A Confined Fabrication of Perovskite Quantum Dots in Oriented MOF

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A Confined Fabrication of Perovskite Quantum Dots in Oriented MOF Thin Film Zheng Chen,†,‡ Zhi-Gang Gu,*,† Wen-Qiang Fu,† Fei Wang,† and Jian Zhang*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China ‡ College of Materials Science and Engineering, Fujian Normal University, Fuzhou, Fujian 350007, P. R. China S Supporting Information *

ABSTRACT: Organic−inorganic hybrid lead organohalide perovskites are inexpensive materials for high-efficiency photovoltaic solar cells, optical properties, and superior electrical conductivity. However, the fabrication of their quantum dots (QDs) with uniform ultrasmall particles is still a challenge. Here we use oriented microporous metal−organic framework (MOF) thin film prepared by liquid phase epitaxy approach as a template for CH3NH3PbI2X (X = Cl, Br, and I) perovskite QDs fabrication. By introducing the PbI2 and CH 3 NH 3 X (MAX) precursors into MOF HKUST-1 (Cu3(BTC)2, BTC = 1,3,5-benzene tricarboxylate) thin film in a stepwise approach, the resulting perovskite MAPbI2X (X = Cl, Br, and I) QDs with uniform diameters of 1.5−2 nm match the pore size of HKUST-1. Furthermore, the photoluminescent properties and stability in the moist air of the perovskite QDs loaded HKUST-1 thin film were studied. This confined fabrication strategy demonstrates that the perovskite QDs loaded MOF thin film will be insensitive to air exposure and offers a novel means of confining the uniform size of the similar perovskite QDs according to the oriented porous MOF materials. KEYWORDS: confined fabrication, perovskite, porous thin film, quantum dots, luminescent

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INTRODUCTION

As crystalline microporous materials, metal−organic frameworks (MOFs) contain ordered pores for loading various guests, including metal ions,14,15 coordination polymers,16 metallic nanoparticles (NPs),17−20 quantum dots,21 and dye molecules.22 Particularly, HKUST-1 (Cu3(BTC)2, BTC = 1,3,5benzene tricarboxylate) is a typical 3-D MOF with nanosized pores, which is easy to prepare oriented thin film by liquid phase epitaxial method. Although perovskite MAPbX3 QDs have been reported with different micrometer or nanometer in recent literatures,23,24 so far MAPbX3 QDs prepared in the pores of MOF have never been explored. In this work, we present a successful strategy for fabricating uniform nanosized MAPbX3 QDs into the interior of pores of a MOF thin film. Because of the synthesis of perovskite MAPbI2X by mixing the MAX and PbI2 is very fast, the size of MAPbI2X QDs is hard to control via this in situ mixing method. A stepwise approach should be considered to load MAPbI2X QDs into the pores of MOF. In this procedure, one of the precursors (ions or nuclei) is first introduced into the MOF cavities, which can be dispersed in the micropores to confine the reaction and the growth of nanoparticles. The fabrication of perovskite

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Since cesium ions (Cs ) were replaced by methylammonium cations (CH3NH3+) successfully in the structure of perovskites CsPbX3 (X = Cl, Br, and I), these organic−inorganic hybrid lead organohalides (CH3NH3PbX3) have attracted considerable attention for high-efficiency photovoltaic solar cells,1−4 optical properties,5,6 and superior electrical conductivity.7,8 In particular, the perovskite CH3NH3PbX3 has excellent photoluminescent properties including high quantum yield, composition-tunable emission wavelength, and short irradiative lifetime. 9,10 As a simple perovskite containing organic CH3NH3+ (MA) cations, MAPbX3 quantum dots (QDs) can be prepared at low cost and show useful optical properties. Up to now, the MAPbX3 nanocrystals and QDs have been synthesized and exploited in the applications of light-emitting diodes (LEDs) and lasing devices.11,12 Conventional synthetic strategies of QDs have relied on interactions between the surfactants of adjacent QDs in liquid media. However, the use of organic surfactants prevents close packing of QDs and affects the properties of QDs to some extent. Therefore, the sizes of MAPbX3 QDs are uncontrollable and nonuniform, making a significant challenge for the luminescent application of MAPbX3 QDs. One way to overcome this issue is to synthesize the QDs in highly ordered cavities of porous materials.13 © 2016 American Chemical Society

Received: September 14, 2016 Accepted: October 10, 2016 Published: October 10, 2016 28737

DOI: 10.1021/acsami.6b11712 ACS Appl. Mater. Interfaces 2016, 8, 28737−28742

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Schematic Illustration of a Confined Synthesis of MAPbI2X (X = Cl, Br, and I) in the Interior Pores of Oriented MOF Thin Film

Figure 1. Powder MAPbI2Br perovskite prepared by mixing CH3NH3Br and PbI2 solution: (a) XRD, (b) SEM image, (c) TEM image, (d) SAED analysis, and (e) HRTEM images with interlattice spacing.

MAPbI2X QDs in HKUST-1 thin film was performed as shown in Scheme 1. A presynthesized HKUST-1 thin film was immersed into a PbI2 solution, so that the PbI2 guests were dispersed and encapsulated in the pores of HKUST-1 thin film (denoted as PbI2@HKUST-1). The resulting PbI2@HKUST-1 thin film was rinsed with pure ethanol for removing the residual PbI2 on the surface. Next, a CH3NH3X (MAX, X = Cl, Br, and I) ethanolic solution was added on the PbI2@HKUST-1 thin film to form MAPbI2X@HKUST-1 thin film after rinsing the sample surface with pure ethanol. The as-synthesized MAPbI2X@HKUST-1 thin film was characterized by XRD, IR, SEM, TEM, and EDS electric mapping. The obtained MAPbI2X QDs in the thin film are homogeneous particles with uniform size, which is close to the pore size of HKUST-1 MOF. Furthermore, the perovskite QDs loaded HKUST-1 thin film has high stability even under moist air with 70% humility. This confined fabrication introduced here will be a promising

strategy in the development of other perovskite QDs loaded luminescent sensors and devices.

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EXPERIMENTAL SECTION

Materials and Instrumentation. All reagents and solvents employed were commercially available and used as received without further purification. Powder XRD (PXRD) analysis was performed on a MiniFlex2 X-ray diffractometer using Cu Kα radiation (λ = 0.1542 nm) in the 2θ range of 4°−20° (thin film) and 5°−70° (powder) with a scanning rate of 0.5° min−1. Evaluation of data was done with Rigaku evaluation software JADE 5.0. TEM images and EDS were recorded on a JEM-2010F instrument. SEM images were measured on a JSM6700 instrument. The photoluminescent spectra were measured by using an Edinburgh Instrument FLS920. Synthesis of Powder MAPbI2X (X = Cl, Br, and I). A mixture of PbI2 DMF solution and CH3NH3X (X = Cl, Br, and I) ethanolic solution was sealed in a 10 mL sealed glass bottle and heated at 60 °C for 1 h and then cooled to room temperature. Finally, the perovskite 28738

DOI: 10.1021/acsami.6b11712 ACS Appl. Mater. Interfaces 2016, 8, 28737−28742

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orientations, thickness, and homogeneity of MOF thin films.28 HKUST-1 thin film was grown by spraying Cu(CH3COO)2 and H3BTC sequentially on the functionalized substrate via the LPE layer-by-layer approach. The OH-functionalized Si wafer was selected as the substrate for thin film preparation.29 The main peaks at 11.6° and 17.5° in XRD pattern of presynthesized thin film correspond to the (222) and (333) peaks in calculated XRD of HKUST-1 (Figure 2), which indicate the successful growth of HKUST-1 thin film on OHfunctionalized substrate along [111] orientation.

MAPbX3 were collected after washing with ethanol and dry with nitrogen. Preparation of MAPbI2X (X = Cl, Br, and I) QDs Loaded HKUST-1 Thin Film. Commercially available Si wafers were used for SURMOF preparation. After rinsing with water, the Si substrate is subsequently immersed in a piranha solution consisting of sulfuric acid (H2SO4, 98%) and hydrogen peroxide (H2O2, 30%) with a volume ratio 3:1 at 80 °C for 30 min, then cleaned with deionized water, and dried under nitrogen flux for the next preparation. Then the OHfunctionalized Si substrate was obtained for next sample preparation. HKUST-1 thin films were grown on OH-terminated Si substrate through the liquid-phase epitaxy (LPE) spray method. The spray times were 15 s for the copper acetate ethanolic solution (1 mM) and 20 s for the H3BTC ethanolic solution (0.4 mM). Each step was followed by a 5 s rinsing step with pure ethanol to remove residual reactants. A total of 40 growth cycles were used for the synthesis of HKUST-1 thin film on functionalized substrate. The presynthesized HKUST-1 thin film was immersed in the PbI2 solution (the mixture of DMF and ethanol (1:9) for solvent) for 1 h, and the PbI2@HKUST-1 thin film was obtained after rinsing with pure ethanol. Then the sample was added in a CH3NH3X ethanolic solution for 10 min. The MAPbI2X@ HKUST-1 thin film was given for further characterization.

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RESULTS AND DISCUSSION The typical organic−inorganic perovskite MAPbX3 (X = Cl, Br, and I) were synthesized by mixing CH3NH3X and PbX2 in the polar solvents. Because of the similar synthesis procedures to prepare powder MAPbI2X (X = Cl, Br, and I), only the synthesis of MAPbI2Br perovskite was described in detail. The CH3NH3Br in DMF solution and PbI2 ethanolic solution were mixed together, and the precipitates were formed rapidly. The solid product was filtered out, washed by ethanol, and dried under N2 condition. The structure of the product was further comformed by powder X-ray diffraction (XRD, Figure 1a and Figure S1). XRD results showed that the strong peaks at 14.1°, 20.0°, 23.5°, 24.5°, 28.4°, 31.8°, 34.8°, 40.5°, 43.0°, and 50.0° were identcal to the calculated XRD peaks of isoreticular MAPbX3 at (002), (112) (121), (022), (004), (114), (024), (224), (134), and (044), respectively. The phase of this MAPbI2X was found to be a tetragonal structure with lattice parameters (a = 8.825 Å, b = 8.835 Å, and c = 11.24 Å) corresponding to the structure of perovskite CH3NH3PbX3.11,25 The SEM showed the as-synthesized perovskite powder was crystalline and microsized material (Figure 1b). The TEM and SAED images showed that the tetragonal perovskite was stable under the HRTEM beam irradiation and had a d spacing of 3.2 Å, corresponding to the (200) and (002) crystal faces (Figures 1c−e). Similar procedures were employed to synthesize powder MAPbI2Cl and MAPbI3 samples (Figure S2). Moreover, we also demonstrated that the synthesis of HKUST-1 did not affect the formation of these perovskites. The mixture of Cu(CH3COO)2 (1 mM), H3BTC (0.2 mM), CH3NH3Br (1 mM), and PbI2 (1 mM) in solution produced HKUST-1 and perovskite together, as revealed by the XRD data (Figure S8). That provides an opportunity for the preparation of perovskite CH3NH3PbX3 in HKUST-1. In order to synthesize uniform perovskite MAPbI2Br QDs, the HKUST-1 thin film is chosen for confining QDs growth. HKUST-1 contains three-dimensional (3-D) ordered micropores with the diameter of ∼2 nm for guest loading.26 The thin film of HKUST-1 is easy to make, and it is stable to load different guests. Here, we adapt a liquid-phase epitaxy (LPE) layer-by-layer procedure to grow HKUST-1 thin film on the substrate, which is a typical method to grow SURMOFs.27 This LPE approach is an efficient method to control the growth

Figure 2. XRD patterns of presynthesized HKUST-1 thin film by the LPE approach, PbI2@HKUST-1 thin film, and MAPbI2Br@HKUST-1 thin film.

A stepwise approach is applied to form and load perovskite QDs in the HKUST-1 thin film. First, the pristine HKUST-1 thin film was immersed in the PbI2 solution (1 mM, DMF/ ethanol (1:9) solvent) to encapsulate PbI2 precursors. The homogeneous HKUST-1 thin film has large open channels, so that PbI2 can be easily diffused into the pores. The adsorption of PbI2 tends to saturation after ∼15 min through checking Pd/ Cu ratio in EDS data (Figures S9−S12). Thus, the HKUST-1 thin film was immersed in the PbI2 solution for 1 h to make full encapsulation of PbI2. XRD results showed that the thin film still kept its crystallinity after 1 h immersion (Figure 2). Because of the perovskite QDs are loaded in to MOF pores with small sizes, the XRD peak of perovskite QDs is not observed in this case. However, the TEM images, EDS, and electron mapping further demonstrated that the PbI2 was encapsulated homogeneously in the pores of HKUST-1 thin film (denoted as PbI2@HKUST-1; Figure 3). The PbI2@ HKUST-1 thin film was added into a CH3NH3Br ethanolic solution for 10 min, resulting in the formation of final MAPbI2Br@HKUST-1 thin film. This thin film was characterized by XRD, SEM, TEM, and EDS. During the preparation procedure, MAPbI2Br QDs are formed in the HKUST-1 pores without disturbing the framework of HKUST-1, resulting in a maintained crystallinity. The unchanged crystallinity of MAPbI2Br@HKUST-1 thin film is demonstrated by XRD (Figure 2). Similar procedures were employed to synthesize MAPbI2Cl and MAPbI3 loaded in HKUST-1 thin film samples, and their crystallinity characterization (Figures S4 and S5) showed a successful approach. The SEM images showed the compact and homogeneous surface of MAPbI2Br@HKUST-1 thin film (Figure S6). The TEM images, electron mapping, and EDS showed that nanosized MAPbI2Br QDs were dispersed into HKUST-1 thin film successfully (Figure 4 and Figure S7). Since the crystalline HKUST-1 film has no lattice fringe in the 28739

DOI: 10.1021/acsami.6b11712 ACS Appl. Mater. Interfaces 2016, 8, 28737−28742

Research Article

ACS Applied Materials & Interfaces

Figure 3. TEM images (a, b), TEM-EDS (e), and electron mapping (c, d, f−h) of Pb, I, C, and O in PbI2@HKUST-1 thin film.

Figure 4. TEM image (a) and HRTEM with lattice spacing (b) of MAPbI2Br@HKUST-1 thin film. (c) Particle size distribution of MAPbI2Br QDs prepared by the template of HKUST-1 thin film.

Figure 5. Solid-state photoluminescent emission spectra of the MAPbI2X@HKUST-1 thin films and their powder samples: (a) X= Cl, (b) X = Br, and (c) X = I.

substrates and after preparing MAPbI2X (X= Cl and I) QDs in the MOF pores. The optical properties of MAPbI2X (X = Cl, Br, and I) QDs in HKUST-1 thin film are further studied. At room temperature, MAPbI2Cl@HKUST-1 film has an intense photoluminescent emission at 536 nm (Figure 5a), which is almost the same as its powder sample. MAPbI2Br@HKUST-1 film has an emission at 655 nm upon excitation at 468 nm (Figure 5b), which is 80 nm blue-shift from its powder sample (λem = 735 nm). This blue-shift in the photoluminescent spectra may be attributed to the sizes of MAPbI2Br QDs, because the photoluminescent property of MAPbI2Br is tunable via the particle size.30,31 With the excitation at 420 nm, the MAPbI3@

TEM images (Figure S3), it is easy to determine the size of perovskite QDs from HRTEM images. HRTEM images of the MAPbI2Br QDs showed that the lattice spacing (0.32 nm) is in accord with that of powder MAPbI2Br (Figure 4b), which also demonstrated the successful formation of MAPbI2Br QDs in HKUST-1 thin film. We measured 300 particles by random from the HRTEM micrograph (Figure 4a), and the particle size distribution is shown in Figure 4c. The summarized bar graph displayed that most of MAPbI2Br QDs have diameters of 1.5− 2.0 nm, which match the pore size of HKUST-1 (1.66 nm).18 Similar procedures were carried out to prepare MAPbI2Cl@ HKUST-1 and MAPbI3@HKUST-1 thin films, respectively. Here the XRD of HKUST-1 thin film grown on −OH Si 28740

DOI: 10.1021/acsami.6b11712 ACS Appl. Mater. Interfaces 2016, 8, 28737−28742

Research Article

ACS Applied Materials & Interfaces HKUST-1 film exhibits an emission at 715 nm (Figure 5c), which shows a 85 nm blue-shift from its powder sample (λem = 800 nm). The lifetimes (τ) of powder MAPbI2X (X = Cl, Br, and I) samples are 1.872, 1.949, and 2.03 μs, respectively; however, the corresponding MAPbI2X@HKUST-1 thin films have the luminescent lifetimes (τ) being 1.992, 2.175, and 2.100 μs, respectively (Figure S17a−c). It is interesting that those MAPbI2X@HKUST-1 thin films have longer lifetimes. These results revealed that the luminescent lifetimes of perovskite MAPbI2X can be enhanced after encapsulating into porous MOF as homogeneous QDs. Furthermore, the stability of those MAPbI2X@HKUST-1 thin films and powder MAPbI2X was investigated. All samples were exposed to the moist air with 70% humidity for 4 days. The XRD intensities of powder MAPbI2Br and MAPbI3 samples were obviously decreased after exposing to moist air (Figures S13 and S14). In contrast, MAPbI2X QDs loaded HKUST-1 thin film (here X = Br, for example) can keep its structure, and there are no obvious changes on XRD (Figure S15). In addition, the TEM (Figure S16) also showed the QDs with uniformed size after sample exposing for 4 days and revealed the identical lattice spacing to that of MAPbI3. These positive results demonstrate the high stability of these MAPbI3@HKUST-1 thin films and reveal a bright future for the application of these materials.32

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CONCLUSION In summary, through a stepwise approach to load perovskite precursors (PbI2 and CH3NH3X, X = Cl, Br, and I) into HKUST-1 thin film, the perovskite MAPbI2X QDs with uniform diameters of 1.5−2 nm are successfully fabricated into the pores of HKUST-1. Such perovskite QDs loaded MOF thin films are insensitive to air exposure and have high stability. This work opens a new perspective for the development of high performance light emission perovskite QDs by using the template of oriented porous thin film.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b11712. More experimental and characterization details, additional figures and images, XRD patterns, SEM and TEM images (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(Z.-G.G.) E-mail [email protected]. *(J.Z.) E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 973 program (2012CB821705) and NSFC (21425102, 21601189, and 21521061).

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