Ultrastable Carbon Quantum Dots-Doped MAPbBr3 Perovskite with

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Ultrastable Carbon Quantum Dots-Doped MAPbBr3 Perovskite with Silica-Encapsulation Jingxi Wang, Ming Li, Wei Shen, Wei Su, and Rongxing He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b12058 • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on August 28, 2019

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ACS Applied Materials & Interfaces

Ultrastable Carbon Quantum Dots-Doped MAPbBr3 Perovskite with Silica-Encapsulation Jingxi Wang,†,‡ Ming Li,† Wei Shen,† Wei Su,,‡ Rongxing He,†

†

Key Laboratory of Luminescent and Real-Time Analytical Chemistry (Southwest University）, Ministry of Education, C

‡ Guangxi Key Laboratory of Natural Polymer Chemistry and Physics, Guangxi Teachers Education University, Nanning 530001, PR China

Corresponding

author E-mail addresses: [email protected] (W. Su); [email protected] (R. X. He). 1

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ABSTRACT Having suffered from the intrinsic structural lability, perovskite quantum dots (PQDs) are extremely unstable under high-temperature and moisture conditions, which have greatly limited their applications. In this work, we propose a novel method to synthesize ultrastable carbon quantum dots (CQDs) doped methylamine (MA) lead bromide PQDs with SiO2 encapsulation (CQDs-MAPbBr3@SiO2). The kernel CQDs-MAPbBr3 is formed by the interaction of carboxyl-rich CQDs with MAPbBr3 via H-bond, which greatly improves the thermal stability of CQDs-MAPbBr3. Furthermore, highly compact SiO2 encapsulates the proposed CQDs-MAPbBr3 via a facile in situ growth strategy, which effectively enhances the water resistance and air-stability of CQDs-MAPbBr3@SiO2. As a result, the proposed nanomaterial shows extremely high water-stability in aqueous solution for over 9 months and ideal thermal-stability with strong FL emission after 150℃ annealing. Based on the superior stability and ultrahigh FL efficiency of this proposed nanomaterial, a primary sensing method for ions (Ag+ and Zn2+) FL detection has been developed and the mechanism of PQDs-based ions determination has also been discussed, thus exhibits the potential applications of CQDs-MAPbBr3@SiO2 in the area of FL assay and environment monitoring. Keywords: perovskite quantum dots; thermal stability; moisture stability; carbon quantum dots; SiO2

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INTRODUCTION Recently, methylammonium (MA) lead halide perovskite quantum dots (MAPbX3 PQDs, X = Cl, Br, I) are attracting plenty of attention for researchers owing to their fantastic optical properties, such as strong quantum light emission, narrow emission bandwidth and tunable emission wavelength, etc.1-3 These unique properties of PQDs make it stand out from many other photoelectric materials, so it exhibits extensive application prospect in various areas, including solar cells,4,5 light emitting diodes (LEDs),6 photodetectors,7 and FL assays.8 However, these PQDs usually inevitably encounter the biggest frustration of poor stability, which seriously affects the performance of PQDs, resulting in obstacle to expand their industrialization application involving liquid crystal display (LCD) and solid-state lighting.9-11 Thus, it is significantly desirable to pursuit a new strategy for improving the stability and performance of PQDs. In fact, the major challenges for the stability of PQDs include the following two aspects: (i) poor thermal stability of PQDs for their instinct structural lability;12 (ii) chemical instability of PQDs for their low formation energy, which may make them decompose easily in air environment for its reaction with water molecules.13,14 To address these problems, previous researchers have reported some nanocrystals (NC)-doping methods, including SnS doped MAPbI3, PbS doped MAPbI3, which enhanced the thermal stability of PQDs, and these results could be attributed to the improved crystallinity of PQDs materials.15,16 However, these sulfide-based quantum dots suffer from the problems of environmental unfriendliness for their toxicity, thus 3



their applications in stabilizing perovskites are been limited. On the other hand, to solve the chemical instability of PQDs, hydrophobic materials such as polymers and some inorganic materials (TiO2 or SiO2) are chosen as protective layers for PQDs embedding. Dong Y.J. and co-workers synthesized a serious of polymer decorated CsPbX3 composites, which exhibit superior stability and strong FL emission in aqueous solution for more than 50 days.17 Chen C. and his co-workers reported a CsPbBr3 PQDs/ethylene vinyl acetate (EVA) composite with long-time stability. Based on this composite, Chen constructed a green-light LED and claimed that this composite was stable enough to be worked for 1000 cycles.18 In the area of inorganic-encapsulation, Peng H.S. synthesized a highly-stabled silica-coated MAPbBr3 QDs (SMAPB-QDs) through one-pot method, which exhibited great potential in the field of green-light LED.19 Although improved hydrophobicity and oxidation resistance for PQDs are obtained, the insulating polymers may restrict the charge transport of PQDs for further applications. Moreover, the TiO2 or SiO2 prepared in “waterless” environment are mesoporous with poor protection for PQDs.20,21 Therefore, searching for more reliable doped materials or wrapping methods to solve these issues is still an urgent task. In response, we provide a strategy by combining carbon quantum dots (CQDs)-doping and inorganic materials wrapping to prepare an ultrastable perovskite composite of CQDs-MAPbBr3@SiO2, in which CQDs-doped MAPbBr3 are wrapped with SiO2 matrix via one facile in situ growth strategy, as shown in Scheme 1.

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Scheme 1. Schematic illustration of (A) synthesis process of CQDs-MAPbBr3; (B) preparation process of CQDs-MAPbBr3@SiO2; and (C) comparison of CQDs-MAPbBr3 and proposed CQDs-MAPbBr3@SiO2 under air and moisture conditions.

Compared with those sulfide QDs-doped perovskites, carboxyl-rich CQDs are chosen as dopant to prepare the proposed perovskite material. Non-toxic CQDs effectively strengthen the crystallinity of MAPbBr3 due to the H-bond interaction between CQDs and MAPbBr3, greatly improving the thermal stability of CQDs-MAPbBr3.22,23 Moreover, hydrophobic SiO2 as shield are utilized to protect CQDs-MAPbBr3, especially, trace water has been intentionally introduced in the preparation process to enable the generated SiO2 with high compactness for achieving more efficient protection.24 Experimental results also confirm the high stability of proposed CQDs-MAPbBr3@SiO2 as exemplifies strong FL emission still maintains after the proposed perovskite has been treated with high temperature at 150℃, and the perovskite can be stable in water for more than 9 months and retains about 75% of its initial FL intensity. Concluding the advantages of this perovskite, the proposed strategy in our work paves an avenue for improving the stability of other perovskite 5



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materials with ultimate applications in optoelectronics, including photodetectors, LEDs, and solar cells. Notably, we find that Ag+ and Zn2+ could effectively quench the luminescent of the proposed material. As a proof of concept, a simple and sensitive FL sensing system has been constructed for detecting these ions quantitatively, which shows great potential in the field of FL assays and environmental monitoring, and further expands the application of PQDs. RESULTS AND DISCUSSION

Figure 1. TEM images of (A) pure SiO2, (B) CQDs-MAPbBr3@SiO2, (C) partly enlarged of CQDs-MAPbBr3@SiO2, (D) CQDs-MAPbBr3 (insert is a large scale image of CQDs-MAPbBr3).

In

order

to

evaluate

the

morphology

of

the

proposed

nanomaterial,

CQDs-MAPbBr3@SiO2 has been characterized by transmission electron microscope (TEM). As illustrated in Figure 1A, the pure SiO2 bulks presents a cubic morphology

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with smooth surface. Figure 1B presents a large number of PQDs dispersed within the SiO2 bulk material uniformly, which indicates that the PQDs are well encapsulated by SiO2 bulks. Simultaneously, as shown in Figure 1C, the partly enlarged TEM image exhibits the interplanar spacing of 0.12 and 0.25 nm, which relates to the (020) crystallographic plane of CQDs and (210) crystallographic plane of MAPbBr3 QDs, respectively (more details are shown in Figure S1 in Supporting Information). Additionally, the TEM characterization of CQDs-MAPbBr3 is shown in Figure 1D with an average particle size of 5 nm. The UV-vis absorption and FL spectra of MAPbBr3, CQDs-MAPbBr3, MAPbBr3@SiO2 and CQDs-MAPbBr3@SiO2 are compared in Figure 2A and 2B. As shown in Figure 2A, the absorption spectra of these materials are around 515 nm without any detectable changes, which correlates with the reported band edge of MAPbBr3.25 Among them, MAPbBr3@SiO2 (curve c) and CQDs-MAPbBr3@SiO2 (curve d) exhibit stronger absorption intensity compared to that of MAPbBr3 (curve a) and CQDs-MAPbBr3 (curve b), which attributes to the perovskite QDs has been enriched by the SiO2 layer. In addition, as shown in Figure 2B, the FL spectra of CQDs-MAPbBr3 (curve b) and CQDs-MAPbBr3@SiO2 (curve d) are slightly red-shifted compared to that of MAPbBr3 (curve a) and MAPbBr3@SiO2 (curve c) due to perovskite QDs size-changing which causes by CQDs-doping.26 The absolute photoluminescence quantum yields (PLQYs) of these materials are also been detected in this work (shown in Table S1 in Supporting Information). The proposed CQDs-MAPbBr3@SiO2 exhibits ultrahigh PLQY of 95.9%, which demonstrates that 7



CQDs-doping and SiO2-encapsulating could effectively improve the luminescent property of MAPbBr3. The X-ray diffraction (XRD) charaterizations of the nanomaterials are also been investigated. As shown in Figure 2C, curves (a) to (d) are the XRD spectra of MAPbBr3 QDs, CQDs-MAPbBr3 QDs, MAPbBr3@SiO2 and CQDs-MAPbBr3@SiO2, respectively. The XRD spectrum of pristine MAPbBr3 QDs is indistinct and many impurity peaks are obtained (Figure S2 in Supporting Information), which indicates the crystallinity of MAPbBr3 is unsatisfied, and further proves that the pure PQDs is quite unstable (curve a). When the MAPbBr3 is doped by CQDs, the carboxyl groups of CQDs would react with MA part of MAPbBr3 via H-bond, which promotes the crystallinity of PQDs and further increases the stability of CQDs-MAPbBr3 (curve b). As displayed in curve (c) and curve (d), MAPbBr3@SiO2 and CQDs-MAPbBr3@SiO2 both exhibit highly crystallinity with strong and clear XRD peaks at 14.8°, 30.1° and 33.7°, which are consistent with the crystal planes (100), (200) and (210) of MAPbBr3 structure (curve #),27,28 proving SiO2-encapsulating is also beneficial to PQDs stabilization and crystallization. Notably, due to the affection of CQDs-doping, the XRD spectrum of CQDs-MAPbBr3@SiO2 is slight different with that of MAPbBr3@SiO2 (more details are shown in Figure S3 and S4 in Supporting Information). In order to identify the optical property of the proposed materials, time resolved FL spectra are evaluated in this research. As shown in Figure 2D, compared with the FL lifetime of pristine MAPbBr3, the average lifetime of CQDs-MAPbBr3 is slightly 8


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decreased, which demonstrates the doping of conductive CQDs has promoted the rates of exciton-trap recombination and charge-transformation.15,29 Meanwhile, as illustrated in Figure 2E, the in situ generated SiO2 successfully encapsulates PQDs to obtain a size-enlarged composite, which effectively enriches PQDs, resulting to a sharp

increase

of

the

average

lifetimes

for

both

MAPbBr3@SiO2

and

CQDs-MAPbBr3@SiO2 in aqueous solution. The detailed data of FL lifetime are listed in Supporting Information (Table S2).30

Figure 2. (A) UV-vis spectra, (B) FL spectra, and (C) XRD spectra of (a) MAPbBr3 QDs, (b) CQDs-MAPbBr3 QDs and (c) MAPbBr3@SiO2, (d) CQDs-MAPbBr3@SiO2; Time resolved FL spectra of (D) MAPbBr3 QDs and CQDs-MAPbBr3 QDs, and (E) MAPbBr3@SiO2 and CQDs-MAPbBr3@SiO2. 9



To investigate the potential feasibility of the proposed CQDs-MAPbBr3@SiO2 for further application, the stability of these materials under different conditions is studied. As displayed in Figure 3A and 3B, the thermal stability of CQDs-MAPbBr3 (a) and MAPbBr3 (b) has been investigated by annealing in 150°C or leaving in room temperature (RT). Both MAPbBr3 and CQDs-MAPbBr3 exhibit high FL efficiency without any significant differences under RT condition. However, after 5 min annealing, the FL emitting of MAPbBr3 is extremely weak due to its poor thermal stability. On the contrary, the proposed CQDs-MAPbBr3 exhibits superior stability with strong fluorescence after 150°C heating for 30 min due to CQDs passivate the grain boundaries and strengthen the instinct structure of MAPbBr3 via H-bond interaction,23 which indicates that CQDs-doping strategy is an effective way to improve the thermal stability of perovskite (Table S3 and Figure S5 in Supporting Information). Additionally, the stability of MAPbBr3 and CQDs-MAPbBr3 in an open atmosphere has been investigated. After two days storing in the open atmosphere, the CQDs-MAPbBr3 still emitted strong FL emission, while the emission of pristine MAPbBr3 decreased obviously and a trap-generated FL peak had obtained (Figure S6 in Supporting Information). Simultaneously, the water-stability of the proposed CQDs-MAPbBr3@SiO2 in aqueous solution has been researched. As illustrated in Figure 3C and 3D, CQDs-MAPbBr3@SiO2 emits strong fluorescence when being dispersed in aqueous solution, and it remains about 75% of its initial FL intensity even after 9 months, exhibiting superior water resistance and infusive long-term stability than that of the 10


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other previous reported strategies including polymer-coating, SiO2-wrapping, TiO2-wrapping and MOF (metal organic framework)-wrapping, which displays a longest stable time of 12 weeks (Table S4 in Supporting Information).17,31-34 Notably, through the experimental result (Figure 3C), it can be confirmed that the FL intensity tends to be stable at the time of 270 d, and since then no obvious differences has been obtained. Therefore, it can be predicted that this proposed material would remain stable in water for an extended period of time. In addition, a light-stability test of the proposed CQDs-MAPbBr3@SiO2 has been investigated through a UV lamp exposure, and a satisfied result of this test is shown in Supporting Information (Figure S7).

Figure 3. (A) FL spectra of (a) CQDs-MAPbBr3 and (b) MAPbBr3 in RT and after 150℃ annealing; (B) Images of (a) CQDs-MAPbBr3 and (b) MAPbBr3 in RT and after 150℃ annealing (365 nm UV lamp); (C) Stability of CQDs-MAPbBr3@SiO2 in aqueous solution; (D) Images of CQDs-MAPbBr3@SiO2 in aqueous solution for long-time immersing (365 nm UV lamp).

Therefore, encouraged by the superior stability and excellent fluorescence

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efficiency of CQDs-MAPbBr3@SiO2 in aqueous solution, the application of the proposed material in the field of highly sensitive FL chemosensing is investigated. In this research, we notice Ag+ and Zn2+ can effectively quench the FL emission of CQDs-MAPbBr3@SiO2, which inspires us to construct a primary PQDs-based FL sensing system for ions detecting (Figure S8 in Supporting Information). As a result, under the optimal concentration of CQDs-MAPbBr3@SiO2 solution (Figure S9 and S10 in Supporting Information), the proposed FL sensing system has been constructed with a superior sensitivity for Ag+ and Zn2+ detection with wide linear range (Figure 4).

Figure 4. (A) FL responses of CQDs-MAPbBr3@SiO2 with different Ag+ concentrations: 1 μM, 5 μM, 10 μM, 20 μM, 40 μM, 50 μM, 60 μM, 80 μM, 100 μM; (B) the calculated corresponding calibration plot of Ag+ and (C) the corresponding calibration plot of Ag+ by single-point mode; (D) 12


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FL responses of CQDs-MAPbBr3@SiO2 with different Zn2+ concentrations: 100 nM, 200 nM, 250 nM, 300 nM 350 nM, 400 nM; (E) the calculated corresponding calibration plot of Zn2+ and (F) the corresponding calibration plot of Zn2+ by single-point mode.

As illustrated in Figure 4A, the FL intensity decreases with increasing the concentration of Ag+ from 1 μM to 100 μM with a limit of detection (LOD) of 0.683 μM, and the corresponding calibration plot (Figure 4B) indicates an excellent linear relationship between FL intensity and logarithm of Ag+ concentration, the linear regression equation is I = -0.468lgc + 0.99 with a squared correlation coefficient of 0.98. In addition, Figure 4C also depicts the linear relationship of Ag+ concentration which is determinated by a single-point mode of the FL spectrometer (I = -0.465lgc + 1.03). Meanwhile, the FL responses of the sensor with different Zn2+ concentrations are shown in Figure 4D, the FL intensity decreases with the increase of Zn2+ ranges from 100 nM to 400 nM with a LOD of 64.8 nM, and the linear equation is I = -1.67lgc + 4.32 (Figure 4E), which is quite similar to the single-point mode equation of I = -1.71lgc + 4.43 (Figure 4F). How the metal ions quench the FL emission of CQDs-MAPbBr3@SiO2 effctively? Herein, this question can be clearly explained by FL lifetime analyzing.35 As shown in Figure 5A, in the presence of Ag+ (10 μM), the average FL lifetime of CQDs-MAPbBr3@SiO2 solution sharply decreases from 124.9 ns to 74.8 ns, which indicates that Ag+ will react with the excitation state of CQDs-MAPbBr3@SiO2 via energy transfer, resulting in a dynamic quenching and a shortened FL lifetime.36 On a contrary, as shown in Figure 5B, Zn2+ in an ultralow concentration such as 200 nM 13



will chemisorb on CQDs-MAPbBr3@SiO2 surface via Zn-O-Si bond and partly form a Lewis acid-like complex, which destroys the SiO2 shield and then reacts with CQDs-MAPbBr3, resulting in a steady-state quenching and an unaltered FL lifetime.37,38

Figure 5. Time resolved FL spectra of CQDs-MAPbBr3@SiO2 with different ions: (A) Ag+ (10 μM) and (B) Zn2+ (200 nM).

SUMMARY In summary, we develop a new method to synthesize an ultrastable perovskite composite of CQDs-MAPbBr3@SiO2. Non-toxic CQDs strengthen the crystallinity of perovskite and effectively promote its thermal stability as exemplifies at a high temperature of 150°C in air. Moreover, this CQDs-MAPbBr3 is embedded by compact SiO2 to obtain a novel CQDs-MAPbBr3@SiO2 composite with amazing water resistance, which displays ultralong-term stability during immersion in aqueous solution for more than 9 months. Inspired by the unique property of proposed perovskite, we have demonstrated a simple and sensitive sensing system for Ag+ and Zn2+ detecting, which opens a door to access PQDs-based FL assay and further extends the avenue of PQDs applications such as fluorescent materials and solar cells. 14


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Experimental Procedures Reagents and Instruments Lead bromide (PbBr2, 99%), hydrobromic acid (HBr, 40%), oleic acid (OA, 90%), oleylamine (OAm, 90%) and tetraethoxysilane (TEOS, 99%) were brought from Macklin Biochemical Co., Ltd. (Shanghai, China). Methylamine aqueous solution (MA, AR), citric acid (AR) and toluene (AR) were obtained from Taixin Chemical Industry Co., Ltd. (Chongqing, China). The FL emission and FL lifetime spectra were tested by a Florolog-3 Fluorescence Spectrometer (Horiba, USA). The UV-vis spectra were carried out by a TU-1901 UV-vis spectrophotometer (Beijing, China). In addition, CQDs were purified by dialysis membranes (2 K MWCO) from Sangon Biotech Co., Ltd. (Shanghai, China). Preparation of CQDs The CQDs were synthesized as follows. 5 g citric acid powder with 2 mL deionized water were heated at 200℃ for 4 h to obtain yellow CQDs. After that, the CQDs were dispersed into 5 mL deionized water and transported into dialysis membranes (2 K MWCO) at 4℃ for 24 h to obtain the purified CQDs, finally the CQDs solution was kept at 4℃ when not in use. Preparation of CQDs-MAPbBr3 QDs CQDs-MAPbBr3 QDs were prepared according to the method reported previously with a minor modification.15 Firstly, 0.5 mL CQDs solution was dropped into PbBr2 (0.08 g) under a slow stirring for 30 min at 120℃ to acquire CQDs-PbBr2 powder. Then the powder reacted with 0.04 g as-prepared MABr in 10 mL toluene (including 100 μL H2O,39 100 μL OA and 75 μL OAm) with a drastic agitation at room temperature to generate CQDs-MAPbBr3 QDs. After a centrifugation at a speed of 10000 rpm for 5 min, the green-yellow supernate was washed by acetonitrile for several 15



times and was redispersed in 20 mL toluene. Thus the CQDs-MAPbBr3 QDs were prepared. The synthesis of MAPbBr3 has been listed in Supporting Information (Page S-12). Preparation of CQDs-MAPbBr3@SiO2 The CQDs-MAPbBr3@SiO2 was synthesized according to method reported previously with a minor modification.21,40 Briefly, 1 mL TEOS was added in 10 mL CQDs-MAPbBr3 QDs and stirred for 4 h at 40 ℃ . In addition, 100 μL deionized water was introduced in this system intentionally, which could promote the decomposition of TEOS to generate SiO2. While the yellow precipitate was generated, another 0.5 mL TEOS was added and kept stirring for 8 h to ensure the PQDs were encapsulated by SiO2 completely. Finally, the products were washed by toluene for several times until the supernate clear, then the remained precipitate was dried under 120 ℃

and kept at room temperature when not in use. The preparation process of

MAPbBr3@SiO2 has been listed in Supporting Information (Page S-12).

Acknowledgements We are grateful to the Natural Science Foundation of China (91741105), Chongqing Municipal Natural Science Foundation (cstc2018jcyjAX0625) for their financial supports, and the Fundamental Research Funds for the central Universities (Grant No. SWU116043). Conflicts of interest The authors declare no conflict of interest.

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Ultrastable Carbon Quantum Dots-Doped MAPbBr3 Perovskite with

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