Cs4PbX6 (X = Cl, Br, I) Nanocrystals: Preparation, Water-Triggered

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Cs4PbX6 (X = Cl, Br, I) Nanocrystals: Preparation, Water-Triggered Transformation Behavior, and Anti-Counterfeiting Application Xiaoya Yu, Linzhong Wu, Huicheng Hu, Min Chen, Yeshu Tan, Di Yang, Qi Pan, Qixuan Zhong, Thidarat Supasai, and Qiao Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01683 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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Cs4PbX6 (X = Cl, Br, I) Nanocrystals: Preparation, Water-Triggered Transformation Behavior, and Anti-Counterfeiting Application Xiaoya Yu,†,§ Linzhong Wu, †,§ Huicheng Hu,† Min Chen, † Yeshu Tan, † Di Yang,† Qi Pan,† Qixuan Zhong,† Thidarat Supasai, ‡,* Qiao Zhang†,*

†

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-

Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, P. R. China. ‡

Department of Materials Science, Faculty of Science, Kasetsart University, 50 Ngam Wong

Wan Rd, Lat Yao, Chatuchak Bangkok 10900 Thailand.

ACS Paragon Plus Environment

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ABSTRACT

As a promising material, Cs4PbX6 (X = Cl, Br, I) nanocrystals (NCs) have attracted much attention. However, their luminescent property is still under debate. In this work, we first systematically studied the colloidal preparation of Cs4PbX6 NCs. It is found that the critical parameter for the formation of Cs4PbX6 NCs is the ratio between Cs and Pb. Pure Cs4PbX6 NCs are non-luminescent. The luminescence property of previous reported Cs4PbX6 NCs may come from the impurity of luminescent CsPbX3 NCs. No co-existence of both Cs4PbX6 and CsPbX3 phase has been found in one single nanoparticle. The water-triggered transformation from nonluminescent Cs4PbX6 NCs to luminescent CsPbX3 NCs has been quantitatively studied. The potential application of Cs4PbX6 NCs in humidity sensor and anti-counterfeiting have been demonstrated. This work is important because it not only confirmed the non-luminescent nature of Cs4PbX6 NCs but also demonstrated the potential application of such NCs.


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INTRODUCTION

Over the past several years, cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite nanocrystals (NCs) have drawn much attention because of their excellent photophysical properties, including high photoluminescence quantum yield (PLQY), tunable emission wavelength, narrow emission bandwidth, and so on.1-3 Since the first report on the hotinjection synthesis of CsPbX3 NCs by Kovalenco et al. in 2015,4 CsPbX3 NCs with different morphologies have been prepared through diverse approaches, such as ion exchange,5,

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room-temperature

reprecipitation,7

microwave

assisted

synthesis,8

solvothermal,9 ultrasonication,10 post-treatment,11-14 and CVD method.15,

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More

importantly, the potential applications of such NCs have been demonstrated in several fields, including photodetectors,17, 18 solar cells,19-21 sensors,22 and light emitting diodes (LEDs).23, 24 To date, some great achievements have been made. For example, the power conversion efficiency (PCE) of solar cell based on CsPbI3 quantum dots has been raised to 14.4%.25 The external quantum efficiency (EQE) of LEDs using CsPbBr3 quantum dots as the emitting layer has reached a record of 11.6%.26

Besides CsPbX3 NCs, some other ternary compounds, e.g., CsPb2X523,

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and

Cs4PbX611-14 NCs, have also been investigated. For example, Yu and co-workers reported the successful preparation of CsPb2X5 nanoplates with excellent photoluminescence property.28 Zhang and co-workers prepared Cs4PbX6 NCs, which also exhibit bright photoluminescence.30 Recently, Cs4PbX6 plates with size of micrometres have also been reported.31 Although much progress has been achieved in this field, many questions are not well resolved yet. For instance, there is a debate on the luminescence property of


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Cs4PbX6 NCs. It has been reported by several groups that different from the threedimensional (3D) crystal structure of cubic or orthorhombic CsPbX3 NCs, CsPb2X5 and Cs4PbX6 NCs are categorized as two-dimensional (2D) and zero-dimensional (0D) phase, respectively.32-34 As a result, it has been reported that Cs4PbX6 NCs are not photoluminescent, and the luminescence originates from the impurity of CsPbX3.12, 14

Recently, the nonluminescent nature of Cs4PbX6 NCs has been verified partially by the transformation between Cs4PbX6 and CsPbX3 NCs. It has been reported that highly luminescent CsPbX3 NCs can be prepared by treating Cs4PbX6 NCs with additional PbX2, in which the Cs4PbX6 NCs can be regarded as a Pb-deficit structure.12 Furthermore, a reversible transformation between Cs4PbX6 and CsPbX3 can be achieved through a mild thermal annealing if a proper amine is used.35 The luminescent CsPbX3 NCs can be converted to nonluminescent Cs4PbX6 NCs through a ligand-assisted transformation process.13 Our group recently found that monodisperse CsPbX3 NCs with bright luminescence could be obtained by treating Cs4PbX6 NCs dispersed in non-polar solvent (such as hexane, cyclohexane, and chloroform) with water.14 Additionally, this interfacial reaction has been employed to prepare Janus CsPbX3/oxide NCs with high stability.36 Similar to the water treatment process where Cs4PbX6 NCs are regarded as a CsX-rich structure, CsPbX3 NCs can be prepared by using Prussian blue as a capture of CsX.11 Despite the improved understanding, there are still many challenges. For example, if the luminescence of Cs4PbX6 NCs is due to the presence of CsPbX3 NCs, how do the impurities co-exist with the major component? In addition, a systematic study on the preparation of pure Cs4PbX6 NCs and the transformation between Cs4PbX6 and CsPbX3 NCs is still needed.


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In this work, two different approaches, which denoted as one-pot and two-step synthesis, have been developed to figure out the optimum condition for the preparation of pure Cs4PbX6 NCs. In the one-pot synthesis, the precursors with different ratios of Cs to Pb were mixed and injected at certain temperature to get the product. In contrast, in the twostep synthesis, CsPbX3 NCs were prepared first, followed by the addition of excessive CsOL. Both two methods have confirmed that pure Cs4PbX6 NCs are nonluminescent. No evidence has been found for the co-existence of CsPbX3 and Cs4PbX6 in a single nanoparticle. A systematic and quantitative research has been conducted to study the water-triggered transformation velocity of Cs4PbX6 to CsPbX3. The potential application in humidity sensor and anti-counterfeit material of Cs4PbX6 has also been demonstrated. EXPERIMENT SECTION Materials Chemicals: Cesium carbonate (99.998%), lead halide (PbX2, ultradry, 99.999%), oleic acid (OA, tech. 90%), oleylamine (OAm, 80-90%), 1-octadecene (ODE, tech. 90%) were obtained from Alfa Aesar. Cyclohexane was purchased from Tokyo Chemical Industry. All chemicals were used as received without any further purification. Synthetic procedures Preparation of cesium-oleate solution. 0.16 g Cs2CO3 (0.49 mmol), 1 mL OA and 16 mL ODE were added into a 50 mL three-neck flask, stirred for 30 min under vacuum at 120 o

C, and then heated to 150 oC under N2 until cesium-oleate (CsOL) solution formed.

Preparation of samples with different Cs to Pb ratios. In a typical synthesis, OAm (1 mL), OA (1 mL), ODE (10 mL) and PbBr2 (0.2 mmol) were added into a 25 mL threeneck flask and stirred under vacuum for 30 min. Then the reaction was heated to 140 oC,


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and hot CsOL solution (150 oC) with volume of 1.2, 1.6, 2.4, 3.2, 4.4 mL (the molar ratio of CsOL and PbBr2 was 0.375, 0.500, 0.750, 1.000, 1.375, respectively) was rapidly injected into the PbBr2 solution. 7 seconds later, the reaction mixture was immediately cooled by an ice-water bath. For the synthesis of Cs4PbX6 NCs with different halides, PbBr2 was simply replaced by PbX2. The reaction duration should be a little longer until the solution became cloudy to ensure the complete reaction of precursors. Preparation of samples with different Cs to Pb ratios by two-step method. In a typical synthesis, ODE (5 mL), OA (1 mL), OAm (1 mL), PbBr2 (0.0734 g, 0.2 mmol) were added in a three-neck flask and dried under vacuum for 30 min at 120 oC. After PbX2 salt dissolved completely, the reaction temperature was raised to 140 oC under N2 atmosphere. Subsequently, 0.8 mL prepared CsOL solution was quickly injected into the solution. 7 seconds later, the reaction mixture was rapidly cooled down by an ice-water bath. Then the reaction was heated again and hot CsOL solution (0 mL, 0.8 mL, 1.6 mL, 4.0 mL) was injected again into the solution (The molar ratio of CsOL in total to PbBr2 was 0.250, 0.500, 0.750 and 1.375). 7 seconds later, the reaction was immediately terminated by an ice-water bath and purified by centrifugation. Isolation and purification of the product. The final solution was extracted from the crude solution by centrifuging at 11000 rpm for 5 min. Then the supernatant was discarded and the precipitates were redispersed in 5 mL cyclohexane. Water-triggered transformation process. The transformation process was conducted under ambient condition. Typically, the diluted solution (0.02 mL Cs4PbX6 NCs was dispersed in 2 mL cyclohexane) was first added into a cuvette and then 0.02 mL water was added. The PL intensity was then acquired simultaneously.


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Preparation of Cs4PbBr6 film. 1 mL Cs4PbBr6 solution was firstly washed with 1 mL methyl acetate. After centrifugation, the precipitation was redispersed in 0.5 mL octane. The film was made of 0.03 mL Cs4PbBr6 solution by spin coating (3000 r/min, 60 s). Characterization. Transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) data were obtained on a 200 kV TECNAI G2 F20 with a Gatan SC200 CCD camera from FEI, USA. Powder X-ray diffraction (XRD) patterns were measured on a desktop diffractometer (D2 PHASER, Bruker, Germany) with a Cu Kα source. All the absorption spectra were recorded by using the UV spectrometer (SPECORD S 600). Photoluminescence (PL) measurement was collected by using a fluorescence spectrophotometer (FLUOROMAX-4). RESULTS AND DISCUSSION We first examined the influence of Cs:Pb ratio using a modified hot-injection method.4 In a typical experiment, PbBr2, oleic acid (OA), oleylamine (OAm), 1-octadecene (ODE) were loaded into a three-neck flask and dried for 30 minutes at 120 oC under vacuum. The reaction system was further heated to 140 oC under N2 atmosphere to make sure the full dissolution of PbBr2. Certain amount of as-prepared CsOL was then rapidly injected into the PbBr2 solution to initiate the reaction (see more details in Supporting Information). 7 s later, the reaction system was quenched quickly by immersing the flask into an ice-water bath. As shown in Figure 1a, a yellowish solution could be obtained when the molar ratio of Cs:Pb was 0.375. Under UV light irradiation, a bright green emission was identified, suggesting the great photoluminescence property. When the molar ratio of Cs:Pb was increased from 0.375 to 1.000, the green emission became weaker gradually. Colorless solution was obtained when the molar ratio increased to 1.375. No emission in the visible


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range was observed under UV light irradiation, indicating no photoluminescent product was prepared.

Figure 1. (a) Photographs of cesium lead bromide NCs with different Cs:Pb molar ratios (from left to right: 0.375, 0.500, 0.750, 1.000, 1.375) under daylight (top) and UV light (λ=365 nm, bottom). (b-f) TEM images of cesium lead bromide NCs with different Cs:Pb molar ratios of (b) 0.375, (c) 0.500, (d) 0.750, (e) 1.000 and (f) 1.375. Insets in (b) and (c) show the corresponding HRTEM images. The scale bars in the insets are 5 nm.

To figure out the product change, the samples were characterized using TEM. Cube-like CsPbBr3 NCs with size of around 10.1 nm were obtained when the molar ratio of Cs:Pb was 0.375. The HRTEM image (inset in Figure 1b) shows a clear lattice spacing of 0.42 nm, which can be indexed to (110) plane of cubic CsPbBr3. A mixture of cubic and cuboctahedral NCs were obtained when the molar ratio of Cs:Pb was 0.500. The average particle size of nanocubes was about 9.8 nm, while the cuboctahedral NCs have a relatively larger particle size of 17.1 nm. The corresponding particle size histograms were


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presented in Figure S1. To figure out the crystal structure of as-obtained products, we examined many NCs with HRTEM. Interestingly, NCs with different morphologies show completely different crystal structures. As shown in the inset in Figure 1c, an obvious lattice spacing of 0.31 nm can be seen in the cuboctahedral NCs, which is consistent with the (214) planes of rhombohedral Cs4PbBr6 phase. On the other hand, all the cube-like structures were confirmed as the cubic CsPbBr3 NCs. It is worth pointing out that we cannot find any particle containing both cubic CsPbBr3 and rhombohedral Cs4PbBr6 phase. With the increasing molar ratio of Cs:Pb, the majority of products became cuboctahedral NCs. No cube-like structure can be found when the molar ratio of Cs:Pb was 1.375, suggesting the disappearance of CsPbBr3 NCs, which is consistent with the change of photoluminescence property.


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Figure 2. (a) XRD patterns, (b) absorption and (c) PL emission spectra (λex = 400 nm) of products obtained with different molar ratios of Cs:Pb.

To further figure out the possible change of product, XRD characterization has been conducted. As depicted in Figure 2a, the product is pure cubic CsPbBr3 phase when the molar ratio of Cs:Pb was 0.375 because the XRD pattern (red line at the bottom) was in good agreement with cubic perovskite phase (JCPDS card No. 54-0752). In the presence of extra Cs source (Cs:Pb = 0.500, orange line), although the major peaks were still the same, some new peaks appeared, implying the formation of new materials. These new peaks can be attributed to the formation of rhombohedral Cs4PbBr6 phase (JCPDS card No. 73-2478). With increasing ratio between Cs and Pb, the intensity of new peaks increased gradually. Accordingly, the intensity of cubic CsPbBr3 phase decreased and disappeared when the molar ratio of Cs:Pb reached 1.375, which was consistent with our TEM characterization. In our previous study, we have confirmed that Cs4PbBr6 NCs are non-luminescent and display a sharp absorption peak at 314 nm, while CsPbBr3 NCs are highly luminescent that have an absorption/emission peak at around 510 nm. The luminescence change has also been carefully studied. As shown in Figure 2b, an absorption shoulder around 489 nm can be observed when the molar ratio of Cs:Pb was 0.375. In the presence of more Cs source (Cs:Pb = 0.500), a new absorption peak at 314 nm appeared, suggesting the formation of Cs4PbBr6 phase. Accordingly, the intensity of the PL emission peak at 497 nm dropped rapidly (Figure 2c). The intensity of the new absorption peak at 314 nm increased gradually with the


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increasing Cs/Pb ratio, along with the decreased intensity of absorption at 485 nm. Meanwhile, the PL emission at 497 nm became weaker. When the molar ratio of Cs:Pb was 1.375, the characteristic absorption peak of CsPbBr3 phase disappeared completely and no emission in the visible range can be detected, confirming the disappearance of CsPbBr3 phase. On the basis of the above-mentioned results, we can conclude that the ratio between Cs and Pb is critical for the formation of CsPbBr3 and Cs4PbBr6 phase.

Figure 3. (a) TEM images of the products obtained in the first step (Cs:Pb = 0.250, no reheating process). (b-e) TEM images of the products obtained by the two-step method with the ratio of Cs:Pb of (b) 0.250, (c) 0.500, (d) 0.750 and (e) 1.375. The scale bars are 50 nm. (f) Corresponding photographs of the products under daylight (top) and UV light (λ=365 nm, bottom). (g) Corresponding XRD patterns of the products. In addition to the one-pot synthesis approach, we further studied the influence of excessive Cs source on the crystal change using a two-step method. In a typical experiment, uniform CsPbBr3 NCs were prepared first through a hot-injection method4 and the crude solution was cooled down to room temperature. For the second step, the crude solution was heated to 140 oC again under N2 atmosphere. Certain amount of CsOL


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was then rapidly injected into the solution, which was cooled down to room temperature after 7 s. To rule out the influence of the reheating process, a reference solution was prepared, in which no additional CsOL was added and the other parameters were kept identical. From the TEM image in Figure 3a, uniform CsPbBr3 nanocubes were obtained from the first step through the hot-injection method, which is consistent with previous reports37 . When the solution was reheated from room temperature to 140 oC, the nanocubes transformed into ultrathin nanoplates (Figure 3b). Because this thickness of nanoplate is smaller than their Bohr radius (7 nm), a strong quantum confinement effect can be observed.38-40 As a result, the absorption peak and the emission peak blue-shifted (Figure S3). This morphology evolution can be attributed to the low stability of CsPbBr3 nanocubes. Alivisatos group and Yang group have also reported similar phenomenon.41, 42 In contrast, when additional CsOL was introduced, the product was a mixture of nanoplates with some irregular particles (Figure 3c). When the molar ratio of Cs:Pb increased to 0.750, the majority of products became quasi-spherical particles (Figure 3d). Only cuboctahedral particles with size of around 20.6 nm could be observed when the molar ratio of Cs:Pb further increased to 1.375 (Figure 3e). To rule out the influence of the re-heating process in the second step, a reference solution was prepared, in which no additional CsOL was added and the other parameters were kept identical. As shown in Figure 3f, under UV light irradiation, the emission changed from green to blue-greenish, implying the morphology change. A sharp absorption peak at 477 nm was observed (Figure S3a). Compared with the original CsPbBr3 NCs, the absorption peak blue-shifted. Accordingly, the emission peak blue-


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shifted from 496 nm to 492 nm (Figure S3b). Although the photophysical properties changed, no difference was observed in crystal structure, as confirmed by the XRD measurement (Figure 3g). As shown in Figure 3f, with the increasing amount of additional CsOL, a clear change from yellowish to colorless under daylight was observed. Under UV light irradiation, the emission color changed from blue-greenish to colorless accordingly. In line with the naked-eye observation, the UV-vis absorption spectra demonstrated an obvious change. The sharp absorption peak at 477 nm disappeared gradually with the increasing amount of CsOL, while a new peak at 314 nm emerged and became more pronounced accordingly (Figure S3a). The emission spectra showed the same trend. The intensity of emission peak at 492 nm dropped quickly with the increasing amount of CsOL and disappeared completely when the ratio of Cs:Pb was 1.375. Similar to the one-pot method, XRD characterization showed the crystal structure change from CsPbBr3 NCs to Cs4PbBr6 NCs with the increasing Cs:Pb ratio from 0.25 to 1.375, as depicted in Figure 3g. It is worth pointing out that the same trend can be obtained in both the one-pot and twostep synthesis. Several conclusions could be drawn from the results. First, Cs4PbBr6 NCs are non-luminescent and CsPbBr3 NCs are luminescent. Second, no nanoparticle containing both Cs4PbBr6 domain and CsPbBr3 domain can be found, which may be attributed to the highly ionic nature of perovskite nanocrystals that leads to the fast reaction. Third, the most important parameter for the formation of Cs4PbBr6 and CsPbBr3 NCs is the ratio of Cs:Pb. We noticed that the sizes of previous reported luminescent Cs4PbBr6 particles were generally in the range of micrometre. We therefore hypothesize


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that there might be some CsPbBr3 nanodomains in the large crystals, leading to the emission property.

Figure 4. Change in PL intensity of the transformation velocity of Cs4PbX6 to CsPbX3 NCs by water triggered process in the presence of (a) different halides and (b-c) various room humidity. The insets in (c) show the color change of Cs4PbBr6 film under UV light. Recently, our group reported that non-luminescent Cs4PbX6 NCs can be converted to highly luminescent CsPbX3 NCs upon treating with water. A CsX-stripping mechanism was proposed. Cs4PbX6 NCs can be regarded as a CsX-rich structure. During the transformation, Cs4PbX6 NCs in the nonpolar solvent can be converted into CsPbX3 NCs through an interfacial reaction, in which CsX will be stripped out from the Cs4PbX6 NCs and dissolved in water. Due to the high solubility of CsX in water as well as the ionic nature and high ion diffusion property of Cs4PbX6 NCs, monodisperse and air-stable CsPbX3 NCs can be obtained, which possess controllable halide composition, tunable


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emission wavelength covering the full visible range, narrow emission width, and high photoluminescent quantum yield (up to 75%). In our previous study, we have noticed a difference in the transformation rate when the halide ions were different. Here, a more quantitative study has been carried out, in which the other parameters were kept the same. In a typical experiment, 2 mL of cyclohexane solution containing 0.02 mL final Cs4PbX6 NCs was mixed with 0.02 mL of water. The photoluminescence intensity was monitored by taking aliquots out during the reaction process. Figure 4a plotted the intensity change as a function of time. Consistent with the naked-eye observation, the transformation of Cs4PbCl6 NCs showed the highest rate. Upon treating with water, a sharp increase in the PL intensity could be observed. The intensity reached the highest point within around 18 min. A linear fit has been applied to the change of intensity as a function of time with a slope is 0.057/min. Accordingly, the transformation processes for both Cs4PbBr6 and Cs4PbI6 NCs can also be treated through the same method. And the slopes for Cs4PbBr6 and Cs4PbI6 NCs are 0.032/min and 0.018/min, respectively. As a result, the transformation rate for the Cs4PbX6 NCs is Cs4PbCl6 > Cs4PbBr6 > Cs4PbI6 NCs. This phenomenon might be attributed to the different solubilities of CsX because the transformation process can be expressed as: Cs4PbX6 (hexane) →CsPbX3 (hexane) + CsX (water)

Eq. (1)

The solubilities of CsCl, CsBr, and CsI in 100 g water at 25 oC are 191 g, 123 g, and 84.8 g, respectively.43 We therefore hypothesize that the different transformation rates of Cs4PbX6 came from the different solubilities of CsX. Because the transformation process from Cs4PbX6 to CsPbX3 is mainly determined by the stripping of CsX, the transformation should also be sensitive to the humidity of


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environment. Here, we try to utilize this sensitivity in the different humidity. We examined the photoluminescence change in three different humidity: 40%, 60% and 80%, respectively. Cs4PbBr6 NCs were deposited on a transparent glass slide through a spin coating approach. When the glass slide was put in the box with a humidity of 80%, a color change can be observed in a very short time. Under UV light irradiation, a green emission can be clearly seen. As shown in Figure 4b, the intensity increased sharply with prolonged treating time. When the humidity was 60%, the color change became much slower. As plotted in Figure 4c, almost no color change can be observed in the first four hours. A quasi-linear increase can then be realized. When the humidity further decreased to 40%, no color change can be detected over 5 hours, further confirming the sensitivity to high humidity. Based on the above results, this phenomenon might be used to fabricate humidity sensors in the future.

Figure 5. Patterns produced by depositing Cs4PbX6 NCs on a glass substrate after humidifier spray treatment under UV light (λ=365 nm). (a) barcode. (b) morse code. (c) quick response (QR) code. (d) panda pattern. (e-g) multicolour patterns.


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The unique water-triggered transformation property of Cs4PbX6 NCs makes them promising materials in many fields. For example, nowadays, novel materials with some reliable features are more attractive in anti-counterfeiting by moisture (or other chemicals)-triggered method.44-46 Since Cs4PbX6 NCs can be well dispersed in non-polar solvent, they can be used to prepare various patterns by either hand painting or ink printing onto diverse substrates. Then, after treating the patterns with moisture, colourful patterns can be observed under UV light irradiation. As shown in Figure 5a-d, when Cs4PbBr6 NCs were used as the precursor, green patterns of (a) barcode, (b) morse code, (c) QR code, and (d) panda pattern can be prepared. When Cs4PbX6 NCs with different halides were used as the precursors, multi-coloured patterns can also be prepared (Figure 5e-f). More importantly, ink-printing makes this material promising in practical applications (Figure 5g). As shown in the video in Supporting Information, the colourless pattern became colourful rapidly upon a moisture sprayer under UV light. We believe this feature can be exploited in the study on security materials and anti-counterfeiting technology in the near future. CONCLUSIONS In conclusion, a systematic study on the colloidal preparation and water-triggered transformation of Cs4PbX6 NCs has been carried out. It is found that the critical parameter for the formation of Cs4PbX6 NCs is the ratio between Cs and Pb. It is believed that pure Cs4PbX6 NCs are non-luminescent. No co-existence of both Cs4PbX6 and CsPbX3 phases has been found in one single particle. The water-triggered transformation of Cs4PbX6 NCs is sensitive to their halide component. The potential application of Cs4PbX6 NCs in sensor and anti-counterfeiting field has been demonstrated. This work is important because it not


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only confirms the non-luminescent nature of Cs4PbX6 NCs but also demonstrates the potential application.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: The following files are available free of charge. Additional data about the size distributions; HRTEM image and UV/vis absorption and PL emission spectra of product (PDF) Video S1, the transformation from Cs4PbBr6 to CsPbBr3 in the presence of spraying moisture (AVI) AUTHOR INFORMATION Corresponding Author ‡,

*Email: [email protected] (T.S);

†,

*Email: [email protected] (Q.Z)

ORCID Qiao Zhang: 0000-0001-9682-3295 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. §These authors contributed equally.


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by the Ministry of Science and Technology of the People's Republic of China (2016YFE0129600), National Natural Science Foundation of China (21401135, 21673150). We acknowledge the financial support from the 111 Project, Collaborative Innovation Center of Suzhou Nano Science and Technology (NANO-CIC), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). REFERENCES (1)

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SYNOPSIS The ratio of Cs:Pb plays an important role in the preparation of Cs4PbX6 NC. Because nonluminescent Cs4PbX6 NCs can be easily turned into luminescent CsPbX3 NCs by the stripping of CsX in the presence of moisture, they exhibit great potential in anti-counterfeiting application. By ink printing Cs4PbX6 NCs onto a substrate and followed by treating the patterns with moisture, colourful patterns can be observed under UV light irradiation.

ToC figure (For Table of Contents Only)


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Cs4PbX6 (X = Cl, Br, I) Nanocrystals: Preparation, Water-Triggered

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