Aqueous Synthesis of Lead Halide Perovskite Nanocrystals with High

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Functional Nanostructured Materials (including low-D carbon)

Aqueous synthesis of lead halide perovskite nanocrystals with high water stability and bright photoluminescence Zha Li, Qingsong Hu, Zhifang Tan, Ying Yang, Meiying Leng, Xiuli Liu, Cong Ge, Guangda Niu, and Jiang Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16471 • Publication Date (Web): 27 Nov 2018 Downloaded from http://pubs.acs.org on November 28, 2018

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ACS Applied Materials & Interfaces

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Aqueous synthesis of lead halide perovskite nanocrystals with high water stability and bright photoluminescence

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Zha Li,† Qingsong Hu,† Zhifang Tan,† Ying Yang,† Meiying Leng,† Xiuli Liu,‡ Cong

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Ge,† Guangda Niu*,† and Jiang Tang*,†

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†Sargent

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(WNLO), Huazhong University of Science and Technology, Wuhan, 430074, China

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‡Britton

9

Optoelectronics (WNLO), Huazhong University of Science and Technology

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joint research center, Wuhan National Laboratory for Optoelectronics

Chance Center for Biomedical Photonics, Wuhan National Laboratory for

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ABSTRACT: Lead halide perovskites nanocrystals (NCs) have attracted intense

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attentions because of their excellent optoelectronic properties. The ionic nature of

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halide perovskites makes them highly vulnerable to water. Encapsulation of perovskite

4

NCs with inorganic or organic materials have been reported to enhance the stability,

5

however they often suffer from large aggregation size, low water-solubility and

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difficulty for further surface functionalization. Here, we report a facile aqueous process

7

to synthesize water-soluble CsPbBr3/Cs4PbBr6 NCs with the assistance of fluorocarbon

8

agent, which features a novel mechanism of the perovskite crystallization at oil/water

9

interface and direct perovskite NCs/fluorocarbon agent self-assembly in aqueous

10

environment. The products exhibit a high absolute photoluminescence quantum yield

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(PLQY) of ~80% in water with the photoluminescence lasting for weeks. Through

12

successive ionic layer adsorption and reaction (SILAR), BaSO4 was further applied to

13

encapsulate the NCs, and greatly enhanced their stability in phosphate buffered saline

14

solutions. The high stability in water and saline solution, high PLQY and tunable

15

emission wavelength, together with the successful demonstration of brain tissue

16

labelling and photoluminescence under X-ray excitation, make our perovskite NCs a

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promising choice for X-ray fluorescent bio-labels.

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KEYWORDS: CsPbBr3 • perovskite • nanocrystals • water soluble • multicolor

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INTRODUCTION

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Lead halide perovskite NCs, such as CsPbX3 (X = Cl, Br, I), emerge as a new class

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of luminescent materials and exhibit outstanding optical properties such as high PLQY

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(>70%), narrow full width at half maximum (FWHM: 12-25 nm) and wide color gamut

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(~150% NTSC).1-6 The PLQY values could be even enhanced to near unity through

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effective surface passivation, such as employing bidentate ligands and BF4-, and

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embedding CsPbBr3 in Cs4PbBr6 phase etc.7-11 Nevertheless, due to the ionic nature,

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perovskites are highly sensitive toward water or other polar solvents, resulting in

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degradation when exposing to these chemicals and hindering their biology related

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application.

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In order to improve the stability toward water, many materials such as small

12

molecules,12 polymers,13 silica,14 and siloxane,15 have been coated onto perovskite NCs.

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However, the as-prepared products tend to aggregate together out of the solution, which

14

is attributed to the formation of macro-/micro-sized bulks by the coating materials.

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These aggregations prevent many potential applications. For example, colloidal NC

16

solutions are necessary to fabricate uniform films for light emitting diode (LED); the

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fabrication of biomarker also requires aggregation-free NCs to enable the

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endocytosis.16 Recently, interfacial synthesis of individually dispersed CsPbX3-oxides

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janus particles have been demonstrated, but the incomplete encapsulation renders the

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particles still vulnerable to liquid water.17 The perovskite NCs above are typically

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insoluble in water, excluding their further functionalization such as successive ionic 3 ACS Paragon Plus Environment


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layer adsorption and reaction (SILAR), or graft to active molecules. It remains a grand

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challenge to produce perovskites NCs that could be individually dispersed into water

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or buffer solution.

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We posit that the perovskite NCs can directly form in the aqueous environment with

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water-proof molecules assembling onto NCs as ligand or surfactant, leading to aqueous

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dispersed perovskites NCs without tedious phase transfer if biology related application

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is targeted. The fluorocarbon agent (FCA) is promising as water-proof molecules, since

8

the fluorocarbon chains within FCAs are of super hydrophobicity due to the low surface

9

energy of -CF2- and -CF3 groups as 18 and 16 mN/m, respectively.18 Conventionally,

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FCAs are able to self-assembly onto particles for water-proof, and their amphiphilic

11

structure could facilitate the colloidal dispersion.19-20 This feature enables the synthesis

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of perovskite NCs directly in aqueous solution, which is in sharp contrast with previous

13

studies that the synthesis has to be conducted tightly in moisture-free oil phase.1, 3-4, 21

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In addition, the ionic environment in water can easily feature charges on the surface,

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hence providing electrostatic repulsions to prevent aggregations and produce colloidal

16

dispersion in water.

17

In

this

work,

we

find PFOTES),

that

perfluorooctyltriethoxylsilane

18

(C6F13CH2CH2Si(OCH2CH3)3,

perfluorooctyl

methacrylate

19

(C6F13CH2CH2OOCCH=CH2, PFOMA), C6F6H7Si(OCH2CH3)3 (HFHTES) and

20

C3F6CH2CH2OOCCH=CH2 (HFBMA) can all serve as fluorocarbon agents for the 4 ACS Paragon Plus Environment

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synthesis of perovskite NCs. HBr solution and dimethyl formamide (DMF) are utilized

2

as the solvent. The Br- rich environment facilitates the co-crystallization of Cs4PbBr6

3

phase together with CsPbBr3 in the NCs, while Cs4PbBr6 passivate the defects of

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CsPbBr3. The presence of co-crystals of CsPbBr3 and Cs4PbBr6 (CsPbBr3/Cs4PbBr6)

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are verified by X-ray diffraction (XRD) and high resolution transmission electron

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microscopy (HRTEM). Our products indeed exhibit a high absolute PLQY of ~80%

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with the PL lasting for weeks in water. The water-solubility comes from the self-

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assembly of FCAs with the non-polar fluorocarbon chains encapsulating perovskites to

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resist water degradation, and polar silanol heads extending towards water phase to

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ensure the aggregation-free colloidal dispersion.19 Furthermore, BaSO4 is engineered

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to encapsulate perovskite NCs through a SILAR method, rendering these NCs even

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stable in phosphate buffered saline solutions, which paves the way for potential biology

13

related application.

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EXPERIMENTAL SECTION

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Materials. Cesium Bromide (99.5%), lead bromide (99.5%), oleic acid (85%), oley

16

amine (80-90%) were from Aladdin. Dimethyl formamide (DMF, analytical grade),

17

HBr solution (>40%), toluene (analytical grade) were from Sinopharm Chemical

18

Reagent Co., Ltd, China. FCAs (technical grade) were purchased from Xeogia China,

19

including perofluorooctyltriethyloxylsilane (C6F13CH2CH2Si(OCH2CH3)3, PFOTES),

20

perfluorooctyl

21

hexafluorohexyltriethyloxylsilane,

methacrylate

(C6F13CH2CH2OOCCH=CH2, (C6F6H7Si(OCH2CH3)3,

PFOMA),

HFHTES)

and 5

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hexfluorobutyl methacrylate (C3F6CH2CH2OOCCH=CH2, HFBMA). NH2-PEG-

2

COOH, PBS and MES buffer were from Sigma–Aldrich. The secondary antibody goat

3

anti-rabbit IgG H&L were from abcom. All the reagents were used without further

4

purification.

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Synthesis of CsPbBr3/Cs4PbBr6 NCs by aqueous emulsion process. CsBr and

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PbBr2 were dissolved in HBr solution separately as the mother solution (0.5M CsBr

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and 0.5 M PbBr2 in 40% HBr). 0.125 ml CsBr and 0.125 ml PbBr2 mother solution

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were mixed together, and 1 ml DMF was added into the mixture to prepare a clear

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solution. Then 0.25 mL FCAs (PFOTES) were added into the clear solution and the

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solution was stirred within ice-water bath for around one hour until the emulsion

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solution became greenish. Consecutively, 0.25 ml OLA and 50 ml water were added

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into the emulsion, and the solution was incubated at room temperature for 2 hours. The

13

use of ice-water bath favors the perovskite precipitation due to the decreased precursor

14

solubility in the solvent with the temperature reduction. If we conducted the process at

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room temperature, much less NCs could be obtained. Without the use of HBr, no stable

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fluorescent NCs solution could be obtained, while the probable reason is that HBr

17

enhanced the solubility of CsBr and PbBr2 in the precursor and also enriched the Br-

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content for Cs4PbBr6 crystallization to passivate CsPbBr3 NCs.

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After centrifugation at 6000 rpm for 5 min, the precipitates were discarded and the

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supernatant was taken out as aqueous CsPbBr3/Cs4PbBr6 NCs solution. The supernatant 6 ACS Paragon Plus Environment

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could be further purified by high speed centrifugation at 10000 rpm for 15 min twice to

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remove unreacted precursors and the precipitate can be re-dispersed in water.

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Secondary coating of BaSO4 onto NCs. During above aqueous emulsion process,

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0.25 ml OLA and 0.25 mL H2SO4 (0.1 M) were added into the solution after the

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emulsion solution became greenish and perovskite NCs formed. Then, 50 ml water

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were injected into the solution, and the solution was incubated at room temperature for

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2 hours. After centrifugation at 6000 rpm for 5 min, the supernatant was taken out as

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CsPbBr3/Cs4PbBr6 NCs solution. Then 2.5 mL of 0.01M Ba(NO3)2 solution was added

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into the supernatant and the solution was incubated for 10 minutes to promote the

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formation of BaSO4. Then the colloidal solution could be used for stability

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measurement in PBS solution.

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RESULTS AND DISCUSSION

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The aqueous synthesis process is described in Figure 1A. Firstly, a mixture of HBr

14

solution and DMF (1:4 v/v) served as “water phase”, and CsBr and PbBr2 were

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dissolved in this solution by stirring. PFOTES was added into the solution, serving as

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the separated oil phase. Secondly, under stirring, aqueous emulsion formed. After one

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hour’s incubation in ice-water bath, the greenish crystals occurred at the oil/water

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interface in the emulsion. The process is also applicable to other FCAs. Then,

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oleyamine (OLA) was added into the emulsion, following by the injection of excessive

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amount of water (30 times volume or more). Both OLA and PFOTES are indispensable

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for successful synthesis of water-soluble perovskite NCs, while independent use of

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OLA or PFOTES all lead to the failure of the synthesis. We speculate that OLA is

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protonated as OLA+ and attach to the surface of perovskite nanocrystals through

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hydrogen bond between –NH3+ and Br-, and thus effectively suppress the surface

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defects, which have been verified in many previous papers.3,5,

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hydrophobic chains of OLA also helps for the packing of FCA over perovskite

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nanocrystals through hydrophobic interactions.23-24In the final step, the emulsion

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solution was incubated for 2 hours at room temperature, and then centrifuged or filtered

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to discard bulk precipitates. The supernatant was collected as colloidal NCs in water.

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The supernatant could be further purified by high speed centrifugation to remove

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unreacted precursors. As-prepared solution showed strong green fluorescence under

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UV illumination.

22

The presence of

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The mechanism and some key factors are illustrated as follows. At the beginning,

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PFOTES possess highly hydrophobic fluorocarbon chain and only intermediate polar

15

head (Si-OC2H5), which could be seen as oil phase. Under strong stirring, the initial

16

mixture of water and oil phases turns into emulsion and generates amounts of oil/water

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interfaces. In the presence of water, PFOTES is gradually hydrolyzed into more

18

amphiphilic PFOTHS (perfluorooctyltrihydroxylsilane) with polar heads of Si-OH

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(Figure 1B). The hydrolyzed product of PFOTES was collected as PFOTHS, which was

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confirmed by Fourier transformation infrared transmission (FTIR) spectroscopy, where

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a deep valley signal at 3720 cm-1 was attributed to Si-OH in PFOTHS. In contrast, the 8 ACS Paragon Plus Environment

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original PFOTES exhibited smooth line in this region (Figure 1C, S2A). At the

2

interface, the silanol polar heads of PFOTHS extend toward water phase and

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hydrophobic fluorocarbon chains point to oil phase.19 When the precursor ions in water

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phase approach the water/oil interface by diffusion, crystallization of perovskites

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occurs due to the hydrophobic microenvironment of fluorocarbon chains. By keeping

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the emulsion still, we could clearly observe perovskite solids at the water/oil interface,

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verifying above assumption (Figure S1).

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Finally, the added excessive water could convert the emulsion into one homogeneous

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phase of colloid or solution, where PFOTHS can self-assemble onto perovskite NCs in

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water. The peak at 3720 cm-1 in FTIR spectrum of Si-OH remains in the final dried

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sample of perovskite NCs, indicating the final structure of perovskite NCs/PFOTHS

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(Figure 1C). Considering the high hydrophobicity of the fluorocarbon chain in

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PFOTHS, the hydrophobic effect is an important driving force for the self-assembly

14

according to classical colloid chemistry theory.18,25 We further obtained the X-ray

15

photoelectron spectroscopy (XPS) spectrum for the FCA/NCs powder (Figure S2B),

16

where the binding energy of F 1s peak shifted from 689.2 eV for neat FCA to 688.9 eV

17

for NCs/FCA. The shift toward lower binding energy indicates the interaction between

18

F atoms and negative ions, while in this case we attribute to Br-. Additionally, we also

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record Br 3d5/2 and Br 3d3/2 at 69.1 eV and 68.1 eV for NCs/FCA composite (Figure

20

S2C), which was higher than the reported perovskite without FCA (68.5 eV and 67.6



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eV).26-27 Such shift also consolidate the interaction (halogen bond) between F atoms

2

and Br-. The binding morphology is illustrated as in Figure S2D.

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The possible configuration of PFOTHS onto NCs is that the fluorocarbon chains in

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FCA bind to NCs and pack as a hydrophobic layer with OLA as additional ligand

5

(Figure 1A).22,28 Moreover, the hydrophobic interaction between hydrophobic chains

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of OLA and fluorocarbon chains of FCAs helps for the cross-link to form a dense and

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efficient shell layer.23,24

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The silanol polar heads extend towards water phase, and probably also the attached

9

OLA, could adsorb H+ from the acid solution, hence providing electrostatic repulsions

10

to prevent aggregations and achieve stable colloidal dispersion in water. This is

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supported by the fact that Malvern Zetasizer measurement revealed the positive zeta

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potential (+36 mV) of the aqueous perovskite NCs. According to DLVO theory, the

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electrical double layer provides repulsion force for colloidal dispersion.29 In order to

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further support this, excessive amounts of HBr or H2SO4 were added into the final NCs

15

solution. Some fluorescent solid aggregates could be observed by naked eyes, while

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their PL was preserved without intensity loss (Figure S2F). Fundamentally, HBr and

17

H2SO4 are strong electrolyte and can enhance the ionic strength in water to compress

18

the electric double layer and weaken the repulsion force for dispersion. The PL was

19

preserved because that the fluorocarbon chains on the NCs still protected perovskites

20

from water. Above observation also verified the packing of PFOTHS onto perovskite


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NCs. Hence the PFOTHS plays an important role as water-proof layer, surfactant and

2

ligand for perovskite NCs with the final self-assembly structure illustrated in Figure

3

1A.

4 5

Figure 1. (A) The schematic illustration of FCA assisted aqueous synthesis of

6

CsPbBr3/Cs4PbBr6 NCs. (B) The hydrolysis reaction of PFOTES. (C) The IR spectrum

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of PFOTES, PFOTHS and the dried NCs sample as NC/PFOTHS self-assembly.

8

We conducted two reference experiments to understand the protection mechanism:

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using FCA or OLA as the only ligand. For the FCA only case, colourless non-

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fluorescent solution was obtained, suggesting the failed production of perovskite NCs. 11 ACS Paragon Plus Environment


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For the OLA only case, emissive NCs were obtained but with much less stability (25%

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loss PL intensity over 4 hours storage, Figure S2E) and lower PLQY (around 40%).

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The OLA was protonated easily as cation OLA+ and bonded to the NCs surface by the

4

hydrogen bond with Br- or even occupied the A site on the surface.26,27 The hydrophobic

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chains of OLA extended around the NC serve as a water-proof layer, and the positive

6

charge associated with OLA+ cation provides additional electrostatic repulsion for the

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colloidal dispersion stability. Clearly, OLA alone could work as the ligand and water-

8

proof layer. However, only with the assistance of FCA could stable and highly emissive

9

perovskite aqueous dispersion be obtained. As discussed above, the interaction between

10

hydrophobic chains of OLA and fluorocarbon chains of FCAs strengthens the

11

compactness of the shelling layer, and the hydrophilic silonal heads helps their

12

dispersion in water.

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The crystalline structure of NCs was studied by X-ray diffractions (XRD). As shown

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in Figure 2A, XRD result contains diffraction peaks from CsPbBr3 and Cs4PbBr6, which

15

are well fitted by standard patterns of JCPDS 01-075-0412 and 073-2478.30 The

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formation of CsPbBr3/Cs4PbBr6 is due to that HBr enriches the Br- in the solution which

17

favors the crystallization of Cs4PbBr6.31 Transmission electron microscopy (TEM)

18

analysis shows that our perovskite NCs have a rectangle shape with the length ranging

19

from 20-100 nm (Figure 2B). The high resolution TEM (HRTEM) analysis of a single

20

NC reveals its high crystallinity nature (Figure 2B), while the fast Fourier

21

transformation and the HRTEM feature reveals both the cubic phase and hexagonal 12 ACS Paragon Plus Environment

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phase (Figure 2C). The spots marked by green circle belong to hexagonal diffraction

2

patterns, and the lattice distance calculated from the diffraction spots is 3.4 Å,

3

corresponding to the (220) plane of Cs4PbBr6 phase. The red circle marked spots belong

4

to typical cubic phase of CsPbBr3, with the lattice distance calculated from the

5

diffraction spots as 5.8 and 4.2 Å, corresponding to (100) and (110) planes,

6

respectively. In addition, from the HRTEM, we directly observed the lattice spacing of

7

CsPbBr3 as 2.9±0.1 Å and a Moire fringe pattern with a spacing of 8 Å (Figure 2D),

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which is from the overlapping lattice fringes between CsPbBr3 and Cs4PbBr6. The

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Moire fringe spacing could be calculated according to the following equation (d113 and

10

11

12 13

d416 are the lattice spacings of CsPbBr3 and Cs4PbBr6 respectively): 𝑑2113

d𝑀𝑜𝑖𝑟𝑒 = 2(𝑑416 ― 𝑑113)

(1)

The Moire fringe spacing is calculated as 8.4 Å with characteristic lattice spacing of Cs4PbBr6 as 3.4 Å, consistent with the measured result.30

14

The Moire fringe also indicates the intimate contact of the two phases. Many papers

15

also reported the well matched lattice to form composite and passivation between

16

CsPbBr3 and Cs4PbBr6.30, 32-33 All these observations prove the co-existence of CsPbBr3

17

and Cs4PbBr6 in the NCs, with CsPbBr3 passivated by the neighboring lattice-matched

18

Cs4PbBr6. Furthermore, the ratio of CsPbBr3 to Cs4PbBr6 in the product could be

19

controlled by adjusting the ratio of CsBr/PbBr2 in the precursor solution. We utilized

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X-ray fluorescence measurements to determine the atomic ratio of Cs/Pb in the product, 13 ACS Paragon Plus Environment


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and the ratio of CsPbBr3 to Cs4PbBr6 can be calculated accordingly. In our experiments,

2

the molar ratio of CsPbBr3 to Cs4PbBr6 could be controlled as 4, 1.3 and 0.875 by using

3

CsBr/PbBr2 ratio as 1, 4 and 8, respectively.

4

We have to admit that the quality of TEM images in Figure 2B is not good. Beside

5

the limitations of our TEM facility, one possible reason is the formation of our

6

perovskite NCs at ice water temperature, which might result in slightly degraded

7

crystallinity. The irregular morphology is caused by the overlapping between

8

nucleation and growth as the formation of our perovskite NCs takes a few hours to

9

complete. Further work should introduce a nucleation burst and subsequent slow

10

growth to promote morphology uniformity and size distribution. In addition, we also

11

conducted energy dispersive spectroscopy (EDS) analysis on individual perovskite

12

NCs, and found the presence of Cs, Pb, Br and Si in the products, suggesting the capping

13

of perovskite NCs by PFOTHS (Figure S2G). However, due to the amorphous nature

14

and atomic thickness, we could hardly directly observe PFOTHS around the

15

synthesized products by TEM.


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Figure 2. Characterization of as-prepared water-soluble CsPbBr3/Cs4PbBr6 NCs. (A)

3

XRD spectra of CsPbBr3/Cs4PbBr6 NCs. (B) TEM images of CsPbBr3/Cs4PbBr6 NCs,

4

with

5

transformation of the single CsPbBr3/Cs4PbBr6 NC in Figure 2B. (D) HRTEM image

6

of a single NC.

the HRTEM image of a single rectangle NC at right corner. (C) The fast Fourier

7

Our aqueous solution of CsPbBr3/Cs4PbBr6 NCs showed an excitonic absorption

8

peak at 515 nm and a PL peak at 520 nm with a FWHM of 16 nm (Figure 3A).3, 11 The

9

PLQY was measured as 79.2% through an integrating sphere. We believe the co-

10

presence of CsPbBr3 and Cs4PbBr6 is beneficial for the final high PLQY, narrow 15 ACS Paragon Plus Environment


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FWHM, small stokes shift and symmetrical peak shape, which have been studied and

2

confirmed in many previous studies.11,

3

photoluminescence excitation (PLE) spectrum reveals a deep valley around 300 nm,

4

where the light is fully absorbed by Cs4PbBr6 yet contribute nothing to PL due to its

5

strong electron-phonon coupling dissipating all absorbed energy. The band gap of

6

Cs4PbBr6 is ~4 eV, which echoes with the deep valley around 300-310 nm, consistent

7

with the previous reports.34 Thanks to the super hydrophobicity of fluorocarbon chains

8

within PFOTHS, the PL intensity of our perovskite NC solution preserves above 80%

9

of initial value after 1 week’s storage at room temperature (Figure 3B). When the

10

colloidal solution was diluted by water, the PL intensity showed linear correlation with

11

the NCs concentration (Figure S3), which further proved no degradation of NCs during

12

the dilution. Additionally, as-prepared NCs can be precipitated and re-dispersed in

13

water to remove abundant free ions and ligands (Figure 3C). Such stability again

14

confirmed the well protection of perovskite NCs by PFOTHS. Notably, the PL intensity

15

slightly increased after 25 hours storage. This phenomenon has also been observed in

16

several previous papers,35-36 which is ascribed to the structural reorganization leading

17

to defect suppression and PL enhancement.

31,

33

Notably, in Figure 3A, the

18

Surprisingly, our PFOTHS capped perovskites CsPbBr3/Cs4PbBr6 NCs are capable

19

of halide exchange. When mixing the solution with HI (HCl) solutions, PL spectrum

20

exhibited red (blue) shift due to the halide exchange, thus producing perovskite NCs

21

with various emission colors (Figure 3D). The underlying mechanism for effective 16 ACS Paragon Plus Environment

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protection from water but capable of halide exchange is still not clear. It is speculated

2

that due to the high hydrophobicity of fluorocarbon agents, water molecules are

3

immobilized around the fluorocarbon group and kept away from the perovskite NCs. 37

4

Ions however can transport through the nanopores in the hydrophobic fluorocarbon

5

layers by diffusion, in analogy to the reported ionic transport in the nanopores or

6

nanochannels in the hydrophobic membrane, leading to halide exchange.38

7

We also used other FCAs to encapsulate perovskite NCs. For example, commercial

8

perfluorooctyl methacrylate (C6F13CH2CH2OOCCH=CH2, PFOMA), were used as the

9

encapsulation layers, which could also be hydrolyzed into fluorocarbon-chain alcohols

10

with hydroxyl group as the hydrophilic head (Scheme S1). The produced perovskite

11

NCs were also dispersible in water and exhibited similar stability. FCAs with shorter

12

fluorocarbon

13

C3F6CH2CH2OOCCH=CH2 (HFBMA) could also enable the water stability of

14

perovskite NCs (Scheme S1, Figure S4). Clearly, these results demonstrate that the

15

fluorocarbon chain length is not crucial for the stability and short fluorocarbon chain

16

down to 4 carbon atoms is enough to guarantee the water-stability of perovskite NCs

17

because of its super hydrophobicity.

chains

like

C6F6H7Si(OCH2CH3)3

(HFHTES)

and

18



Page 18 of 32

1 2

Figure 3. (A) UV-Vis absorption spectrum (green), PL spectrum (purple) and PLE

3

spectrum (yellow) of CsPbBr3/Cs4PbBr6 NCs in water. (B) The normalized PL intensity

4

evolution of CsPbBr3/Cs4PbBr6 NCs in water at room temperature. (C) The PL of

5

CsPbBr3/Cs4PbBr6 NCs before and after purification. (D) The normalized PL spectra

6

of CsPbBr3/Cs4PbBr6NCs after halide exchange (Cl- and I-) in water.

7

The stability of perovskite NCs in saline solutions is the prerequisite for their

8

potential biology-related applications. Phosphate buffered saline (PBS) is one of the

9

most widely used saline solution, containing 8 mM Na2HPO4, 136 mM NaCl, 2 mM

10

KH2PO4 and 2.6 mM KCl. Unfortunately, when we mixed the synthesized NCs with

11

PBS, the PL peak quickly blue shifted and the PL intensity rapidly decreased (Figure

12

S5). The blue shift of PL was due to the anion exchange between NCs and free Cl- in

13

PBS, while other cations (K+, Na+) induced the collapse of perovskite structure and 18 ACS Paragon Plus Environment

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1

eventually completely quenched the emission.38 Therefore, the stability of our NCs

2

needs to be further improved to enable their survival in PBS.

3

To ensure the stability in saline solution, we consider further coating of inorganic

4

layers onto the prepared NCs. The positive charges of our NCs provided reaction sites

5

for successive ionic layer adsorption and reaction (SILAR), enabling the fabrication of

6

one additional inorganic layer to fully block the ionic exchange through channels in the

7

packed fluorocarbon layer. Herein we integrate BaSO4 coating into the aqueous

8

emulsion process. In the final step during the synthesis, just before adding excessive

9

water, a certain amount of H2SO4 was added into the mixture, initiating the adsorption

10

of SO42- onto positively charged NCs. Then, after the addition of excessive water and

11

centrifugation, the supernatant was collected, followed by adding Ba(NO3)2 solutions.

12

Ba2+ would simultaneously adsorb onto NCs and react with the abundant SO42- to form

13

BaSO4 encapsulation layers onto the NCs.



Page 20 of 32

1 2

Figure 4. (A) The PL spectra of CsPbBr3/Cs4PbBr6/BaSO4 NCs in PBS before (green)

3

and after (blue) 1 day’s storage. (B) The PL spectra of multi-color

4

CsPbX3/Cs4PbX6/BaSO4 NCs (3-color NCs/BaSO4) in PBS. (C) HAADF image (left)

5

and EDS mapping of Ba, Cs, Pb and Br elements of CsPbBr3/Cs4PbBr6/BaSO4 NCs.

6

(D) The fluorescence image of the mouse brain tissue labelled by the biomarker NC-

7

antibody through the conventional immune-labelling method under the irradiation of

8

405 nm laser.

9

After coating, the stability of CsPbBr3/Cs4PbBr6/BaSO4 NCs in PBS has been greatly

10

improved, preserving 90% of initial fluorescence intensity after one day’s storage at

11

room temperature (Figure 4A), which was in sharp contrast with NCs without BaSO4 20 ACS Paragon Plus Environment

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1

coating (Figure S5). Meanwhile, the PL spectra of CsPbBr3/Cs4PbBr6/BaSO4 NCs in

2

PBS showed no significant shift or broadening compared with CsPbBr3/Cs4PbBr6 NCs

3

in water within a storage time of 24 hours (Figure S6).

4

Furthermore, the inorganic coating is also applicable to other perovskite NCs with

5

different compositions. After inorganic layer coating, the anion exchange could be

6

completely suppressed. By simply mixing the three kinds of CsPbX3/Cs4PbX6/BaSO4

7

NCs into PBS, the co-existence of multi-color PL emission was observed (Figure 4B).

8

These phenomena demonstrate that additional BaSO4 coating efficiently suppress the

9

anion exchange between the NCs (Cl-, Br- and I-) and free ions (K+, Na+, Cl-) in PBS

10

buffer. To directly verify the presence of BaSO4 coating layers, we applied the scanning

11

transmission electron microscopy and high-angle annular dark-field (STEM–HADDF)

12

and energy dispersive spectroscopy (EDS) mapping for CsPbBr3/Cs4PbBr6/BaSO4

13

NCs. The heavy elements like Ba, Cs, Pb, Br could be clearly recognized (Figure 4C).

14

The overlapping of Ba element along with Cs, Pb and Br confirmed the successful

15

coating of BaSO4 shell onto NC surface rather than forming isolated BaSO4

16

nanoparticles. Overall, this achievement laid the foundation to use perovskite NCs as

17

multi-color biomarkers.

18

To demonstrate the potential application of the water soluble perovskite NCs, we

19

conjugate perovskite NCs with antibody as the fluorescent biomarker. By conventional

20

immuno-labeling method, in the mouse brain tissue, some neuron fibers were



Page 22 of 32

1

fluorescently labeled by the NCs biomarker under the irradiation of 405 nm laser

2

(Figure S7). It is encouraging that the NCs survive during the conjugation process.

3

Further studies are still undergoing to better resolve the brain tissue. In addition, the as-

4

prepared perovskite NCs can also serve as scintillators, which exhibits the photo-

5

response to the X-ray with the light yield as 3645 photons/MeV (Figure S7), thanks to

6

the strong stopping capability of the heavily component and the high PLQY. The good

7

stability of the products guarantees the long-term use in practical applications. Thus,

8

the demonstrated bio-compatibility, as well as the strong photoluminescence under X-

9

ray excitation, indicate that our NCs can be a promising candidate for X-ray fluorescent

10

bio-label in the future.

11

12

CONCLUSION

13

In summary, we have reported the water-soluble CsPbX3/Cs4PbX6 NCs through an

14

aqueous emulsion process with the assistance of fluorocarbon agents. In our study, the

15

crystallization and the self-assembly of NCs occurs slowly with the existence of water,

16

which allows the fluorocarbon agents to self-assemble onto NCs in aqueous

17

environment, thus providing a new strategy to directly synthesize highly luminescent

18

CsPbX3/Cs4PbX6 NCs which are dispersible in water and buffer solution. The as-

19

prepared CsPbBr3/Cs4PbBr6 NCs exhibited a PLQY of ~80%, a FWHM of 16 nm, and

20

excellent stability in water for weeks. The co-existence of CsPbBr3/Cs4PbBr6 resulted 22 ACS Paragon Plus Environment

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1

in good defect passivation and hence high PLQY. Highly hydrophobic fluorocarbon

2

chains of FCAs served as the water-proof layer, and the hydrophilic heads of FCAs

3

provide the colloidal dispersion in water. Additional coating of NCs by inorganic

4

materials (BaSO4) was applied to improve the NCs stability in saline solutions like PBS.

5

We further explored their application for brain tissue labelling and scintillators, which

6

paves the way for their potential applications as X-ray fluorescent bio-labels.

7

ASSOCIATED CONTENT

8 9

Supporting Information. The supporting information is available and free of charge via the Internet at http://pubs.acs.org.

10 11

Characterization methods, experiment, hydrolysis reactions and measurement data are included in supporting information.

12

AUTHOR INFORMATION

13

Corresponding Author

14

[email protected] & [email protected]

15

Funding Sources

16

This work was financially supported by the National Natural Science Foundation of

17

China (51761145048, 61725401, 51702107), the National Key R&D Program of China

18

(2016YFB0700702), the HUST Key Innovation Team for Interdisciplinary Promotion

19

(2016JCTD111), by the Fundamental Research Funds for the Central Universities



Page 24 of 32

1

(2017KFXKJC0020) and the Open Fund of State Key Laboratory of Luminescence and

2

Applications (SKLA-2016-08).

3

Notes

4

The authors declare no competing financial interest.

5

ACKNOWLEDGMENT

6

We thank Prof. Huanping Zhou and Dr. Ligang Wang at Peking University for HRTEM

7

measurement and discussion, and Dr. Jun Su at WNLO for TEM measurement. The

8

authors thank the Analytical and Testing Center of HUST and the facility support of

9

the Center for Nanoscale Characterization and Devices, WNLO.

10

ABBREVIATIONS

11

NCs,

12

Reprecipitation; PLQY, photoluminescence quantum yield; TEM, transmission

13

electron microscopy; FWHM, full width at half maximum; PBS, phosphate buffered

14

saline;

15

hydrophiliclipophilic balance; PFOMA, perfluorooctyl methacrylate; STEM-EDS,

16

scanning transmission electron microscopy and energy dispersive X-ray spectroscopy;

17

HAADF, high-angle annular dark-field.

18

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Quantum

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PFOTES,

Dots

FCAs,

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for

Lighting

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Displays:

LARP,

OLA,

Ligand

Assisted

oleylamine;

Room-Temperature

HLB,

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Aqueous Synthesis of Lead Halide Perovskite Nanocrystals with High

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