Journal of Materials Chemistry C This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online PAPER View Journal | View Issue Room temperature synthesis and characterization of novel lead-free double perovskite nanocrystals Open Access Article. Published on 23 November 2020. Downloaded on 3/24/2021 2:36:02 PM. Cite this: J. Mater. Chem. C, 2021, 9, 158 with a stable and broadband emission† Yingying Tang,a Leyre Gomez,ab Marco van der Laan,a Dolf Timmerman,c Victor Sebastian, def Chia-Ching Huang,a Tom Gregorkiewicza and Peter Schall*a Low-dimensional and lead-free halide perovskites are of great interest for their wide application potential for optoelectronic applications. We report on the successful synthesis of novel lead-free colloidal Cs3BiBr6 nanocrystals (NCs) with an ultra-small size of B1.5–3 nm by a room temperature antisolvent process. From crystallographic characterization we show that it is critical to precisely control the ratio of precursors to Received 14th September 2020, obtain the pure 3-1-6 phase. The synthesis process is facile and repeatable and results in Cs3BiBr6 NCs that Accepted 23rd November 2020 display stable blue emission around 438 nm with a relatively broad linewidth of 92.1 nm. Low-temperature DOI: 10.1039/d0tc04394j photoluminescence (PL) measurements displayed a red-shift of bandgap with decreasing temperature, which might be attributed to the thermal expansion of the lattice. In addition, the NCs demonstrate high rsc.li/materials-c stability at ambient conditions. Introduction For above reasons, there is a strong research effort into finding lead-free perovskite materials that have similar Lead-halide perovskite nanocrystals (NCs) have recently beneficial optoelectronic properties as their lead-containing emerged in various applications like solar cells1,2 and light- counterparts. So far, several works have reported on lead- emitting diodes.3,4 The benefits of NCs in this context are the free perovskites; among these are Bi, Sn, and Ge-based tunability of their optoelectronic properties by variation of perovskites.10–17 However, oxidation of Sn2+ and Ge2+ in ambi- their size, composition and surface ligands5 and the high ent conditions limits their synthesis and application in natural photoluminescence quantum yield (PLQY). Other interesting environments. Bismuth-based perovskites do not suffer from properties observed in these materials include efficient carrier this effect and are strong candidates for non-toxic and stable multiplication,6 activation of a strong light-induced forbidden perovskite material. Currently, most of the Bi-based perovskites exciton transition due to the influence of polaron in confined are low-dimensional structures, which could be accompanied space,7 and an efficient triplet energy-transfer efficiency to by exotic and novel optical properties. For instance, recent pyrene complexes.8 Despite these interesting features, the reports in defect-tolerant double perovskites Cs2ABX6 (with A: important limitation of these materials in applications is the Ag, Cs, Na; B: In, Bi, Sb; X: Cl, Br, I), showed slow hot-carrier toxicity of lead; Pb2+ is soluble in water, and exposure to this relaxation, dual-color emission, coexistence of direct–indirect has a serious impact on health.9 bandgap and single emitter based warm-white emission.18–21 Especially, for NCs in the quantum confinement regime, optical a Institute of Physics, University of Amsterdam, Science Park 904, and electronic properties are strongly altered. For example, 1098 XH Amsterdam, The Netherlands. E-mail: P.Schall@uva.nl Cs3Bi2I9 NCs showed emission from exciton and defect states b Catalan Institute of Nanoscience and Nanotechnology, CSIC, BIST, inside the bandgap, and induced broadband emission, as well and CIBERBBN, 08193 Bellaterra Barcelona, Spain c as enhanced Raman scattering due to the strong exciton–ligand Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan electronic coupling.22 Several structures have been reported in d Department of Chemical Engineering and Environmental Technology, Universidad the A–Bi–X perovskite series, however, only a few of them are in de Zaragoza, Campus Rı́o Ebro-Edificio I + D, 50018 Zaragoza, Spain the form of colloidal NCs. For example, it has been shown that e Instituto de Nanociencia y Materiales de Aragón (INMA), relatively large NCs of Cs3BiCl6 could be obtained by a hot- University of Zaragoza-CSIC, c/Marı́a de Luna 3, 50018 Zaragoza, Spain f injection method.23 Networking Research Center on Bioengineering Biomaterials and Nanomedicine (CIBER-BBN), Madrid, Spain Here, we report on the synthesis of stable bismuth-based † Electronic supplementary information (ESI) available. See DOI: 10.1039/ double perovskite NCs, Cs3BiBr6, using an antisolvent precipita- d0tc04394j tion method at room temperature. Cs3BiBr6 features an isolated 158 | J. Mater. Chem. C, 2021, 9, 158--163 This journal is © The Royal Society of Chemistry 2021 View Article Online Paper Journal of Materials Chemistry C BiBr6 octahedra structure, which can be considered a double for absorption by using a multi-scan method.25 The structure perovskite.24 The synthesized NCs have extremely small particle was solved using direct methods and refined by full-matrix sizes in the range of 1.5–3 nm, depending on the precursor least-squares fitting on F2 by SHELX-97.26 All non-H atoms were ratio, and display blue emission around 438 nm, independent refined using anisotropic thermal parameters. The final refined This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. of this ratio. Furthermore, they show an excellent stability at structural parameters were checked by using the PLATON room temperature for at least 3 weeks. The synthesis is repro- program.27 ducible, and the NCs are obtained in a wide range of precursor ratios. Optical measurements The absorption spectra was obtained by measuring the trans- Open Access Article. Published on 23 November 2020. Downloaded on 3/24/2021 2:36:02 PM. mission on A LAMBDA 950 UV/vis/NIR spectrophotometer Experimental section (PerkinElmer). The separately measured absorption spectrum Materials of the solvent (IPA) was subtracted from the NC spectrum. PL BiBr3 (Z98%), CsBr (99.9%), oleic acid (OA, 90%), hydrobromic spectra were determined with a Jobin Yvon FluoroLog spectro- acid (HBr, 48%) and dimethyl sulfoxide (DMSO, Z99.9%) were fluorometer (Horiba). As excitation source, a 450 W xenon lamp all purchased from Sigma-Aldrich. Isopropanol (IPA, Z99.5%) coupled to a monochromator was used. The PL data was was purchased from EMD Millipore company. All materials collected for excitation wavelength of 355 nm. All spectra are were used as received without further purification. corrected for the spectral sensitivity of the spectrofluorometer. The determination of the PLQY was based on an integrating Synthesis of Cs3BiBr6 NCs sphere methodology.28 Time-resolved PL measurements were performed using a LifeSpecII time correlated single photon For the synthesis of ligand-free Cs3BiBr6 NCs, 120 mmol CsBr counting (TCSPC) spectrometer series, 100 ps was used as (25.5 mg) and 30 mmol BiBr3 (13.45 mg) were dissolved in 2 mL excitation source. A right angle between the excitation and DMSO at room temperature or 70 1C to form the precursor emission beam paths was maintained to reduce detection of solution. Alternatively, 90 mmol CsBr (19.2 mg), 30 mmol BiBr3 scattered excitation light. For the low temperature PL setup, (13.45 mg) and HBr (100 mL) were dissolved in 2 mL DMSO to a xenon lamp (L2273 Hamamastu) coupled to a Solar MSA 130 form precursor solution. 200 mL of the above precursor solution double grating monochromator was used as an excitation was injected into 5 mL IPA, under vigorous stirring for one source. The collected emission was dispersed by an M266 (Solar minute. Then the solution was centrifuged at 10 000 rpm for LS) monochromator coupled to a silicon CCD (Horiba scienti- 5 min to discard the large particles. After that, clear colloidal fic). The sample was cooled in a CCC1104 (Oxford inst.) cryo- NCs dispersed in IPA were obtained. For the synthesis of OA-capped stat, which was used in combination with a closed-cycle helium Cs3BiBr6 NCs, all the procedures were the same with the only compressor (Cryomech). The sample was placed on a cold difference being the addition of 300 mL OA into the precursor. finger in a static exchange gas to ensure the temperature setting Measurement and characterization was obtained. A 355 nm excitation wavelength was used to collect the low temperature PL spectra. Materials characterizations. Samples were prepared by drop- casting the NCs containing solution onto a Lacey grid for measurements. The grids made from Cu were used as support Results and discussion materials for Transmission Electron Microscope (TEM) ima- ging. TEM imaging was conducted at 300 kV on an ultra-high The double perovskite Cs3BiBr6 crystallizes in the orthorhom- resolution imaging Titan Cube 0 Transmission Electron Micro- bic system in a space group of Pbcm, with a = 8.689(2) Å, scope from FEI Company. The microscope is equipped with a b = 13.628(1) Å, c = 27.694(9) Å.24 Each Bi is coordinated with six SuperTwins objective lens and a CETCOR Cs-objective correc- Br atoms, forming BiBr6 octahedra. BiBr6 octahedra are sepa- tor from CEOS Company allowing a point to point resolution of rated from each other, forming a zero-dimensional perovskite 0.08 nm. High-angle annular dark-field-scanning transmission structure, as shown in Fig. 1a. In the present work, the Cs3BiBr6 electron microscopy (HAADF-STEM) and energy dispersive NCs were synthesized by an antisolvent precipitation method. X-ray (EDX) measurements were also performed on the same For the synthesis of the nanomaterials, a precise control of system. The phase and purity of all samples were characterized the precursor ratios between BiBr3 and CsBr was required. As by powder X-ray diffraction (XRD) with Cu Ka1 (l = 1.5406 Å) control experiments, different ratios of precursors were tried to radiation (Rigaku MiniFlex-II). X-ray Photoelectron Spectro- obtain the NCs. To exclude the effects of excess of Cs+ on the scopy analyses were carried out with a Kratos Axis SUPRA synthesis, HBr was instead used to introduce excess Br while spectrometer using a monochromatic Al K(alpha) source keeping the stoichiometric ratio between CsBr and BiBr3. (15 kV). Small crystals of Cs3BiBr6 obtained from previous work Cs3BiBr6 NCs could also be obtained using HBr as precursor were selected and data collections were performed on a Bruker (Fig. 1b). Controlling the ratios of the precursors is critical APEX-II CCD diffractometer equipped with graphite monochro- for the formation of the NCs, as we confirmed by a series of mated Mo Ka radiation (l = 0.71073 Å) at 120 K.24 The data sets control experiments. As displayed in Fig. S1a (ESI†), when the were corrected for Lorentz and polarization factors as well as ratio (CsBr/BiBr3) was below 4, Cs3Bi2Br9 NCs or a mixture of This journal is © The Royal Society of Chemistry 2021 J. Mater. Chem. C, 2021, 9, 158--163 | 159 View Article Online Journal of Materials Chemistry C Paper This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 23 November 2020. Downloaded on 3/24/2021 2:36:02 PM. Fig. 1 (a) Crystal structure of Cs3BiBr6. (b) XRD pattern of Cs3BiBr6 NCs with addition of CsBr or HBr precursors and the simulated XRD pattern (CCDC, #1877001). (c) XRD pattern of Cs3BiBr6 for several precursor ratios of CsBr/BiBr3 at room temperature and at 70 1C. (d) Representative TEM image for Cs3BiBr6-5.5 NCs. (e) HRTEM image with an atomic resolution for a Cs3BiBr6-5.5 NC. Cs3Bi2Br9 and Cs3BiBr6 NCs were obtained. In contrast, for peaks can be well identified by the simulated Cs3BiBr6 XRD ratios between 4 and 7.5, we clearly observed the Cs3BiBr6 pattern. Yet, in Fig. 1c, many of the weak diffraction peaks are phase alone, as shown by the XRD patterns in Fig. 1c, which hidden by the pronounced peaks with high intensity in these match well with its simulated pattern. When the ratios are three samples. In the HRTEM image of Cs3BiBr6-5.5 NCs, the larger than 7.5, several dominant peaks are displayed, i.e. (136), clear lattice fringes of 0.21 nm and 0.25 nm in a single NC could (410), (331), (130), (025) crystalline planes for Cs3BiBr6.24 Peaks be related to the crystalline planes (412) and (244) of Cs3BiBr6, from CsBr and BiBr3 precursor impurities can be excluded by respectively, while a lattice spacing of 0.22 nm corresponds to comparison with their XRD peaks. We thus consider that the the (111) plane (Fig. S2d, ESI†). In order to check the chemical morphology of the NCs may differ for lower precursor ratios. In composition of these NCs, XPS measurements on Cs3BiBr6-5.5 order to check this, we characterized the NCs with different NCs were conducted, which indeed show characteristic peaks ratios of precursors. For convenience we use Cs3BiBr6-4, of Cs, Bi and Br (Fig. S4, ESI†). The detailed elemental ratio Cs3BiBr6-5.5, Cs3BiBr6-7.5, Cs3BiBr6-10, Cs3BiBr6-20 to indicate between Cs, Bi, and Br elements was calculated as Cs2.3BiBr6.1, molar ratios of CsBr/BiBr3 precursors of 4, 5.5, 7.5, 10, 20, which is close to the reported chemical formula. respectively. Corresponding XRD patterns plotted in Fig. 1c The optical properties of Cs3BiBr6 NCs are shown in Fig. 2 show that the peaks are robust. Furthermore, to investigate if and Fig. S5 (ESI†). Fig. 2a shows that the absorption peak of the the Cs3BiBr6 phase is still stable at high temperature, the NCs appears around 385 nm, which is similar to the previous reaction temperature was increased to 70 1C. The corres- result through a halide exchange reaction from Cs3BiCl6,23 and ponding XRD pattern shows that Cs3BiBr6 NCs could also be its behaviour is independent of the precursor ratio (Fig. 2b). obtained. The absorption spectra for Cs3Bi2Br9 NCs display an absorption Fig. 1d and e present HRTEM images of Cs3BiBr6-5.5 NCs. onset of around 440 nm (Fig. S1b, ESI†), which is different from The individual NCs could be observed with an average size of the onset at 385 nm for Cs3BiBr6 NCs. This further excludes the around 1.6 nm (inset of Fig. S2a, ESI†). For Cs3BiBr6-10, the possibility of impurities from large amount of Cs3Bi2Br9 NCs. average size NCs is about 2.3 nm (Fig. S2b and d, ESI†). Also, a We thus hypothesize that it may be the crystallization pathway branch-like morphology of NCs was observed (Fig. S2c, ESI†), that makes it necessary to mix with a molar ratio of 4 : 1 or more which might induce the dominant orientation of NCs corres- to obtain the pure phase. The PL peak is located at 438 nm ponding to the pronounced XRD diffraction peaks, for example, and the bandgap was determined from the Tauc plot in Fig. S6 the XRD diffraction peak at B291 in the XRD pattern. We (ESI†) as 2.89 eV (429 nm). Similar robustness as the absorption conclude by comparison with simulated XRD patterns (CsBr, spectra is observed for the PL spectra of the different materials Fm3% m; CsBr, Pm3% m; and BiBr3, P21/c) and Cs3BiBr6 that these as shown in Fig. 2c, which are nearly identical. The peak dominant XRD diffraction peaks e.g. XRD diffraction peak at positions display no obvious changes upon increasing CsBr, B291, cannot result from the precursors (Fig. S3a, ESI†). Mean- which is consistent with the TEM results. This can be well while, as shown in the zoomed-in XRD for Cs3BiBr6-7.5, explained by the small Bohr radius of Cs3BiBr6 NCs (B0.5 nm).29 Cs3BiBr6-10, Cs3BiBr6-20 NCs (Fig. S3b, ESI†), all the dominant The as-synthesized Cs3BiBr6 NCs with size of 1.5–3 nm are 160 | J. Mater. Chem. C, 2021, 9, 158--163 This journal is © The Royal Society of Chemistry 2021 View Article Online Paper Journal of Materials Chemistry C passivation, or direct emission from the OA ligands. Future studies on carrier dynamics will be necessary to point out the exact origin. One important characteristic for understanding their photo- This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. physical properties, especially those depending on photo- excited charge carriers and thus applicability of photonic or optoelectronic applications, is the absorption cross-section. We have determined the absorption cross-section s from linear absorption measurements using the following equation: Open Access Article. Published on 23 November 2020. Downloaded on 3/24/2021 2:36:02 PM. T = e( lsn) (1) where T is the transmittance, l is the path length and n is the density of the NCs. For the cuvette used in our lab, the path length is l = 1 cm. From the measured transmittance T = 0.35 and the NC concentration of 80 mg mL 1, the Fig. 2 Optical properties for ligand-free Cs3BiBr6 NCs: (a) absorption absorption cross-section for the Cs3BiBr6 NCs is calculated to spectra and PL spectra for Cs3BiBr6-5.5 NCs. (b) Absorption spectra for be 2.4  10 17 cm2.35 This value is about an order of magnitude different ratios of precursors. (c) PL spectra for the addition of different lower than what would be estimated from extrapolating the ratios of precursors. (d) TR-PL kinetics of Cs3BiBr6-5.5 NCs with and without OA capping measured with TCSPC (375 nm pump) fitted with a size-dependent cross-section of the well-known CsPbBr3 QDs.36 bi-exponential function. Due to the low PLQY of Cs3BiBr6 at room temperature, we also performed PL measurements at low temperatures in order to obtain information about processes that are responsible for around 3 to 6 times larger than this Bohr radius, and are thus PL quenching at higher temperatures. The PL spectra recorded not expected to display any significant quantum confinement for a temperature range of 50–300 K are displayed in Fig. 3a and effects, resulting in robust PL emission peaks within this size Fig. S8 (ESI†). They reveal two important features: an increase regime. We have determined the PLQY for the Cs3BiBr6 NCs of the PL intensity for lower temperatures, accompanied by a passivated by OA ligands as 22%. However, we found that the red-shift. The integrated PL intensity as a function of reciprocal OA itself has a strong emission band partly overlapping with temperature is depicted in Fig. 3b. We use this to determine the the emission of the NCs, as shown in Fig. S7a (ESI†). For activation energy of the non-radiative process responsible for comparison, the PL spectra of OA-capped NCs are also shown the thermal quenching, by fitting with the Arrhenius formula: in Fig. S7b (ESI†), which reveal a strong emission signal in the range of 340–500 nm. A similar phenomenon has also been PL(T) = I0/(1 + A  exp( e/kBT)) (2) reported previously.22 We therefore prepared fresh samples where I0 and A are free parameters, e is the effective activation without ligands. These demonstrated a considerably lower energy, and kB is Boltzmann’s constant. The indicated fit gives PLQY, which was difficult to determine accurately. an activation energy of 41 meV, which is of the same order of To obtain more insight, we studied the photophysics of both magnitude as that of other perovskites.37–39 The red shift in ligand-free and OA-capped Cs3BiBr6-5.5 NCs by time-resolved band gap at low temperature has been found in CsPbBr3 photoluminescence (TRPL) under excitation of a 375 nm laser. quantum dots, which was attributed to the thermal expansion The results are shown in Fig. 2d. The PL decays were fitted of the lattice limiting the interaction between valence orbitals with a bi-exponential function, both with a fast component and leading to the narrowing of the valence bandwidth and (t1 = 1.0 ns) and a slower component of 2.8 and 4.3 ns for the increase of the forbidden gap.40 we have measured the crystal ligand-free and OA capped NCs, respectively (ligand-free: A1 = structure of the Cs3BiBr6 single crystals at 120 K based on the A2 = 0.5; OA-capped: A1 = 0.55, A2 = 0.45). The longer lifetime is previously reported work.24 At 120 K, Cs3BiBr6 single crystals comparable to other reported lead-free perovskite NCs: average lifetimes of 4.3 ns in Cs3Sb2Br9 NCs25 and 2.1 ns in Cs3Bi2Br9 NCs30 have been obtained. The increase in lifetime for OA-capped NCs cannot fully explain the much higher PLQY that we observe as compared to ligand-free NCs. It is known from studies on carrier dynamics in other lead-free NCs that they typically show an ultrafast ({100 ps) carrier trapping process that strongly limits their PLQY.31–34 Passivation by OA alleviates this process and increases the PLQY. Since the time resolution of our system is longer than such and ultrafast trapping process, we do not observe it in the PL dynamics. Fig. 3 (a) Temperature-dependent PL spectra of OA-capped Cs3BiBr6 However, we note that the longer decay time we observe for NCs at the temperature range of 50–300 K. (b) Integrated PL intensity of OA-capped NCs indicates that it is either due to OA-related Cs3BiBr6 NCs versus inverse temperature and the Arrhenius fitting. This journal is © The Royal Society of Chemistry 2021 J. Mater. Chem. C, 2021, 9, 158--163 | 161 View Article Online Journal of Materials Chemistry C Paper and generates NCs with an ultra-small size of B1.5–3 nm. In order to obtain pure Cs3BiBr6 NCs, an excess amount of Br is necessary. Otherwise, another bismuth phase, Cs3Bi2Br9, will also be one of the products. The Cs3BiBr6 NCs display stable This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. blue emission, centred at 438 nm, with a broad linewidth of 92.1 nm. At ambient conditions, the NCs display a high stability against the solvent and ambient air. This work proposes a promising low-dimensional candidate for lead-free opto- electronic applications. Open Access Article. Published on 23 November 2020. Downloaded on 3/24/2021 2:36:02 PM. Conflicts of interest There are no conflicts to declare. Acknowledgements We are grateful for the Dutch Technology Foundation STW, The Fig. 4 Stability for Cs3BiBr6 ligand-free NCs. (a) UV-vis absorption spectra Netherlands Organization for Scientific Research (NWO), and after wash treatment with IPA twice. (b) XRD pattern after washing treatment by IPA twice. (c) XRD patterns for fresh and aged samples after the Joint Solar Program (JSP III, 680-91-011) of The NWO for 7 days. financial support. We acknowledge Arnon Lesage and Dido Van der Gon for their kind help. V. S. acknowledges the LMA-INA for offering access to their instruments and expertise. display the same structure (space group, Pbcm with a = 8.637(3) Å, b = 13.495(6) Å, c = 27.532(6) Å), as at room temperature (space group of Pbcm with a = 8.689(2) Å, b = 13.628(1) Å, and Notes and references c = 27.694(9) Å), see Table S1 (ESI†). This indicates that at low 1 A. Swarnkar, A. R. Marshall, E. M. Sanehira, B. Chernomordik, temperature, no phase transition occurs. D. T. Moore, J. A. Christians, T. Chakrabarti and J. M. Luther, The stability of NCs is critical for their potential applica- Science, 2016, 354, 92. tions. We investigated various aspects of their stability 2 L. M. Wheeler, E. M. Sanehira, A. R. Marshall, P. Schulz, which are displayed in Fig. 4. First, the stability against an M. Suri, N. C. Anderson, J. A. Christians, D. Nordlund, IPA washing treatment was investigated and as shown in D. Sokaras and T. Kroll, J. Am. Chem. Soc., 2018, 140, 10504. Fig. 4a. Before the treatment, the absorption peak for Cs3BiBr6 3 J. Sun, J. Yang, J. I. Lee, J. H. Cho and M. S. Kang, J. Phys. NCs appears at around 385 nm. After one additional washing Chem. Lett., 2018, 97, 1573. treatment by IPA, the absorption peaks for all samples are still 4 J. Xing, Y. B. Zhao, M. Askerka, L. N. Quan, X. W. Gong, at the same position, consistent with the different kinds of W. J. Zhao, J. X. Zhao, H. R. Tan, G. K. Long, L. Gao, ratios of precursors and also for higher temperature. This Z. Y. Yang, O. Voznyy, J. Tang, Z.-H. Lu, Q. H. Xiong and demonstrates the stability of Cs3BiBr6 NCs against washing E. H. Sargent, Nat. Commun., 2018, 9, 3541. treatment with IPA solvent. Additionally, we used XRD to check 5 Y.-F. Li, J. Feng and H.-B. Sun, Nanoscale, 2019, 11, 19119. the chemical composition after the IPA treatment (Fig. 4b), 6 C. de Weerd, L. Gomez, A. Capretti, D. M. Lebrun, after single and double washing. Compared to the XRD in E. Matsubara, J. H. Lin, M. Ashida, F. C. M. Spoor, Fig. 1b, no impurities or other phases were found in the XRD L. D. A. Siebbeles, A. J. Houtepen, K. Suenaga, Y. Fujiwara data, supporting the conclusion that these NCs are stable and T. Gregorkiewicz, Nat. Commun., 2018, 9, 4199. against IPA solvent washing. Another important factor for 7 D. Rossi, H. Wang, Y. T. Dong, T. Qiao, X. F. Qian and applications is the air stability. The XRD spectra for a fresh D. H. Son, ACS Nano, 2018, 12, 12436. and aged sample, after storage at ambient condition for 7 days, 8 X. Luo, R. C. Lai, Y. L. Li, Y. Y. Han, G. J. Liang, X. Liu, were investigated as shown in Fig. 4c. In addition, the optical T. Ding, J. H. Wang and K. F. Wu, J. Am. Chem. 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