by the antenna effect between the molybdate ions and the layered europium hydroxide (LEuH). The efficient energy transfer from molybdate ions to LEuH allowed 0.1 g LEuH to detect concentration as low as 100 ppm in 30 min. According to the exploration of different experimental conditions, the photoluminescence (PL) intensity of Eu3þ is gradually enhanced with the increased adsorption for molybdate ions. When the quantity of molybdenum adsorbed is within a certain range, the multiple of PL increase has an exponential function relation with the adsorption quantity providing a new idea to detect molybdate ions in industry.

1. Introduction As a new kind of inorganic layered compound with the general for­ mula of R2(OH)6–mAm⋅nH2O (0.5 � m � 2.0, R is a rare earth element and A denotes guest anions), layered rare-earth hydroxides (LRHs) [1] pro­ vide more opportunities for designing and synthesizing the new func­ tional materials used in the fields of biology, medicine and optics [2,3], due to the outstanding optical properties of rare earth ions and the ad­ vantages of the controllability of the interlayer guest anions. However, due to the quenching effect of the coordinated water molecules and hydroxyl groups on the R3þ, the photoluminescence (PL) intensity of LRHs is usually weak [4].Therefore, in order to enhance the PL, LRHs are usually calcined to the corresponding oxides [5] or modified to replace the hydroxyl groups by some groups [6]. Meanwhile it is well known that the antenna effect is an effective method for PL enhancement of the rare earth ions, which means the effective intra­ molecular energy transfer from ligands to rare earth ions [7,8]. The guest anions in the interlayer of LRHs can be exchanged with other anions as sensitizers using the antenna effect for enhancing the PL through intercalation chemistry [9]. So far, the organic anions as the sensitizers, such as aromatic anions, heteroaromatic anions and so on, intercalated into the LRHs have been extensively reported [10–14].

While the LRHs sensitized by the inorganic metal-oxyanions get very little attention. At present, the PL enhancement of rare earth ions by antenna effect has been widely used in the fields of lasers, light-energy conversion devices and photosensitive materials [15,16]. It is also essential for the detection of the metal-oxyanions or pollutants in aqueous solutions taking advantage of the antenna effect, but rarely reported except for the follows. Byeon realized the selective PL detection of tungstate anions [17] and vanadate anions [18] by Tb-doped layered yttrium hydroxide (LYH:Tb) and Eu-doped layered gadolinium hy­ droxide (LGdH:Eu) based on antenna effect, which is an important leap in the field of PL detection of LRHs. In order to protect the water re­ sources and treat the sewage, Kim found that the PL enhancement of LYH:Tb can be used for detecting chromium pollution in natural surface water at low and medium concentration [19]. As a transition element, molybdenum is necessary for human body [20], animals and plants and widely used in machinery [21], electronics [22] and medicine fields [23]. Molybdate anions (MoO24 ) are the main form of molybdenum in aqueous solution and important source of mo­ lybdenum uptake by organisms. At present, chromogenic reaction [24], spectrophotometry [25,26] and ion chromatography [27,28] are widely used as the methods for detecting molybdate ions. However, the ions interference and finite conditions show the obvious disadvantages in the

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (F. Cui), [email protected] (X. Yang). https://doi.org/10.1016/j.optmat.2019.109597 Received 20 October 2019; Received in revised form 25 November 2019; Accepted 1 December 2019 Available online 18 December 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.

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above methods besides tedious operations. To meet the demand for detecting molybdate ions in food analysis, wastewater treatment and other fields in our daily life, a preferable method to detect molybdate ions is of vital importance. In this study, we realized a simple PL detection of molybdate anions in aqueous solution by the PL enhancement of the MoO24 absorbed LEuHs. The molybdate anions in aqueous solutions at the concentration as low as 100 ppm can be detected in 30 min using 0.1 g LEuHs. The PL sensitivity of LEuHs for MoO24 among different metal-oxyanions in aqueous solutions was found and showed a PL saturation phenomenon. The PL enhancement of the sample at 614 nm is exponentially related with the absorption amount of Mo by LEuHs. The mechanism of the PL sensitivity of LEuHs for MoO24 is discussed to be the energy transfer from Mo6þ to Eu3þ based on the PL excitation and emission spectra of different samples. Therefore, it is a new style quantitative optical detection optical detection method for detecting MoO24 ions with outstanding application potential.

precipitates were dried at 40 � C after centrifugation. 2.7. PH dependence of adsorption for molybdate ions Six parts of 0.1 g NO3ˉ-LEuHs were separately added to six aqueous solutions of MoO24 (20 ml, 100 ppm) with the different pH of 2, 4, 6, 8, 10 and 12 regulated by the aqueous solutions of HNO3 and NaOH and stirred at room temperature for 0.5 h. Then the precipitates were dried at 40 � C after centrifugation. 2.8. Characterizations The X-ray diffraction (XRD) patterns were tested by a Philips X’Pert Pro MPD diffractmeter with Cu-Kɑ radiation at room temperature (2θ ranging from 4.5 to 70� ). The scanning step size of wide angle XRD was 0.0165� with the X-ray generator set to 40 kV and 40 mA. Fouriertransform infrared spectra (FT-IR) patterns of the samples were collected by the Nicolet 380 FT-IR spectrometer with pressed-disk technique. Photoluminescence (PL) spectra were obtained by the FS5 (Edinburgh Instruments) Spectrofluorophotometer. The quality of mo­ lybdenum element was calculated by the results of inductively coupled plasma (ICP) atomic emission spectroscopy. The Hitachi S-4800 mi­ croscope was used to measure the scanning electron microscope (SEM) images.

2. Experimental 2.1. Preparation of Eu(NO3)3⋅6H2O A mixture of Eu2O3 (3.0 g), HNO3 (5.0 ml) and distilled water (10 mL) in a round bottom flask (50 mL) was heated and stirred for 3 h at 80 � C water bath [29]. After that, vacuum-rotary evaporation was used to remove the solvent and remain solid precipitates. Adding distilled water to dissolve the precipitates and repeated vacuum-rotary evaporation procedure until the pH of the solution was 3–4. Eu(NO3)3⋅6H2O was obtained after rotation-drying under the irradiation of infrared light.

3. Results and discussion 3.1. PL sensitivity of LEuHs for MoO24 The XRD patterns of the NO3ˉ-LEuHs before and after adsorbing various metal-oxyanions at room temperature for 12 h are depicted in Fig. 1A. The NO3ˉ-LEuHs presents three obvious diffraction peaks at 2θ of 11, 21 and 28� , corresponding to (002), (004) and (220) planes of the layer structured NO3ˉ-LEuH with the basal spacing of 0.83, 0.42 and 0.31 nm [30], as seen curve (a) in Fig. 1A. After absorbing CrO24 ions, the (002) and (004) diffractions of the sample move to the smaller 2θ angles consistently and the basal spacing is enlarged to 0.94 and 0.47 nm, which can be attributed to the larger sized CrO24 , compared with the size of NO3ˉ, intercalated into the interlayers of LEuHs, as shown in Fig. 1A(b). The decrease of the diffraction intensity of the CrO24 absorbed sample was resulted by the fewer layer number of the sample compared with that of NO3ˉ-LEuHs. The diffraction patterns of the WO24 absorbed samples are almost the same as those of NO3ˉ-LEuHs, maybe caused by the failed intercalation of WO24 anions into the interlayer of LEuHs in Fig. 1A(c). The MoO24 absorbed sample shows two new diffraction peaks with plane spacing of 0.92 and 0.46 nm left beside the original (002) and (004) diffractions, except for the diffractions peaks of NO3ˉ-LEuHs, which indicates that the partial NO3ˉ ions have been exchanged by MoO24 ions in the interlayers of LEuH within 12 h at room temperature. All the samples present the (220) diffraction peaks at the same 2θ angle with the basal spacing of 0.31 nm, indicating the inner-layer structure of LEuHs is hardly changed after the absorption of metal-oxyanions [10]. Fig. 1B indicates the SEM images of the samples. The original LEuHs morphology (Fig. 1B(a)) is a spherical secondary particle, composed of wrinkled nanosheets (Fig. 1C(a)), which exhibit the layered structure of LEuHs. Although the basal spacing was mark­ edly changed after the adsorption as shown in XRD patterns (Fig. 1A), no noticeably change could be found in morphology (Fig. 1B and C). The PL excitation and emission spectra of NO3ˉ-LEuH before and after adsorbing various metal-oxyanions at room temperature for 12 h are obtained in Fig. 2. A series of PL excitation peaks of the samples attributed to the intra-4 f [6] electric transitions of Eu3þ in LEuHs are observed in the excitation spectra (Fig. 2A), among which the excitation peaks at 394 nm wavelength correspond to 7F0-5L6 transition of Eu3þ [31]. Compared with others, the MoO24 absorbed sample presents an obvious enhanced excitation peak at around 260 nm in the excitation

2.2. Preparation of NO3ˉ-LEuHs NaNO3 (13 mmol), hexamethylenetetramine (1 mmol), Eu (NO3)3⋅6H2O (1 mmol) and deionized water (80 mL) were added in a Teflon-autoclave and kept for 12 h at 90 � C. After being cooled to room temperature, the target products were centrifuged and washed with distilled water and ethanol, finally vacuum dried at 40 � C. 2.3. Adsorption of metal-oxyanions by NO3ˉ-LEuHs Three parts of 0.1 g NO3ˉ-LEuHs were separately added to the aqueous solutions (20 ml, 0.1 mol/L, pH > 7) of CrO24 , WO24 and MoO24 and stirred at room temperature for 12 h. Then the samples were centrifuged, washed and dried at 40 � C. 2.4. Time dependence of adsorption for molybdate ions In order to investigate the sensitivity of LEuHs for molybdate ions, six parts of 0.1 g NO3ˉ-LEuHs were added to six aqueous solutions of MoO24 (20 ml, 100 ppm, pH � 7) and stirred at room temperature for 0.25, 0.5, 1, 3, 6 and 12 h respectively. Then the samples were centrifuged, washed and dried at 40 � C. 2.5. Concentration dependence of adsorption for molybdate ions Five parts of 0.1 g NO3ˉ-LEuHs were added to five aqueous solutions of MoO24 (pH � 7, 20 ml) and stirred at room temperature for 0.5 h with different concentrations of 50, 100, 400, 800 and 1200 ppm respec­ tively. And then the samples were centrifuged, washed and dried at 40 � C. 2.6. Temperatures dependence of adsorption for molybdate ions Five parts of 0.1 g NO3ˉ-LEuHs were separately added to five aqueous solutions of MoO24 (20 ml, 100 ppm, pH � 7) with the different tem­ peratures of 0, 20, 40, 60 and 80 � C and stirred for 0.5 h. Then the 2

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Fig. 1. (A) XRD patterns, (B) SEM images and (C) magnification of (B) for NO3ˉ-LEuH (a) before and after absorption in 100 ppm aqueous solutions of (b) CrO24 , (c) WO24 and (d) MoO24 for 12 h.

Fig. 2. PL excitation (A) and emission (B) spectra of (a) NO3ˉ-LEuH before and after absorption in 100 ppm aqueous solutions of (b) CrO24 , (c) WO24 and (d) MoO24 for 12 h.

between MoO24 and Eu3þ ions. While the PL enhancement by the energy transfers between the other metal-oxyanions and Eu3þ ions are not detected, which means the PL selectivity of LEuHs for MoO24 .

spectra. There is no the notable charge-transfer band of LEuHs at the range of 200–300 nm, so the broad excitation peak at 260 nm should be assigned to the energy-transfer from the O → Mo LMCT (ligand-to-metal charge-transfer) band to the Eu3þ ion [32]. The PL emission spectra of the samples were excited under the 394 nm laser except for the MoO24 absorbed sample excited under 260 nm laser (Fig. 2B). All of the samples emerge the emission peaks of char­ acteristic transitions of Eu3þ with different PL intensities. The emission peaks at 580, 595, 614, 652 and 700 nm corresponded to the charac­ teristic transitions (5D0→7FJ (J ¼ 0, 1, 2, 3, and 4)) of Eu3þ [31]. The PL intensity of the CrO24 absorbed sample decreased compared with that of the original NO3ˉ-LEuHs, due to the quenching effect of CrO24 on the f-f transitions of Eu3þ [17]. The role of CrO24 could be explained as that CrO24 and Eu3þ have a competitive effect on energy absorption rather than energy transfer, as blocks the energy absorption of the Eu3þ, causing the PL quenching of the Eu3þ in LEuHs [33,34].The PL in­ tensities of WO24 absorbed samples are almost the same as those of NO3ˉ-LEuHs, probably due to the failed intercalation of WO24 into the interlayer of LEuHs, as seen the XRD results in Fig. 1A, implying failed antenna effect between them and Eu3þ ions. Compared with that of the original NO3ˉ-LEuHs, the PL intensity of MoO24 adsorbed sample at 614 nm is greatly enhanced, which can be attributed to the antenna effect

3.2. Time dependence of PL sensitivity of LEuHs for MoO24 Fig. 3A depicts the XRD patterns of NO3ˉ-LEuHs before and after absorption of MoO24 at room temperature for different time. All the samples present the same diffraction peaks of (002), (004) and (220) planes of the layered NO3ˉ-LEuHs with 2θ at 11, 21 and 28� . However, the 12 h absorbed sample presents two new diffraction peaks at left side of the (002) and (004) plane diffractions with basal spacing of 0.92 and 0.46 nm, indicating that the bigger sized MoO24 ions have successfully entered the interlayer of LEuHs upon 12 h absorption but not completely exchanged with NO3ˉ ions. The vibration absorption of chemical bonds or functional groups in the samples can be detected by the FT-IR transmittance spectra. The FTIR spectra of NO3ˉ-LEuHs are shown in Fig. 3B after the MoO24 ab­ sorption at room temperature for different time. The FT-IR absorptions at 596, 839, 1384 and 1637 cm 1 of the MoO24 absorbed samples (curve (a)–(f) in Fig. 3B) were attributed to the vibration of Eu–O [35,36], MoO24 [37,38], NO3ˉ [39,40] and H2O [41–43]. The absorption peaks 3

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Fig. 3. (A) XRD patterns and (B) FT-IR transmittance spectra of NO3ˉ-LEuH after absorption in 100 ppm aqueous solutions of MoO24 for (a) 0.25, (b) 0.5, (c) 1, (d) 3, (e) 6, and (f) 12 h ((B) FT-IR transmittance spectra of (g) sodium molybdate.).

of MoO24 at 839 cm 1 of the samples gradually increase in intensity along with the absorption time from 0.5 to 12 h, which means that the MoO24 ions have gradually entered the interlayer of LEuHs with pro­ longing of the absorption time or been adsorbed on the surface of the LEuHs. However, the vibration absorption of NO3ˉ at 1384 cm 1 can still be observed for all the samples indicating the incomplete exchange of NO3ˉ by MoO24 within 12 h at room temperature, which was consistent with the results of XRD. The PL excitation and emission spectra of MoO24 adsorbed LEuHs for different time at room temperature were detected in Fig. 4A and B. The emission peaks of the characteristic transitions of Eu3þ are nearly the same except for the gradually increased peak at ~260 nm of the MoO24 adsorbed samples with the increase of the absorption time due to the energy transfer from MoO24 to Eu3þ. The emission spectra of the sam­ ples present the same peaks attributed to the 5D0→7FJ transitions of Eu3þ but with different PL intensity especially at 614 nm wavelength, which are gradually enhanced along with the increase of the MoO24 absorption time. When the MoO24 absorption time reached 1 h, the PL intensity of the emission peak at 614 nm reaches to the highest and then keeps this

intensity almost unchanged for the samples with 3, 6 and 12 h absorp­ tion time. In order to furtherly investigate the PL sensitivity and enhancement of LEuHs by MoO24 anions, the digital photographs of the MoO24 adsorbed sample with different absorption time was taken under 365 nm irradiation, as shown in Fig. 4C. It is obvious that the intensity of the red colored luminescence gradually increases with the increase of the ab­ sorption time. The PL sensitivity of LEuHs for MoO24 anions can be used as a method for detecting and distinguishing the MoO24 anions in aqueous solutions at a relatively low concentration of 100 ppm. Sur­ prisingly, it only takes half an hour to detect the presence of MoO24 ions in aqueous solutions because of the distinct red color change of MoO24 absorbed sample, which is consistent with PL emission spectra in Fig. 4B. 3.3. The absorption amount of Mo by the LEuHs The PL sensitivity of LEuHs for the MoO24 is definitely related to the absorption amount of Mo by the LEuHs, which can be deduced from the above results and discussion. The factors of concentration, pH and

Fig. 4. PL excitation (A) and emission (B) spectra of NO3ˉ-LEuH before and after absorption in 100 ppm aqueous solutions of MoO24 for 0.25, 0.5, 1, 3, 6, and 12 h. Photographs (C) of MoO24 solutions at 100 ppm added with 0.1 g NO3ˉ-LEuHs after different absorption time under 365 nm irradiation. 4

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temperature for the adsorption amount of Mo by LEuHs have been further investigated, since the concentration and temperature acting on the diffusion rate of MoO24 and the pH on the configuration of MoO24 in aqueous solution. The XRD patterns of the MoO24 adsorbed samples under different conditions of concentration, pH and temperature are shown in Figs. S1–S3. The relations of absorption amount of Mo by the LEuHs with factors of absorption time, concentration, pH and temperature are shown in Fig. 5. The adsorption amount of molybdenum element by the LEuHs linearly increases as the increase of adsorption time and concentration under the experimental conditions, except for an inflection in Fig. 5A. The adsorption amount of molybdenum by the LEuHs does not vary along with the variation of pH at room temperature, as shown by Fig. 5C. The molybdate anions exists in various forms due to polycondensation reaction in aqueous solution with different pH values (As the pH is greater than 6, MoO24 is the main form whereas Mo7O624 and Mo8O426 are) [32]. However, the pH has little effect on the adsorption amount of molybdenum under the conditions, which was probably caused by the failed interaction and the insufficient driving force of MoO24 into the interlayers of LEuHs under the conditions of room temperature, 0.5 h absorption time and 100 ppm concentration, as seen curve (b) in Fig. 3A and curve (b) in Fig. 3B. Therefore, it can be speculated that the adsorption amount of molybdenum is independent of the forms of the oxyanions. The adsorption amount of molybdenum slowly increases with the increase of temperature under the conditions of pH � 7, 0.5 h absorption time and 100 ppm concentration, as shown in Fig. 5D. However, as the temperature increases from 20 to 80 � C, the change in the adsorption amount of molybdenum is not obvious, which indicates that the MoO24 detection by the PL enhancement of LEuHs at room temperature is appropriate for practicality.

conditions affect the adsorption amount of molybdenum by LEuHs thus cause different enhancement of PL intensity of the samples. In order to explore the relation between the PL sensitivity of LEuHs and the adsorption amount of molybdenum, the enhancement of PL intensity is taken to indicate the PL sensitivity. The PL enhancement of the MoO24 absorbed samples was characterized by the ratio of PL intensity at emission of 614 nm of the samples after (I) and before (I0) MoO24 adsorption in this study. The relation of I/I0 and the adsorption amount of molybdenum (mMo/mg) is demonstrated in Fig. 6. The ratio I/I0 increases exponen­ tially as the increase of the adsorption amount from 0 to 12 mg, as shown the inset in Fig. 6. The exponential relation of them have been fitted by calculation and can be expressed as follows. I = I0 ¼ 0:4836 � expðmMo = 4:169Þ Therefore, the concentration of Mo in the aqueous by the ratio of I/I0 can be inferred, which provides us an optical method of Mo detection through the enhancement of PL intensity of LEuHs. The ratio I/I0 keeps steady at 8.8 and is no longer increasing when the adsorption amount is above 12 mg, which may be by reason of the PL saturation of LEuHs for MoO24 and consistent with the results shown in Fig. 4B. 3.5. The mechanism of PL enhancement of LEuHs by MoO24 From the above research in Figs. 5C and 6, the enhancement of PL is not affected by the different forms (Mo7O624 and Mo8O426 ) of MoO24 , which indicates that all forms of the oxyanions for molybdenum can transfer energy to LEuH. At the same time, from the study results of the adsorption for other ionic forms (HCrO4 , Cr2O27 , W7O626 and W8O426 ) of CrO24 and WO24 in Figs. S4 and S5, it can be inferred that the CrO24 and WO24 cannot transfer energy to the LEuHs no matter what forms they are in, suggesting the importance of energy level matching such as the MoO24 and Eu3þ. Two possible factors determining the combination of molybdate and layers are electrostatic interaction and coordination bond. Molybdate ions are extremely easy to bond to the positively

3.4. The dependence of PL sensitivity of LEuHs on the absorption amount of Mo From the above studies, it is apparent that different adsorption

Fig. 5. The adsorption amount of molybdenum (mMo/mg) as a function of (A) time at room temperature, pH � 7 and 100 ppm concentration, (B) concentration at room temperature, pH � 7 and 0.5 h absorption time, (C) pH at room temperature, 100 ppm concentration and 0.5 h absorption time and (D) temperature at 100 ppm concentration, pH � 7 and 0.5 h absorption time. 5

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the schematic electron transitions of Eu3þ in Scheme 1. The key to the energy transfer is that the middle state 3T2 energy level of MoO24 ions can match the excited states energy level of Eu3þ, which determines whether it is an effective energy transfer between them. Other anions can’t enhance the PL of LEuHs caused by the unmatched energy level, indicating the importance of energy level matching. Therefore, com­ parison with the original NO3ˉ-LEuHs, MoO24 absorbed LEuHs have the more intense red light emission attributed to the role of molybdate ions in energy transfer. 4. Conclusions In conclusion, a sensitive and simple method for the detection of molybdate anions in aqueous solutions with low trace concentration and a wide pH range is realized by the antenna effect between the molybdate ions and the NO3ˉ-LEuHs. The molybdate-absorbing NO3ˉ-LEuHs present the intense red emission, attributed to the selective and effective energy transfer between molybdate ions and NO3ˉ-LEuHs, which shows an exponential relation between the enhanced photoluminescence in­ tensity and the absorption amount of Mo by LEuHs. This is a new paradigm that the antenna effect is applied to PL detection of layered rare earth hydroxides. The present detection method for molybdate is of advantage, including simple to operate and easy to implement without expensive equipment, compared with common detection methods mentioned in the introduction section, such as ion chromatography and spectrophotometry, as well as it works over a wide pH range and solves the obvious disadvantages of ion interference in the chromogenic reaction.

Fig. 6. The enhancement of PL intensity for MoO24 adsorbed LEuHs at emis­ sion of 614 nm as a function of the adsorption amount of molybdenum (mMo/ mg). (The enhancement of PL intensity was characterized by the ratio of PL intensity of the samples after (I) and before (I0) MoO24 adsorption.).

charged LEuH layers through electrostatic interaction, which causes that molybdate ions were intercalated into the interlayer or adsorbed on the surface of the LEuH layers. Then the coordination bonds between molybdate ions and LEuH layers can form in the form of Eu3þ–O–Mo6þ through the lone pairs of molybdate ions which occupy the empty orbit of Eu3þ [32]. These interactions allow efficient energy transfer from MoO24 to Eu3þ known as the antenna effect. The MoO24 ions absorb energy and pass it on the Eu3þ ions, which causes the strong emissions ascribed to the characteristic f-f transitions of Eu3þ. For further inves­ tigating the antenna effect, the possible PL enhancement mechanism of LEuHs by MoO24 is schematically illustrated in Scheme 1. The PL excitation peaks at ~260 nm by the MoO24 absorbed LEuHs in Figs. 2A and 4A can be attributed to the absorption of the MoO24 ions by comparing the PL excitation spectra of the different samples. So that the MoO24 absorbed LEuHs can be excited by the laser at ~260 nm and the electrons are lifted from ground state 1A1 to exited state 1T1 then transfer to middle state 3T2 through interband transition as seen the schematic electron transition of Mo6þ in Scheme 1 [44–46].After that, the energy absorbed by MoO24 should transfer to the excited states of Eu3þ and then radiate out in the form of Eu3þ characteristic luminescence resulting in the PL enhancement in the MoO24 absorbed LEuHs, as seen

Author contribution statement The completion of this manuscript benefits from the cooperation of the authors. Specific contributions are as follows: Baiyi Shao: Conceptualization, Methodology, Investigation, Data curation, Writing-Original draft preparation. Xiaobao Zhang: Valida­ tion, Investigation, Data curation, Formal analysis. Xinying Wang: Resources, Data curation. Fangming Cui: Formal analysis, WritingReview and Editing Project administration. Xiaojing Yang: Supervi­ sion, Writing-Reviewing and Editing, Project administration, Resources. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence

Scheme 1. Schematic diagram of electron transitions of Mo6þ and Eu3þ. 6

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the work reported in this paper.

[17] [18] [19] [20]

Acknowledgments

[21]

This work was supported by the National Natural Science Foundation of China (Grants. 51572031 and 51272030) and Programs for Chang­ jiang Scholars and Innovative Research Team in University.

[22]

Appendix A. Supplementary data

[23] [24] [25]

Supplementary data to this article can be found online at https://doi. org/10.1016/j.optmat.2019.109597.

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