Encapsulation of CH3NH3PbBr3 Perovskite Quantum Dots in MOF-5

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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Encapsulation of CH3NH3PbBr3 Perovskite Quantum Dots in MOF‑5 Microcrystals as a Stable Platform for Temperature and Aqueous Heavy Metal Ion Detection Diwei Zhang,† Yan Xu,‡ Quanlin Liu,† and Zhiguo Xia*,† †

The Beijing Municipal Key Laboratory of New Energy Materials and Technologies, School of Materials Sciences and Engineering, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China ‡ Department of Chemistry, College of Science, Northeastern University, Liaoning 110819, People’s Republic of China ABSTRACT: The stability issue of organometallic halide perovskites remains a great challenge for future research as to their applicability in different functional material fields. Herein, a novel and facile two-step synthesis procedure is reported for encapsulation of CH3NH3PbBr3 perovskite quantum dots (QDs) in MOF-5 microcrystals, where PbBr2 and CH3NH3Br precursors are added stepwise to fabricate stable CH 3 NH 3 PbBr 3 @MOF-5 composites. In comparison to CH3NH3PbBr3 QDs, CH3NH3PbBr3@MOF-5 composites exhibited highly improved water resistance and thermal stability, as well as better pH adaptability over a wide range. Luminescent investigations demonstrate that CH3NH3PbBr3@MOF-5 composites not only featured excellent sensing properties with respect to temperature changes from 30 to 230 °C but also exhibited significant selective luminescent response to several different metal ions in aqueous solution. These outstanding characteristics indicate that the stable CH3NH3PbBr3@MOF-5 composites are potentially interesting for application in fluorescence sensors or detectors.

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INTRODUCTION Hybrid organic−inorganic halide perovskites, such as CH3NH3PbX3 (X = Cl, Br, I), have attracted considerable attention due to their intriguing applications in the field of photovoltaic solar cells, light-emitting diodes (LEDs), lasers, photodetectors, sensors, and so on.1−6 The recent surge of interest in colloidal perovskite CH3NH3PbX3 nanocrystals (NCs), also called quantum dots (QDs), has emerged from their excellent luminescent properties, including high quantum yield, composition-tunable emission wavelength, narrow bandwidth, and short irradiative lifetime.7,8 However, despite the fact that the remarkable performance of CH3NH3PbX3 has been demonstrated, their practical applications are still greatly hindered by the inherent instability of organic CH3NH3+ cations, which undergo extremely fast degradation under different circumstances, such as moisture, UV light, oxygen, and high temperature. 9 Therefore, the instability of CH3NH3PbX3 QDs bring about a significant challenge for their photoluminescence application. To improve the stability of perovskite NCs, one possible strategy is to design and fabricate hybrid composite materials containing the perovskite QDs. Metal−organic frameworks (MOFs), as a fascinating class of porous crystalline materials, have been extensively studied due to their tunable pore sizes and high specific areas. Moreover, they can be conveniently designed and self-assembled.10−12 These outstanding features make MOFs a promising platform for host−guest chemistry, providing ideal accommodation for a variety of guest species.13,14 Up to now, a wide variety of QDs © XXXX American Chemical Society

have been incorporated into MOFs to integrate many novel composite materials, with combined properties superior to those of the individual components.15−17 Although perovskite CH3NH3PbX3 QDs have been reported to be loaded in various hosts to improve the relative stability,14,18−20 the combination of stability and exploration of their synergistic application is urgently needed; especially, the fundamental properties, mechanisms, and potential new applications of CH3NH3PbX3 QDs encapsulated in MOFs are still rare and challenging. In this work, we report a feasible two-step synthesis method for perovskite CH3NH3PbBr3 QDs embedded in MOF-5. The precursor PbBr2@MOF-5 was first prepared with Zn2+, Pb2+, and Br− ions and 1,4-benzenedicaboxylic acid (H2BDC) by solvothermal methods in N,N-dimethylformamide (DMF) solution. Then, CH3NH3Br ethanol solution was introduced and reacted with PbBr2@MOF-5 to form CH3NH3PbBr3 QDs in situ at room temperature. In comparison to CH3NH3PbBr3 QDs, the resultant CH3NH3PbBr3@MOF-5 composites show improved water resistance and high thermal stability and also possess an excellent sensing function with respect to temperature change from 30 to 230 °C on the basis of the fluorescence measurements. These versatile composites also featured excellent tolerance for a wide range of pH values and are very sensitive to different metal ions, so that they can be used as aqueous heavy metal ion detectors. In addition, their potential fluorescence sensing mechanism has also been elucidated. Received: February 7, 2018

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DOI: 10.1021/acs.inorgchem.8b00355 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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EXPERIMENTAL SECTION

Materials and Preparation. All of the chemicals were commercially purchased and used without further purification. The CH3NH3Br and PbBr2@MOF-5 precursors were synthesized by a method modified from that in the literature.21,22 Typically, 30 mL of methylamine (33 wt % in methanol) and 35 mL of HBr (48 wt % in water by weight) were mixed in a beaker in an ice bath (1−2 °C) with stirring over 2 h. Most water was then evaporated on a hot plate in a hood. The yellowish product was collected by filtration and washed three times with 20 mL of diethyl ether. Finally, the CH3NH3Br crystals were dried at 60 °C in a vacuum oven for 12 h. In addition, Zn(NO3)·6H2O (1.666g, 5.6 mmol) and H2BDC (0.334g, 0.2 mmol) were dissolved in 25 mL of DMF, and PbBr2 powder was dissolved in 5 mL of DMF, respectively. Then the solutions were sonicated for 10 min to ensure absolute dissolution, and then the mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave, which was heated at 120 °C for 12 h. The reaction mixture was slowly cooled to room temperature at a rate of 5 °C/min. The product was separated from the final reaction system by filtration, and then the white precipitates were collected, washed several times with 15 mL of DMF, and immersed in 20 mL of CH2Cl2 overnight. Finally, the products were activated under a vacuum oven at 120 °C for 12 h. Then, N2 adsorption isotherms of 0.107 g of MOF-5 and 0.0596 g of CH3NH3PbBr3@MOF-5 were measured at 77 K. Elemental analyses for Pb and Br were determined by using inductively coupled plasma atomic emission spectrometry; 0.025 g of MOF-5, PbBr2@MOF-5, and CH3NH3PbBr3@MOF-5 were dissolved in HNO3 solution and then transferred and diluted to 250 mL. Different standard solutions of Zn (10, 20, 50, and 100 ppm), C (10, 20, 50, and 100 ppm), Pb (1, 2, 5, and 10 ppm), and Br (1, 2, 5, and 10 ppm) were selected to measure the contents of Pb and Br in these composites. Characterization. X-ray powder diffraction was performed on a PANalytical X’Pert3 powder diffractometer equipped with a Cu Kα radiation (λ = 0.15406 nm) source and operated at 40 kV and 40 mA. The crystallinity and morphology of the as-prepared products were determined with a scanning electron microscope (SEM, JEOL JSM6510A). Transmission electron microscopy (TEM) and the corresponding elemental mapping were performed on a JEM-2010 instrument operated at 120 keV on 200 mesh carbon-coated nickel grids. Inductively coupled plasma atomic emission spectrometry (ICPAES) measurements were recorded on a PerkinElmer Optima 7000dv apparatus. A Quantachrome Autosorb gas sorption analyzer (Autosorb-iQ-2MP, USA) was used to perform N2 adsorption measurements. Gas adsorption measurements was carried out at 77 K in a liquid nitrogen bath. UV−vis−NIR absorption spectra of colloidal solutions were collected at room temperature using a Varian Cary 5 spectrophotometer. The steady-state photoluminescence (PL) spectra were measured using an F-4600 fluorescence spectrophotometer (Hitachi, Japan) with a photomultiplier tube operating at 400 V and a 150 W Xe lamp. The variable-temperature luminescence was measured on the same device, which was coupled with a self-made heating attachment and a computer-controlled electric furnace. Fluorescence microscopy images were recorded on a Nikon fluorescence stereomicroscope (ECLIPSE LV100ND, DS-U3). Simultaneous thermal analysis (TG-DTA) was performed on a TA Q500 thermal analyzer with the sample held in a platinum pan under a continuous nitrogen flow atmosphere.

Figure 1. (a) Schematic diagram of the two-step approach to CH3NH3PbBr3@MOF-5 composites. The first step included the synthesis of PbBr2@MOF-5 precursor by a solvothermal method, and the second step involved the addition of CH3NH3Br solvent for the final CH3NH3PbBr3@MOF-5. (b) Illustrative images of the PbBr2@ MOF-5 precursor in daylight and the CH3NH3PbBr3@MOF-5 composite in daylight and under 365 nm UV light and its fluorescence microscope morphology.

mixture of PbBr2, 1,4-H2BDC, Zn(NO3)·6H2O, and DMF solvent, which was then transferred to a Teflon-lined stainless steel autoclave and heated at 120 °C for 12 h. After that, the white precipitates of PbBr2@MOF-5 were collected. Second, the as-obtained PbBr2@MOF-5 precursor was dispersed into a CH3NH3Br (11.2 mg, 0.1 mmol) ethanol solution (5 mL) for 10 min to give the final CH3NH3PbBr3@MOF-5 composites. The PbBr2@MOF-5 composites turned from white to yellow immediately with the addition of a CH3NH3Br solution (Figure 1b). Eventually, it turned bright green, resulting from the emission of CH3NH3PbBr3@MOF-5 under UV 365 nm light, as was also verified by the optical fluorescence microscope image shown in Figure 1b. To identify the structure and photoluminescence (PL) evolution during the synthesis of the CH3NH3PbBr3@MOF-5 composite, powder X-ray diffraction (XRD) and PL spectra of selected samples were measured, and the results are shown in Figure 2. The diffraction peaks of MOF-5, PbBr2@MOF-5, and CH3NH3PbBr3@MOF-5 are shown in Figure 2a, and the XRD diffraction peaks of as-prepared MOF-5 were found to match well with the simulated pattern. For PbBr2@MOF-5 and CH3NH3PbBr3@MOF-5, only the diffraction pattern of MOF5 could be clearly observed, which should be due to the high crystallinity with strong diffraction peaks of MOF-5 in diffraction patterns, and the diffraction peaks associated with small contents of PbBr2 or CH3NH3PbBr3 QDs were too weak to be observed, as was also found in other systems.23,24 However, their existence was confirmed by ICP-AES analysis, as shown in Table 1. In comparison to MOF-5, the ICP results prove the presence of Pb and Br elements in PbBr2@MOF-5 and CH3NH3PbBr3@MOF-5 composites, and the Pb:Br mole ratios are 1:1.728 for PbBr2@MOF-5 and 1:2.985 for CH3NH3PbBr3@MOF-5 composite, respectively, which are nearly consistent with the chemical constituents of PbBr2 and CH3NH3PbBr3. The result supported the successful incorporation of PbBr2 and CH3NH3PbBr3 in the MOF-5 framework. Therefore, a series of composites with different concentrations of PbBr2 (0.1, 0.5, 1, 1.5, and 2 mmol) have been prepared. The PL spectra of the as-prepared samples under 365 nm excitation are shown in Figure 2b. Obviously, the emission intensities of CH3NH3PbBr3@MOF-5 composite increased first with increasing PbBr2 concentration and found to be the maximum for 1 mmol of PbBr2.25 Figure 2c further displays the XRD patterns of CH3NH3PbBr3@MOF-5 with the addition of different PbBr2 contents. It can be noted that addition of PbBr2 did not change

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RESULTS AND DISCUSSION Synthesis, Morphology, and Structure Characterization. A schematic illustration of the synthesis and the structural evolution diagram for CH3NH3PbBr3@MOF-5 composites are shown in Figure 1. The CH3NH3PbBr3@ MOF-5 composites were prepared by using a two-step synthesis strategy, as demonstrated in Figure 1a, and the CH3NH3PbBr3 QDs can be encapsulated into the porous MOF-5 microcrystal framework. First, the PbBr2@MOF-5 precursors were obtained by a solvothermal method from a B

DOI: 10.1021/acs.inorgchem.8b00355 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. (a) Typical TEM image of as-prepared CH3NH3PbBr3 QDs. SEM (b) and TEM (c) images of PbBr2@MOF-5 precursors, and the elemental mapping of Zn (d), Pb (e), and Br (f) in PbBr2@MOF-5 composites. Figure 2. (a) XRD patterns of the simulated MOF-5, as-synthesized MOF-5 microcrystals, PbBr 2 @MOF-5 precursors, and final CH 3 NH 3 PbBr 3 @MOF-5 composites. (b) PL spectra of CH3NH3PbBr3@MOF-5 composites with different concentrations of PbBr2. The inset presents the variations of the emission intensities versus PbBr2 concentration. (c) XRD patterns of CH3NH3PbBr3@ MOF-5 composites with the addition of different concentrations of PbBr 2 . (d) Normalized PL spectra of MOF-5 (blue line), CH3NH3PbBr3 QDs with an emission peak at 521 nm (green line), and CH3NH3PbBr3@MOF-5 composites with the main emission peak at 533 nm and a minor peak at about 428 nm (red line). The dotted line shows the absorption spectrum of CH3NH3PbBr3@MOF-5 composites.

the crystal structure. Finally, PL spectra of MOF-5, CH3NH3PbBr3 QDs, and CH3NH3PbBr3@MOF-5 composites are compared in Figure 2d. For CH3NH3PbBr3 QDs, a narrow green emission peak at 521 nm was observed (quantum yield 85%) along with the self-launch peak of MOF-5 at about 430 nm. In comparison with the emission peak of CH3NH3PbBr3 QDs and MOF-5, CH3NH3PbBr3@MOF-5 composites (quantum yield 37.49%) show the main emission peak at 533 nm and one minor broad band at about 428 nm, which can be assigned to the band-edge emission of CH3NH3PbBr3 and MOF-5 emission, respectively. The weak PL emission at ∼430 nm is related to the fact that the QD emission is much stronger than the MOF-5 emission. It also can be seen that a small red shift of the main emission maxima from 521 to 533 nm occurs, which might be attributed to the difference in diameters of CH3NH3PbBr3 QDs during the synthesis.26 Additionally, a band edge at 518 nm can be found in the absorption spectrum of CH3NH3PbBr3@MOF-5 composites, which is the same as that of CH3NH3PbBr3 QDs. To further demonstrate the microstructure evolution during this two-step synthesis, SEM, TEM, and high-resolution TEM (HRTEM) images of PbBr2@MOF-5 and CH3NH3PbBr3@ MOF-5 were carried out for comparison, and the results are shown in Figures 3 and 4, respectively. First, as shown in Figure

Figure 4. (a) TEM image of CH3NH3PbBr3@MOF-5. (b) HRTEM image of CH3NH3PbBr3 QDs encapsulated into the MOF-5 host with a typical lattice spacing of 0.32 nm. SEM image (c) and overlapped elemental mapping (d) of CH3NH3PbBr3@MOF-5 microcrystals, and the corresponding elemental distribution of Pb, Br, N, and Zn. (e) N2 adsorption and desorption isotherms of MOF-5 (black) and CH3NH3PbBr3@MOF-5 (red).

3a, we can see that CH3NH3PbBr3 QDs can be obtained on the basis of a similar two-step procedure without the addition of the starting materials of MOF-5. As a comparison, it is found that the addition of PbBr2 did not change the crystalline morphology of MOF-5, as shown by the SEM and TEM images of PbBr2@MOF-5 microcrystals (Figure 3b,c). Moreover, the elemental mapping (Figure 3d−f) showed that PbBr2 was encapsulated into porous MOF-5 microcrystals with a uniform distribution of Zn, Pb, and Br elements. After the introduction of CH3NH3Br solution to PbBr2@MOF-5, the TEM image in Figure 4a,b revealed the phase formation of CH3NH3PbBr3

Table 1. ICP-AES Analysis of MOF-5, PbBr2@MOF-5, and CH3NH3PbBr3@MOF-5 mass % compound

Zn

C

Pb

Br

mole ratio Pb:Br

MOF-5 PbBr2@MOF-5 CH3NH3PbBr3@MOF-5

21.922 21.839 20.915

32.715 33.455 33.773

2.751 2.914

1.838 3.348

1:1.728 1:2.985

C

DOI: 10.1021/acs.inorgchem.8b00355 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

light are also given in the inset of Figure 5, and the green emission can still be clearly observed after 30 days. Such a slow degradation process could be explained in the following section:28 We supposed that there were two stages for the degradation of CH3NH3PbBr3 QDs in MOF-5. In the first stage, the PL intensities of CH3NH3PbBr3@MOF-5 composites changed very little, benefitting from the MOF-5 protective porous structure, which could separate CH3NH3PbBr3 QDs from aggregation and fluorescence degradation in water. Otherwise, CH3NH3PbBr3 would slowly decompose into CH3NH3Br and PbBr2 reversibly in the presence of water (reaction 1). In the second stage, along with the partial decomposition of CH3NH3PbBr3, the equilibrium reaction moves to the right. Then reaction 2 plays a dominant role, and CH3NH3Br will further decompose in aqueous solution.

QDs in the MOF-5 substrate and the HRTEM image verified the high crystallinity of the encapsulated CH3NH3PbBr3 QDs with an interplanar crystal spacing of 3.2 Å, which matches well with the in-plane lattice spacing of the CH3NH3PbBr3 phase in Figure 3a, also demonstrating the successful formation of CH3NH3PbBr3 QDs in MOF-5 microcrystals. Figure 4c,d show the typical SEM image and overlapped elemental mapping of CH3NH3PbBr3@MOF-5 microcrystals, respectively. Moreover, the Pb, Br, and N elements from CH3NH3PbBr3 are equally distributed in the MOF-5 microcrystals, and uniform Zn elements of MOF-5 also suggest the successful formation of the CH3NH3PbBr3@MOF-5 composites. The corresponding pore volume and surface areas from the Barrett−Joyner−Halenda (BJH) method are calculated to be 0.167 cc/g and 522.87 m2 g−1 for the MOF-5 host and 0.164 cc/g and 439.872 m2 g−1 for CH3NH3PbBr3@MOF-5, respectively. It is clear that the pore volume and surface area decrease slightly with the formation of the composite, also suggesting the successful encapsulation of the CH3NH3PbBr3 QDs in MOF-5. Thus, it can be concluded that the two-step procedure can successfully control the encapsulation of the CH3NH3PbBr3 QDs in MOF-5 in combination with TEM, EDS, ICP, and N2 adsorption data mentioned above. Water-Resistance Properties. Due to the photoluminescence sensitivity of the traditional perovskite CH3NH3PbX3 toward moisture,27 we investigated the PL emission property of the as-prepared CH3NH3PbBr3@MOF-5 composites on exposure to water for 30 days to evaluate the stability. Therefore, fluorescence emission spectra (excited under 365 nm light) depending on the length of time in water have been given in Figure 5, and a bar chart of the emission intensities at

H 2O

CH3NH3PbBr3(s) HoooI CH3NH3Br(aq) + PbBr2(s)

(1)

CH3NH3Br(aq) ⇌ CH3NH 2(aq) + HBr(aq)

(2)

Thermal Stability Properties. The results of the temperature-dependent PL properties and thermally induced switching cycles of CH3NH3PbBr3@MOF-5 composites are shown in Figure 6. It can be seen that the photoluminescence of

Figure 6. (a) PL spectra of CH3NH3PbBr3@MOF-5 composites depending on increasing temperature from 30 to 230 °C. (b) Variation of the emission intensity depending on temperature and a linear fitting. (c) Comparison of the fluorescence emission intensities of CH3NH3PbBr3@MOF-5 depending on the heating and cooling temperature. (d) Switching cycles of the emission intensities between 30 and 230 °C.

Figure 5. PL spectra of CH3NH3PbBr3@MOF-5 composites in water for different days (d) (under 365 nm light). The inset shows a bar chart of the emission intensities at different times, as well as photos of CH3NH3PbBr3@MOF-5 solution under 365 nm UV light after 1 and 30 d.

CH 3 NH 3 PbBr3 @MOF-5 at 533 nm can be quenched substantially with increasing temperature from 30 to 230 °C, suggesting a typical thermal quenching behavior (Figure 6a). To quantitatively determine whether the temperature-dependent changes in the emission spectra could be used for accurate temperature sensing, we plotted the variation in logarithmic fluorescence intensity as a function of temperature (Figure 6b). There is a very good linear relationship between the intensity ratio and temperature, which can be fitted as the function

different times has also been provided in the inset. The results show that the CH3NH3PbBr3@MOF-5 composites exhibited good moisture resistance properties even after direct exposure to water for 30 days. It is due to the fact that the porous MOF5 framework, which contributes to the superior stability, can protect the CH3NH3PbBr3 QDs from degradation under a moist atmosphere. Moreover, the representative photos of CH3NH3PbBr3@MOF-5 aqueous solution under 365 nm UV

log I = 4.08 − 0.01T D

(3) DOI: 10.1021/acs.inorgchem.8b00355 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry where I denotes the emission intensity and T is the temperature (°C). The excellent linear relationship with a correlation coefficient (R2) of 0.9944 enables the CH3NH3PbBr3@MOF-5 composites to act as an excellent luminescent thermometer within the tested temperature range from 30 to 230 °C. Hence, the CH3NH3PbBr3@MOF-5 composite can be used as a ratiometric thermometer for temperature monitoring. Interestingly, the photoluminescence intensity of CH3NH3PbBr3@ MOF-5 composites increased again with decreasing temperature from 230 to 30 °C (Figure 6c). This means that there is a reversible process depending on the heating and cooling temperature, and the degraded emission can recover again with a small degradation rate of 3.6%. To further evaluate the reversibility of the CH3NH3PbBr3@MOF-5 composites, temperature-dependent emissions were measured between 30 and 230 °C for seven cycles (Figure 6d), and it was found that the emission intensities at different temperatures for each cycle hardly change, demonstrating the excellent reversibility of this system. In addition, the as-prepared CH3NH3PbBr3@MOF-5 luminescent thermometers have distinct advantages over other conventional thermometers because of their fast response and excellent reusability. Thermogravimetric analyses (TGA) of the as-prepared MOF-5, PbBr2@MOF-5, and CH3NH3PbBr3@MOF-5 samples under atmospheric conditions are comparatively demonstrated in Figure 7a. For MOF-5, there exist four stages for the weight

states of the QD particles.20,29 Therefore, a possible luminescence quenching mechanisms is given in Figure 7b, considering the thermally activated trapping processes involved in the pre-existing trap states. 30 In comparison to CH3NH3PbBr3 perovskite QDs, the conduction band edge of MOF microcrystals is more negative than the flat band potential. Thus, the low-lying conduction band edge of MOF-5 results in an efficient charge storage.31 Hence, when the encapsulated CH3NH3PbBr3 QDs are excited under 365 nm irradiation, a luminescence quenching effect occurs due to the formation of surface states or defects ascribed to the lattice atom mismatch. Meanwhile, a large energy band offset forms, and then the excited electrons will most likely transfer to the empty conduction band of MOF-5 with elevating temperature. Upon the cooling process, the thermally created traps in MOF5 relax back and the original configuration of CH3NH3PbBr3 is restored. Overall, the PL emission intensity depending on variation in temperature can be preserved by using MOF-5 as the host material. Selectivity and Sensing Properties. Due to its good water resistance and thermal stability, CH3NH3PbBr3@MOF-5 may possess potential as a fluorescent probe for metal ions in aqueous media. First, the effect of pH value and different heavy metal ions on the fluorescence emission intensities of the CH3NH3PbBr3@MOF-5 composites were investigated, as shown in Figure 8. Except for the strong acid (pH 1) and

Figure 7. (a) TGA curves of MOF-5, PbBr2@MOF-5, and CH3NH3PbBr3@MOF-5 and DSC curve of CH3NH3PbBr3@MOF-5 under atmospheric conditions. The decomposition temperature (Td) is found to be 483 °C. (b) Schematic diagram of the proposed mechanism of reversible quenching under 365 nm in the CH3NH3PbBr3@MOF-5 composites.

Figure 8. (a) PL spectra of CH3NH3PbBr3@MOF-5 composites at different pH values. The inset shows the peak emission intensities depending on pH values. (b) Comparison of different metal ion selectivities of CH3NH3PbBr3@MOF-5 composites in aqueous solutions.

strong base (pH 13) conditions, the emission intensities had relatively high values over a wide pH range (from pH 3 to pH 11), suggesting its broad range of applications in aqueous solution (Figure 8a), and the inset gives a plot of the luminescence intensities versus pH value with fitting of Gaussian distribution; a pH 7.0 buffer was selected as the optimum condition used in our following study. Accordingly, the effects of different metal ions on the fluorescence emission intensities of CH3NH3PbBr3@MOF-5 were investigated (Figure 8b). The same quantity (4 mg) of the as-prepared composite was dispersed into different aqueous solutions (2 mL) containing 10−1 M of M(NO3)x aqueous solution (M = Na+, K+, Sr2+, Ba2+, Ca2+, Cd2+, Cu2+, Ni2+, Co2+, Al3+, Fe3+, Bi3+) to investigate the sensing properties. Before their fluorescence intensities were tested, all suspensions were sonicated for 3 min to ensure dispersion. It is clear that the fluorescence intensities of the samples are heavily dependent on the species of metal ions, indicating that the sensing platform shows excellent selectivity toward different metal ions. Particularly, Al3+, Bi3+, Co2+, Cu2+, and Fe3+ exhibit a significant quenching effect on the fluorescence intensity for the

loss at about 120, 210, 270, and 500 °C; as a comparison, there are only two obvious stages for the weight loss of PbBr2@ MOF-5 and CH3NH3PbBr3@MOF-5. It is clearly found that CH3NH3PbBr3@MOF-5 possessed good thermal stability with high decomposition temperature in comparison to the others, and the first weight loss platform at about 200−300 °C should be associated with the loss of the organic matter coating layer, which can also explain the fast degradation of emission intensity beyond 230 °C in Figure 6 mentioned above. However, the observed total weight loss of about 99.7% at around 500 °C should be ascribed to the decomposition of the intrinsic metal organic framework of MOF-5. As also demonstrated by the differential scanning calorimetry (DSC) data of CH3NH3PbBr3@MOF-5 composites, an obvious exothermic peak centered at about 483 °C exists, corresponding to the asobserved sharp weight loss during this temperature range, viz. the decomposition temperature (Td). As mentioned above, reversible fluorescence quenching and recovery processes were observed in the CH3NH3PbBr3@ MOF-5 system, and they should be related to the surface defect E

DOI: 10.1021/acs.inorgchem.8b00355 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

surface ligands of methylamino groups (CH3NH3+) in the encapsulated CH3NH3PbBr3 QDs.35 For Cd2+ ions, the PL intensity gradually enhanced with increasing concentration of Cd2+ ions from 2 × 10−3 to 100 × 10−3 M (Figure 9b), which is different from the effect of Cu2+ ions. On the basis of the S−V fitting (Figure 9d), the curve gradually deviated from linearity to nonlinearity with increasing concentration, and the curve can be fitted to F0/F = 1.188− 0.286 exp(−[C]/7.478) + 0.024 exp([C]/46.224) with a correlation coefficient (R2) of 0.9924. Furthermore, a linear curve can be well fitted to F0/F = 0.959 − 0.019[C] with a correlation coefficient (R2) of 0.9928 at low Cd2+ ion concentrations ((2−10) × 10−3 M), as given in the inset of Figure 9d. This enhancing phenomenon of Cd2+ indicates that the luminescence enhancing effect can be attributed to a combination of dynamic and static enhancement, which may be related to the stabilization effect of the CH3NH3PbBr3@MOF5 composites originating from the Cd2+ addition.36,37 Therefore, we consider that the quenching and enhancing effects of Cu2+ and Cd2+ on the CH3NH3PbBr3@MOF-5 mentioned above are controlled by different mechanisms, and the result also reveals that CH3NH3PbBr3@MOF-5 composites could be used as stable and versatile fluorescent sensors for detecting different metal ions in aqueous solutions.

CH3NH3PbBr3@MOF-5 composites used. Especially, as a comparison, the fluorescence emission intensity of CH3NH3PbBr3@MOF-5 increased with the addition of Cd2+ ion. It is proposed that the interactions between the Cd2+ ions and organic ligands or coordinated solvent molecules, which strengthen the stability of the composites, eventually lead to an enhanced luminescence intensity in aqueous solution.32 It is also found that the composites are stable with invariable phase structures after completion of the corresponding fluorescence sensing tests. Herein, we have selected two metal ions, Cu2+ and Cd2+, as examples to study the fluorescence sensing properties. PL spectra of CH3NH3PbBr3@MOF-5 composites depending on different Cu2+ and Cd2+ contents are demonstrated in Figure 9a,b, and the plots of F0/F versus the corresponding

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CONCLUSIONS In summary, we developed a two-step approach to encapsulate perovskite CH3NH3PbBr3 QDs into a MOF-5 matrix, as an effective way to improve the stability of luminescent perovskite QDs. CH3NH3PbBr3@MOF-5 composites also featured excellent water resistance, high thermal stability, and stable photoluminescence over a wide pH range. CH3NH3PbBr3@ MOF-5 composites might offer several advantages for future applications in thermometry and also as stable fluorescence probes for the detection of Al3+, Bi3+, Co2+, Cu2+, Fe3+, and Cd2+ ions in aqueous solution due to their different quenching luminescence performances. The potential fluorescence sensing mechanism for Cu2+ and Cd2+ has also been elucidated. The strategy of encapsulating CH3NH3PbBr3 QDs in MOFs opens up a new pathway for designing novel composite materials with intriguing luminescence properties and high stability for various expected applications.

Figure 9. PL spectra of CH3NH3PbBr3@MOF-5 composites depending on various Cu2+ ion concentrations (a) and Cd2+ ion concentrations (b). Stern−Volmer plots for the F0/F values and different Cu2+ ion contents (c) and Cd2+ ion contents (d). The inset shows the Stern−Volmer plot for the F0/F values and Cd2+ ions at low concentrations.

concentrations are shown in Figure 9c,d. The effect of Cu2+ ions on the luminescence intensity of CH3NH3PbBr3@MOF-5 is shown in Figure 9a. Depending on Cu2+ concentration in the range from 20 × 10−5 to 200 × 10−5 M, the luminescence intensity was significantly quenched by the addition of Cu2+ ion, which was also found to be concentration dependent. Therefore, this system can be used as a sensitive and selective media for Cu2+ ion detection. The luminescence quenching data were further analyzed by the Stern−Volmer (S−V) equation, as shown in Figure 9c. The Stern−Volmer relationship is given by the equation33

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AUTHOR INFORMATION

Corresponding Author

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

Quanlin Liu: 0000-0003-3533-7140 Zhiguo Xia: 0000-0002-9670-3223 Notes

The authors declare no competing financial interest.

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F0 = 1 + K sv[C] (4) F where F0 and F represent the luminescence intensities of CH3NH3PbBr3@MOF-5 in the absence and presence of Cu2+ ion, respectively. [C] is the concentration of Cu2+, and Ksv is a Stern−Volmer constant. As shown in Figure 9c, the S−V curve for Cu2+ ion can be perfectly fitted to F0/F = 1.064 − 0.002[C], with a correlation coefficient (R2) of 0.9843, which indicates the presence of static quenching of the fluorescence in the mechanism of interaction in the system.34 Such a process may be attributed to the interaction between absorbed Cu2+ and the

ACKNOWLEDGMENTS The authors acknowledge the support from the National Natural Science Foundation of China (Nos. 51722202 and 21771031).

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DOI: 10.1021/acs.inorgchem.8b00355 Inorg. Chem. XXXX, XXX, XXX−XXX