Controlled Synthesis of Lead-Free Cesium Tin Halide Perovskite

Jul 10, 2017 - The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02089. Additiona...
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Controlled Synthesis of Lead-Free Cesium Tin Halide Perovskite Cubic Nanocages with High Stability Aifei Wang, Yanyan Guo, Faheem Muhammad, and Zhengtao Deng* Department of Biomedical Engineering, College of Engineering and Applied Sciences, Nanjing National Laboratory of Microstructures, Collaborative Innovation Center of Chemistry for Life Sciences, Nanjing University, Nanjing, Jiangsu, 210093, P. R. China S Supporting Information *

ABSTRACT: Hollow nanocrystals have attracted considerable attention in many areas of materials science because of their unique geometry, large surface area, high loading capacity and low density, and potential applications in catalysis, energy storage, drug delivery, and molecular separation. So far, numerous hollow nanostructured materials, such as noble metals, metallic oxides, hydroxides, and chalcogenides, have been reported. However, there is no report about the hollow metal halide perovskite nanocrystals yet. Herein, for the first time, we report the controlled synthesis of metal halide perovskite hollow nanocrystals, i.e., CsSnBr3 cubic nanocages, through a facile hot-injection colloidal approach. The formation of nanocages was controlled by rationally choosing the precursors and reaction temperature and was achieved through the self-assembly driven process. The as-synthesized CsSnBr3 nanocages exhibit excellent oxygen resistibility under desiccation conditions. Furthermore, treatment of CsSnBr3 nanocages with perfluorooctanoic acid results in an improved stability against moisture, oxygen, and light. This work represents a step toward hollow nanocrystals of metal halide perovskites for optical, electronic, and optoelectronic applications.

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Cl, is gaining momentum in recent years. A majority of researches were initially focused on the fabrication of bulk perovskite films, while the colloidal synthetic routes to prepare hybrid organic−inorganic or fully inorganic perovskite nanocrystals have been very recently developed.18−22 Colloidal metal halides perovskite nanocrystals are of significance because they display a favorable combination of quantum-size effects with respect to their bulk counterparts, versatile surface chemistry, and the nature of the colloidal state allows them dispersion into a variety of media and used as “inks” to fabricate various electronic and optoelectronic devices through a low-cost printing process. Many efforts have been devoted to the synthesis of these nanocrystals with controllable morphologies, such as nanocubes, nanorods, nanowires, and nanoplatelets.23−28 To the best of knowledge, there is no example in the literature for the synthesis of hollow nanocrystals of metal halide perovskites. Metal halide perovskites so far mainly rely on the use of Pb as the divalent metal. However, there are growing concerns regarding the risks that Pb poses to health and the environment.29 Restriction of hazardous substances directive severely limits the use of Pb in consumer electronics. There have been a number of efforts to explore and replace lead with

ollow nanocrystals have attracted considerable attention in many areas of nanotechnology because of their unique geometry, large surface area, high loading capacity and low density, and potential applications in catalysis, energy storage, drug delivery, and molecular separation.1,2 The ability to manipulate the structure and morphology of such porous solids on a nanoscale would enable greater control of the local chemical environment. Many approaches based on different principles have been developed to prepare hollow nanostructures, such as galvanic replacement, the Kirkendall diffusion effect, Ostwald ripening, and self-assembly. So far, numerous hollow nanostructured materials have been reported, such as noble metals (Au, Ag, Pt, and alloy),3−5 metallic oxides (SnO2, CuOx),6,7 hydroxides (Ni(OH)2@Co(OH)2),8 and chalcogenides (CoSx, CdSe).9,10 However, there is no report about the hollow metal halide perovskite nanocrystals yet. Recently, metal halide perovskites have become an exciting topic of materials research.11−13 For instance, as light absorption materials used in photovoltaics, metal halide perovskite rapidly achieved high power conversion efficiencies (over 22%).14 As light-emitting materials in light-emitting diodes (LEDs), they have exhibited internal quantum efficiencies exceeding 15% and tunable light emission spectra.15−17 Due to their immense potential for industrially relevant and hence achieve long-term application, the research in the synthesis of halide perovskites with the general chemical formula ABX3, where A is a monovalent metal cation or molecules, B is a divalent metal, and X is halogen like I, Br, or © 2017 American Chemical Society

Received: May 21, 2017 Revised: July 3, 2017 Published: July 10, 2017 6493

DOI: 10.1021/acs.chemmater.7b02089 Chem. Mater. 2017, 29, 6493−6501

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Chemistry of Materials

Figure 1. (A, B) TEM images of CsSnBr3 cubic nanocages. (C) HRTEM image of typical CsSnBr3 cubic nanocage. (D, E) HAADF-STEM images of CsSnBr3 cubic nanocages. (F) Typical EDS mapping with STEM-HAADF image and corresponding elemental maps of Cs, Sn, and Br.

The synthesis of CsSnBr3 nanocages is carried out by a modified synthetic protocol reported by Protesescu et al.18 In our method, noncoordinating 1-octadecene (ODE) was used as a solvent, and oleylamine and oleic acid as ligands. Particularly, to avoid the use of toxic and expensive alkylphosphines, such as trioctylphosphine (TOP) used in the literature,43 during synthesis and also to prevent the formation of CsBr, we selected stannous 2-ethylhexanoate as the precursor in this study instead of previous TOP-SnBr2.43 Unlike organic halogenated amines which are usually used as bromide source, but their synthesis needs tedious procedures, therefore inorganic salts are preferred to be chosen. Herein, a commercial chemical (MgBr2) was used as the bromide source. The underlying reason behind the selection of Mg2+ salt is its smaller ionic radius, and according to the tolerance factor theory, it cannot penetrate the perovskite structure. Upon the injection of the cesium oleate precursor into the Br-Sn precursor solution, the color of the solution became dark red after a few seconds, indicating the formation of CsSnBr3 nanocages. Figures 1 and S1−S3 show transmission electron microscopy (TEM), high-resolution TEM (HRTEM), scanning TEM (STEM), and energy-dispersive X-ray spectroscopy (EDS) mapping of a typical sample of CsSnBr3 nanocages prepared at 230 °C. The nanocages were found to be hollow cubic nanostructures with a size distribution ranging from 80 to 150 nm and the average shell thickness of 20 nm. It is worth mentioning that a few partly hollowed structures also coexist with the hollow product. (Figure S4). The HRTEM image (Figure 1C) displays the lattice fringes of the CsSnBr3 nanocages which are spaced by 2.9 Å, which matches well with that of the (200) plane. EDS mapping suggests a homogeneous distribution of tin, cesium, and bromide in the nanocage structure with an atom ratio of 1:1:3 (Figures 1F and S3). SEM also shows the same hollow cubic morphology (Figure S3). The crystal structure of CsSnBr3 is shown in Figure 2A. The crystal structural properties of CsSnBr3 nanocages is analyzed using powder X-ray diffraction (XRD). XRD data indicated that

less toxic elements, having an analogous electronic band structure to that of Sn, Ge, Bi, and Sb.30−32 Moreover, tin seems also as an appropriate choice because it is in the same group of the periodic table with lead and thus has the most similar electronic properties.33−39 Beyond being lead-free, tin halide perovskites offer a number of properties which make them attractive for use in photovoltaics including narrower band gaps than their lead analogues,40 low exciton binding energies,41 and long carrier diffusion lengths.42 In this regard, CsSnX3 nanocubes have been reported by Bohm and coworkers.43 However, these nanocubes suffered from rapid degradation in the presence of air, and the fabrication process has to be performed in an inert atmosphere. The instability of divalent tin-based perovskites was attributed to the oxidation processes (Sn2+ → Sn4+), which resulted into the collapse of the crystal structure.44−46 Thus, exploring new strategies to synthesize CsSnBr3 nanocrystals with improved stability is highly demanded. Among various approaches reported so far to improve the stability of Sn-perovskite solar cells, the replacement of divalent (Sn2+) to air stable tetravalent tin (Sn4+) has been effectively explored to synthesize relatively stable Cs2SnI6 instead of CsSnI3 as the hole transporter for air stable solar cells.47−49 Recently, our group reported stable Cs2SnI6 perovskite derivative nanocrystals with different morphologies using a colloidal synthetic approach.50 The addition of bivalent tin halide (SnF2 or SnCl2) strategy was also found to inhibit the moisture ingress and suppresses the trap states and thus improves the solar cell performance of tin-based perovskites.51,52 In order to further advance Sn-based perovskite nanocrystals, we report herein the synthesis of hollow CsSnBr3 perovskite cubic nanocages with improved stability. To the best of our knowledge, this is the first example of the synthesis of hollow metal halide perovskite nanocrystals. Importantly, we demonstrated that the stability of the CsSnBr3 perovskite cubic nanocages is significantly improved with the surface treatments with perfluorooctanoic acid (PFOA). 6494

DOI: 10.1021/acs.chemmater.7b02089 Chem. Mater. 2017, 29, 6493−6501

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Chemistry of Materials

Figure 2. Structural and optical characterizations of CsSnBr3 cubic nanocages. (A) XRD pattern measured under ambient conditions and the standard pattern of bulk CsSnBr3 (red); crystal structure of CsSnBr3 (cubic, space group Pm3m, a = 6.804 Å) is shown. Inset, the red, green, and purple balls represent Cs, Sn, and Br, respectively. (B) Normalized UV−vis absorption and PL excitation and emission spectra of CsSnBr3 nanocages dispersed in hexane. (C) UV−vis reflectance spectra and the corresponding plots of (ahν)2 vs photon energy (hν) of hollow CsSnBr3 nanocages. These spectra were recorded from drop-casting the samples on glass slides using a spectrometer equipped with an integrating sphere. (D) Timeresolved PL decay and fitting curves of the PL emission at 670 nm with the excitation wavelength of 375 nm.

were derived, which is comparable to those of lead halide perovskite (1−22 ns) and tin halide thin films (∼3.9 ns). Moreover, in contrast to their lead analogues, the measured photoluminescence quantum efficiencies (PLQYs) are quite low (2.1%). Temperature-dependent synthesis was carried out, and the resulting product was examined using TEM. Cubic-shaped product with a darker contrast in the middle region, as compared to the edges (Figures 3A and S4A), was observed when the reaction was carried out at 190 °C. Upon further elevating the reaction temperature to 210 and 220 °C, the solid evacuation consequently became much more obvious and the contrast of edges turned darker than the middle region (Figures 3B,C and S4B,C). Extreme synthetic temperatures (>240 °C) will generate large crystals (Figure 3E). Besides the contrast, the crystallinity of the product also improved with increasing the reaction temperature, which is shown in Figure S6E,F; otherwise, an amorphous product was observed at lower temperatures (Figure S6D). Similar to lead analogues perovskites, the band gap of CsSnBr3 can be slightly tuned by reaction temperature owing to the various particle shapes and edge sizes (Figures 3F and S7, and see the summary in the below table).

CsSnBr3 nanocages adopt a perovskite structure, and the obvious diffraction peaks can be indexed at 2θ = 15.2, 21.5, 26.6, 30.8, and 37.9°, which correspond to diffractions from the {100}, {110}, {111}, {200}, and {211} planes. All diffraction peaks illustrated that CsSnBr3 nanocages crystallize in a cubic structure, Pm3m space group, under ambient conditions (JCPDS No. 22-01 99). No detectable impurity was observed. This is consistent with the TEM observations. The optical properties of CsSnBr 3 nanocages were investigated via UV−vis absorption and photoluminescence (PL) excitation and emission spectroscopies under ambient atmosphere as shown in Figure 2B. The absorption data of CsSnBr3 nanocages display a broad peak centered around 655 nm. The PL emission spectrum shows a peak positioned at 685 nm (1.81 eV) with full width at half-maximum of 56 nm (0.15 eV). To study the direct band gap of nanocages, Kubelka− Munk transformations from solid-state diffuse reflectance spectra were performed (Figure 2C). A plot of [F(R)hν]2 versus energy indicates the direct band gap of 1.85 eV for the CsSnBr3 nanocages with a blue shift compared to bulk CsSnBr3 crystals (1.75 eV). The time-resolved PL decay curve is of CsSnBr3 and was fitted to triexponential decay functions as shown in Figure 2D. The average PL decay lifetimes of 6.52 ns 6495

DOI: 10.1021/acs.chemmater.7b02089 Chem. Mater. 2017, 29, 6493−6501

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Chemistry of Materials

Figure 3. (A−E) TEM images of CsSnBr3 nanocrystals synthesized at different reaction temperatures (190, 210, 220, 230, and 240 °C). (F) PL emission spectra of the sample. Below table: Summary of the calculated band gaps of the samples derived from UV−vis reflectance spectra and the PL emission peaks synthesized at different reaction temperatures.

In addition to reaction temperature, the role of Mg2+ and ligands effect in the generation of a hollow structure of CsSnBr3 have also been investigated. A control experiment was performed by adopting oleylamine bromide as the bromide source instead of MgBr2. The TEM image showed a similar cubic nanocage-like structure which was observed in the case of the MgBr2 method (Figure 4A). In order to preclude the presence of Mg2+ in the perovskite structure or outside surface of nanocages, elemental analysis was carried out, and we found no Mg signals in XPS, EDS spectra. Thus, we can safely say that Mg plays no role in the hollowing process and simply acts as a cation to introduce bromide into the system. Moreover, instead of tin 2-ethylhexanoate, we explored ethylhexanoic acid, noctanoic acid, n-hexanoic acid, and oleic acid as coordinating agent for Sn2+. In the case of oleic acid, it was found that only oleic acid was not enough to dissolve and coordinate with SnO; a much stronger coordinating solvent (tri-n-octylphosphine43) or short carboxylic acid is required, as we used in this paper. On the other hand, n-octanoic acid and hexanoic acid generated solid and irregularly shaped products (Figure 4C,D). Similar to tin 2-ethylhexanoate, the use of ethylhexanoic acid also produced a hollow structure, but the product was not uniform and irregular in shape (Figure 4B). The peculiar behavior of tin 2-ethylhexanoate has been previously reported; mechanistically, the branched n-alkanoates ligands dramatically increase the solubility of nanocrystals in

solution due to the interrupted intermolecular hydrocarbon chains and assist the generation of a myriad of diverse nanostructures.53 Compared to n-octanoic and hexanoic in this study, the branched 2-ethylhexanoate with strong steric repulsion leads to high solubility of metals cations, which in turn assist the migration of ions, and eventually, the limited oriented attachment resulted in the formation of a hollow structure.54 In addition to our study, voids in the lead halide perovskite nanostructure have also been recently reported by other research groups, wherein the void generation was explained by a growth driven self-assembling process.55,56 Therefore, we also supposes that growth driven self-assembly could be the process behind the formation of hollow CsSnBr3 nanocages. As mentioned above, lead halide perovskites possess superior optoelectronic properties, but their use is limited by their poor stability and toxicity. We use tin as a substitute to toxic lead in order to reduce the toxicity of perovskite; however, Sn-based perovskites CsSnX3 have been found more sensitive to the atmosphere in comparison to lead-based perovskite owing to the rapid oxidation of Sn2+ to Sn4+. Thus, improving the stability of lead-free CsSnX3 perovskite nanocrystals can be considered a great improvement for their widespread applications in optoelectronic devices. To compare the stability of our sample with previously reported tin-based perovskite nanoparticles, we prepared CsSnBr3 nanoparticles via following 6496

DOI: 10.1021/acs.chemmater.7b02089 Chem. Mater. 2017, 29, 6493−6501

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Chemistry of Materials

acid (PFOA) as a strong electron-withdrawing group to stabilize Sn2+ species. As expected, after a simple treatment of PFOA (Figure 5C), no noticeable decrease in absorption onset (∼620 nm) of PFOA functionalized CsSnBr3 nanocages film was observed within 16 h under ambient conditions, and the absorption spectrum remains the same as observed in the case of as-prepared CsSnBr3 nanocages product, indicating the usefulness of PFOA as a reagent to enhance the stability of the perovskite structure. Moreover, the color of the PFOA functionalized film remained dark red, which further supports the improved stability of CsSnBr3 nanocages film. Our simple strategy can be extended to other perovskite systems for improving their stability. Since the absorption onset at 620 nm can indicate the decomposition process of perovskite, the absorption intensity of the onset peak over time was used as a reference to evaluate the stability.59,60 Further experiments were also performed to estimate the film stability under various conditions (Figure 5D). Under the desiccation condition, the unfunctionalized CsSnBr3 nanocages film showed a slow disintegration (around 30% decomposition) in 48 h, whereas the PFOA functionalized CsSnBr3 nanocages film similarly displayed slow decomposition (25%) in a moisture-free environment. Under illumination and desiccation conditions, we observed that unfunctionalized films showed a rapid degradation (