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Highly luminescent and stable quasi-2D perovskite quantum dots by introducing large organic cations

Herein, ultra-stable quasi-two-dimensional perovskite quantum dots (quasi-2D PQDs) are synthesized by introducing the butylamine cation (BA+) into the methylamine lead bromide perovskite (MAPbBr3). By reducing the dimensionality of the perovskite structure, the quasi-2D perovskite (BA)2(MA)x−1PbxBr3x+1 presents higher luminescence efficiency and better environmental stability than traditional 3D perovskites, which is mainly because the dimensionality-reduced perovskite has higher exciton binding energy and formation energy. Under an optimal MA : BA ratio of 1 : 1, the quasi-2D perovskite exhibits about four times higher luminescence efficiency (PLQY = 49.44%) than pristine MAPbBr3; meanwhile it emits stable luminescence in an environment with 80% humidity for 50 days. Most importantly, carbon quantum dot (CQD) doping has also been applied in this work, which effectively passivates the defects of (BA)2(MA)x−1PbxBr3x+1via H-bond interaction, further improving the stability of the perovskite in water. Inspired by the superior performances of the proposed quasi-2D nanomaterial, a novel colorimetric method based on halide ion exchange has been developed for H2O2 detection, which also demonstrates that PQDs show significant potential in the field of environmental monitoring.


Introduction
Nowadays, perovskite quantum dots (PQDs) have emerged as promising light emitters due to their extraordinary properties such as high photoluminescence quantum yield (PLQY), 1,2 tunable emission wavelength, 3,4 and facile preparation. 5,6 Various PQDs with different light emissions have rapidly risen to the forefront in the areas of optical and electrical devices including light-emitting diodes (LEDs), 7,8 photodetectors, 9,10 and photovoltaics. 11,12 However, PQD research still encounters great challenges, for example (i) difficulty in the deep understanding of the physical and chemical properties of PQDs; (ii) the poor stability of PQDs; (iii) the limited applications of PQDs.
Owing to their ionic nature and low formation energy, PQDs are easily degraded when they are exposed to oxygen and moisture. 13,14 Therefore, to prevent PQDs from being corroded by air and water, hydrophobic coating is a desirable choice. 15,16 Fan's group presented air stable perovskite@polymer nano-bers, formed by covering methylammonium lead halide perovskite (MAPbBr 3 ) with poly(vinylidene uoride) (PVDF) and polymethyl methacrylate (PMMA), which can emit intensive light under humid conditions (humidity $ 60%) and even in water for a month. 17 Additionally, in our previous work, we have synthesized a metal oxide coated MAPbBr 3 @SiO 2 composite, which shows ideal stability in water for more than 9 months. 18 Although hydrophobic-coating has obviously enhanced the stability of perovskites in water, actually, this method is complicated and time-consuming. Thus, it is signicantly desirable to pursue a simple and efficient strategy for improving the stability and performance of PQDs.
It has been reported that steric hindrance has direct effects on the optical bandgap, formation energy and hydrophobicity of PQDs. 19,20 Several methods, such as developing lowdimensional perovskites, have been employed to improve the steric hindrance of PQDs. 21,22 Low-dimensional perovskites such as two-dimensional (2D) and one-dimensional (1D) perovskites possess large steric hindrance, which display higher formation energy and better oxygen-water stability than threedimensional (3D) materials. 23,24 Unique quasi-2D perovskites are obtained by inserting large organic cations into the 3D perovskite lattice, which possess the merits of both 2D and 3D materials in terms of high stability and good luminescence efficiency. 25,26 For instance, Ning and co-workers introduced phenethylamine (PEA) into CsPbBrCl 3 to synthesise a quasi-2D perovskite, which exhibited high stability and luminescence. 27 Kim's group inserted a butylamine cation (BA + ) into the MAPbBr 3 structure under acidic conditions to synthesize a (BA) 2 (MA) xÀ1 Pb x Br 3x+1 quasi-2D perovskite, presenting longterm stability when exposed to moisture. 28 It is worth noting that the method of preparing highly stable quasi-2D perovskites by reducing the dimension is effective and promising, but it still faces some drawbacks such as the complicated operating process and the harsh acidic reaction conditions. Thus, devising a fast and mild method to obtain stable and bright quasi-2D perovskites is essential.
To solve this problem, a novel quasi-2D (BA) 2 (MA) xÀ1 Pb x -Br 3x+1 quantum dot perovskite is prepared by reducing the dimension of the 3D MAPbBr 3 perovskite using BA cations as spacers in an organic environment (Scheme 1). Here, BA layers act as "barriers" and inorganic layers serve as "wells", so that the generated excitons are conned in the multiple quantumwell structures and thus produce superior uorescence (FL). 29 The FL performance of the proposed quasi-2D perovskite is tested, and it exhibits about four times higher FL efficiency (PLQY ¼ 49.44%) than MAPbBr 3 ; meanwhile, it is stable under humid conditions (humidity $85%) for 50 days. Signicantly, carbon quantum dots (CQDs) are further doped in the above quasi-2D perovskite. The hydrogen-bond interaction between the CQDs and perovskite endowed CQDs-(BA) 2 (MA) xÀ1 Pb x Br 3x+1 with superior water stability; specically, the CQDs doped quasi-2D material can emit strong light in water for 3 days. 30 As a proof of concept, CQDs-(BA) 2 (MA) xÀ1 Pb x Br 3x+1 with outstanding stability is applied in the eld of colorimetric assay, 31 which effectively expands the application area of PQDs.
MABr was synthesized by stirring 5 mL of MA, 5 mL of HBr and 10 mL of ethanol in an ice bath for 2 h. Then the solution was heated at 60 C to obtain the crude MABr. The white powder was washed with ethyl ether several times, and then dried in a vacuum oven at 45 C to acquire the puried MABr. Meanwhile, BABr was synthesized using the same method as described above.
Synthesis of (BA) 2 (MA) xÀ1 Pb x Br 3x+1 QDs As shown in Scheme 1(A), the precursor solutions were prepared by dissolving PbBr 2 , MABr, and BABr in stoichiometric ratios in DMF. The Pb 2+ molar concentration was 0.25 M in the solution. The MA : BA ratios were controlled by changing MABr and BABr contents. Briey, 1 mmol PbBr 2 was rstly dissolved in 4 mL DMF (including 50 mL OAm and 100 mL OA). Then, different ratios of MABr and BABr were added into the above solution, the molar ratios of MABr : BABr were 1 : 0, 1 : 0.5, 1 : 1, 1 : 2, 1 : 4, and 0 : 1, respectively. The total concentration of Br À was controlled to be 0.25 M. Aer that, the mixed solution was stirred at 100 C for 1 h and nally a light yellow precursor was obtained. Aer that, 200 mL precursor samples with different MA : BA ratios were added into 4 mL ethyl acetate, respectively. The generated yellow-green sediment was washed several times with ethyl acetate by centrifugation. Finally, the puried (BA) 2 (MA) xÀ1 Pb x Br 3x+1 QDs with different MA : BA ratios were stored in ethyl acetate when not used.

Synthesis of CQDs
The CQD solution was synthesized as reported in our previous work. 18 As illustrated in Scheme 1(B), the mixture of CQDs-PbBr 2 powder was prepared by heating 1 mL CQD colloid and 1 mmol PbBr 2 at 100 C for 1 h (Fig. S1 †). Aer that, the CQDs-PbBr 2 powder, 0.5 mmol MABr, and 0.5 mmol BABr (MA : BA ¼ 1 : 1) were dissolved in 4 mL DMF and stirred at 100 C for 1 h to generate a precursor solution. Subsequently, 200 mL precursor was added into 4 mL ethyl acetate and then washed with the same solvent several times. Finally, the puried CQDs-(BA) 2 (-MA) xÀ1 Pb x Br 3x+1 with blue-green emission was obtained.

Results and discussion
The X-ray diffraction (XRD) patterns of the perovskites with different MA : BA ratios are provided in Fig. 1(A). Curves a-f are the XRD patterns of MAPbBr 3 and (BA) 2 (MA) xÀ1 Pb x Br 3x+1 with Scheme 1 Schematic illustration of (A) the synthesis process of (BA) 2 (MA) xÀ1 Pb x Br 3x+1 QDs; (B) preparation process of CQDs-(BA) 2 (-MA) xÀ1 Pb x Br 3x+1 ; and (C) schematic representation of metal halide perovskites with 3D and quasi-2D structures. different MA : BA ratios, and BA 2 PbBr 4 , respectively. All materials present clear diffraction peaks which indicate the high crystallinity of these perovskites. With the increase of BA content, the XRD patterns of perovskite materials gradually change from 3D MAPbBr 3 to 2D BA 2 PbBr 4 , indicating that the introduction of BA transformed the material into a quasi-2D structure.
Simultaneously, the UV-vis absorption spectra are provided to further clarify the inuence of BA introduction on the structures of the perovskites. When the BA cation is inserted into the MAPbBr 3 lattice, the number of lead halide octahedron layers (n) decreases, which is determined from the amount of BA introduced. The smaller the n-value of the perovskite, the larger the band gaps of the material. As shown in Fig. 1(B), MAPbBr 3 (n ¼ N) exhibits a clear characteristic peak at 513 nm (curve a), which correlates with the reported band edge. 32 With increasing amount of BA cation, new UV absorption peaks of (BA) 2 (MA) xÀ1 Pb x Br 3x+1 are generated and gradually blue shied (curves b-e), which indicates that introducing BA will reduce the number of inorganic layers, thereby generating a variety of quasi-2D perovskites with different band gaps. As a control, the UV-vis peak of 2D BA 2 PbBr 4 has been observed at about 408 nm (curve f), which is quite different from that of (BA) 2 (MA) xÀ1 -Pb x Br 3x+1 , further proving that the (BA) 2 (MA) xÀ1 Pb x Br 3x+1 perovskites have a quasi-2D structure. 33 The corresponding FL emission spectra are presented in Fig. 1(C). The main emission peak of each perovskite exhibits a signicant blue shi when the MA : BA ratio changes from 1 : 0 to 0 : 1 (curves a-f). The change of FL peaks could be mainly attributed to the increase of perovskite band gap. When MA : BA ¼ 1 : 4, the FL performance of the quasi-2D material (curve e) is similar to that of 2D BA 2 PbBr 4 (curve f), which indicates that excessive BA cation will lead to the transformation of the quasi-2D perovskite structure into a 2D perovskite structure.
The PLQYs of the perovskites are provided in Table 1. The prepared MAPbBr 3 shows a low PLQY of 13.70%, which may be attributed to the large amount of defects on its surface. Aer the insertion of BA, the PLQYs of quasi-2D perovskites are signicantly increased (b-d). Notably, under the optimal MA : BA ratio of 1 : 1, the quasi-2D perovskite displays the highest PLQY value of 49.44%, which indicates that the generated quasi-2D perovskite will greatly conne the generated excitons, thereby improving the luminescence property of the materials. Besides, continuously increasing the amount of BA causes the perovskite structure to transform from quasi-2D into 2D, which leads to a decrease in PLQY (e and f).
As shown in Fig. 1(D), the time-resolved FL decay measurements of the perovskites are provided to conrm the improvement of radiative recombination. Compared with the decay time of MAPbBr 3 (curve a), the lifetimes of the BA-introduced perovskites are signicantly increased until the MA : BA ratio is 1 : 2 (curves b-d), which indicates that BA cations ll the defects of the perovskites. Longer the decay time of the perovskite, lesser the traps and defects in the material. However, the lifetime of the quasi-2D perovskite with MA : BA ¼ 1 : 4 (curve e) is close to that of 2D BA 2 PbBr 4 (curve f), further proving that these two materials have similar structures. The decay curves are tted by a double-exponential function, and the corresponding data are listed in Table S1. † Under the optimal MA : BA ratio of 1 : 1, the quasi-2D perovskite (BA) 2 (MA) xÀ1 Pb x Br 3x+1 (1 : 1) exhibits ideal luminescence performance. Herein, this perovskite has been used as a model to investigate the stability of quasi-2D materials. Compared with the 3D perovskite MAPbBr 3 (shown in Fig. S2 †), the quasi-2D material presents obvious improved environmental stability. The air stability of (BA) 2 (MA) xÀ1 Pb x Br 3x+1 (1 : 1) has been provided in Fig. 2(A) and (B). Aer storage for 50 days, the XRD patterns do not change signicantly, while the FL intensity of the material decreases to 65% of the initial intensity. Surprisingly, it has been noticed that CQD doping can effectively improve the stability of perovskites, which is mainly attributed to the hydrogen-bond interaction between CQDs and perovskites. As shown in Fig. 2(C) and (D), the XRD patterns of CQDs-(BA) 2 (MA) xÀ1 Pb x Br 3x+1 (1 : 1) almost remained unchanged aer a long period of storage, and the FL intensities did not decrease over 30 days. Also, CQDs-(BA) 2 (MA) xÀ1 Pb x Br 3x+1 (1 : 1) displays unexpected water stability for 3 days as shown in Fig. S3, † indicating that the doping of CQDs is indeed benecial to improve the environmental stability of the perovskite. Additionally, we have compared the material in this work with other  quasi-2D perovskites in terms of stability, further demonstrating the excellent stability of CQDs-(BA) 2 (MA) xÀ1 Pb x -Br 3x+1 (1 : 1) (Table S2 †). [34][35][36][37] It has been noticed that CQD doping caused the PLQY of the quasi-2D perovskite to decrease from 49.44% to 39.25%, as shown in Fig. S4. † Here we infer that the reason for the decrease in PLQY is energy transfer between the CQDs and perovskite. The related representations of (BA) 2 (MA) xÀ1 Pb x Br 3x+1 (1 : 1) and CQDs-(BA) 2 (MA) xÀ1 Pb x Br 3x+1 (1 : 1) are provided to support this conjecture (Fig. 3). The XRD and UV-vis characterizations of the perovskites are provided in Fig. 3(A) and (B). The corresponding spectra of the CQD-doped perovskite (curve b) are quite close to those of the pristine quasi-2D material (curve a), which indicates that CQD doping will not affect the intrinsic structure of the perovskite. As shown in Fig. 3(C), the FL emission of CQDs-(BA) 2 (MA) xÀ1 Pb x Br 3x+1 (1 : 1) is signicantly blue-shied, which may be attributed to the fact that CQD-doping decreases the particle size of the quasi-2D perovskite. Finally, a slight decrease in the FL lifetime of CQDs-(BA) 2 (MA) xÀ1 Pb x Br 3x+1 (1 : 1) is observed in Fig. 3(D), indicating that an energy transfer process has occurred between the conductive CQDs and the perovskite, which causes decrease in the PLQY of the perovskite.

Applications of the quasi-2D perovskite
Different from water-unstable perovskites such as CsPbBr 3 and MAPbBr 3 , the proposed quasi-2D perovskite shows ideal stability in humid and even aqueous environments, making it promising in the eld of FL analysis. Herein, the FL emission of CQDs-(BA) 2 (MA) xÀ1 Pb x Br 3x+1 (1 : 1) is gradually red-shied in the presence of I À due to a halide exchange reaction, the degree of which depends on the content of I À . As shown in Fig. 5, the   FL emission of CQDs-(BA) 2 (MA) xÀ1 Pb x Br 3x+1 (1 : 1) changes from green to red with increasing amount of I À . In the presence of H 2 O 2 , I À will be oxidized by H 2 O 2 , which causes the degree of FL red-shi to decrease. Thus, based on the halide exchange reaction of CQDs-(BA) 2 (MA) xÀ1 Pb x Br 3x+1 (1 : 1) and the redox reaction between I À and H 2 O 2 (shown in formulae (1) and (2)), a simple and rapid colorimetric method is developed for semiquantitative detection of H 2 O 2 , which extends the application of perovskite QDs in the eld of FL detection, especially in an aqueous environment.

Conclusions
In this work, unique quasi-2D perovskite quantum dots are synthesized by introducing a large organic cation BA into the MAPbBr 3 structure, which not only avoids the drawback of poor stability of traditional 3D perovskites, but also demonstrates the strong luminescence of low-dimensionality perovskites. What's more, the nontoxic and carboxyl-rich CQDs are utilized to passivate the defects of the above perovskite, as a result of which the air and even water stability of the quasi-2D perovskite was further improved. As a proof of concept, the obtained highly luminescent and stable CQD-doped quasi-2D perovskite is successfully applied to develop a novel colorimetric method for monitoring H 2 O 2 . So this work not only provides a novel strategy for developing highly luminescent and stable perovskite materials but also shows great potential in the eld of environmental monitoring.

Conflicts of interest
There are no conicts to declare.