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Article Cite This: ACS Omega 2019, 4, 12948−12954
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Surfactant-Induced Anion Exchange and Morphological Evolution for Composition-Controlled Caesium Lead Halide Perovskites with Tunable Optical Properties Arup Ghorai,† Anupam Midya,*,† and Samit K. Ray*,‡,§ School of Nanoscience and Technology and §Department of Physics, Indian Institute of Technology Kharagpur, Kharagpur 721302, India ‡ S. N. Bose National Centre for Basic Sciences, Kolkata 700106, India Downloaded via 91.243.191.235 on August 1, 2019 at 03:02:08 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
S Supporting Information *
ABSTRACT: Environmentally stable lead halide perovskite nanostructures with engineered composition and morphology are attractive because of their exotic optical properties. Here, we report the synthesis of monodispersed (∼20 nm) CsPbI3 cubic perovskite nanocrystals (NCs) using edible olive oil as a solvent as well as a chelating reagent. Thereafter, bromide anion exchange reaction using the cetyl trimethyl ammonium bromide surfactant in hexane is carried out at relatively lower temperatures to synthesize caesium lead halide perovskites with variable halide compositions and tunable band gaps. Interestingly, because of the formation of micelles, continuous morphology evolution varying from NCs of different sizes to nanowires (NWs) and nanosheets is observed. The anion exchange temperature has a distinct effect on the morphology of the CsPbBr3 nanostructure and the anion exchange reaction rate. Finally, an easy solution-processed photoconductive device is demonstrated using as-grown CsPbBr3 NWs, indicating its potential for optoelectronic applications.
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INTRODUCTION Inorganic caesium lead halide perovskite nanocrystals (NCs) have attracted immense attention because of their incredible optical properties, high quantum yield of emission, and tunable band gap by varying the halide composition1 and cations. Mixed methyl-ammonium and formamidinium (FA) lead halide (MAPbX3, FAPbX3, X = Cl, Br, I) perovskite-based solar cells have been reported to achieve an exciting 22% power conversion efficiency,2 though they suffer from stability toward moisture3,4 and temperature beyond 100 °C.5 Recently, it has been reported that the thin films of organic FA and inorganic caesium (Cs) cation mixtures are structurally and thermally stable above 100 °C.6 Li et al. have reported that the stability of the FAPbI3 can be improved by Cs doping.7 Similarly, Wang et al. demonstrated direct colloidal synthesis of stable FA and caesium-based double cation lead halide perovskite with controllable morphology.8 To improve the thermal stability of the perovskite, the organic part needs to be fully replaced by an inorganic cation such as Cs. The fully inorganic perovskite materials have now drawn much attention for both photovoltaic and light-emitting-based device applications. Colloidal CsPbX3 NCs have a unique ability of anion exchange reaction, which opens a new door to tune the sizecontrolled band gap and for studying interesting chemical and physical properties. By postsynthesis chemical engineering, the composition as well as optical properties9−11 can be tuned by cation and anion exchange reactions. In most of the cases the © 2019 American Chemical Society
precursors are highly reactive; which spontaneously undergo anion exchange, when they come in contact with the NCs. Ramasamy et al.12 reported a postsynthesis anion exchange reaction of CsPbX3 using LiX at room temperature. Benzoyl halide,13 Grignard reagents (MeMgX),9 oleylammonium halides (OAmX),14 and tertiary butyl ammonium halide (TBAX)15 have also been used as alternative precursors for postsynthesis anion exchange reaction. Morphology of the CsPbX3 nanostructures, viz., NCs, nanosheets (NSs), nanowires (NWs), and so on, can be engineered by varying the reaction condition and temperature. There are a few reports on the synthesis of two-dimensional (2D) CsPbX3 NSs by varying the hydrocarbon chain length of the primary amine or tuning the ligand precursor ratio.16,17 Pan1 et al.18 systematically studied the effect of the carbon chain length on the surface chemistry as well as morphologies of the perovskite NCs. Seth and Samanta reported a sequential change in morphology by varying different parameters such as amount of oleylamine, addition of antisolvents, and the reaction time.19 Zhang et al.10 reported the formation of CsPbX3 NWs by anion exchange reaction using CsPbBr3 NWs as the template and PbX2 or OAmX as the anion source. Even incident light had resulted in compositional and structural transition from orthorhombic CsPbBr3 to tetragonal CsPb2Br5 Received: March 26, 2019 Accepted: July 9, 2019 Published: July 31, 2019 12948
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Figure 1. Schematic diagram of CTAB-assisted anion exchange reaction of CsPbI3 to CsPbBr3 at 60 °C.
Figure 2. Evolution of UV−visible absorption spectra during the anion exchange reaction of CsPbI3 in the presence of 45 mg of CTAB at different temperatures (a) at 60, (b) 80, and (c) 100 °C. (d) Change of band gap at different durations at three different reaction temperatures.
Figure 3. Evolution of PL spectra of the anion exchange reaction of CsPbI3 in the presence of 45 mg of CTAB at different reaction temperatures (a) 60, (b) 80, and (c) 100 °C. (d) Digital images of PL emission at different stages of the anion exchange reaction at 60 °C.
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Figure 4. TEM image of CsPbI3 NCs (a), CsPbBr3 NWs at 60 °C (b), CsPbBr3 NSs at 80 °C (c), and degraded CsPbBr3 NCs at 100 °C (d). SAED patterns of CsPbI3 NCs (e), CsPbBr3 NWs (f), CsPBr3 NSs (g), and degraded CsPbBr3 NCs (h). Lattice fringe patterns of CsPbI3 NCs (i), CsPbBr3 NWs (j), CsPBr3 NSs (k), and degraded CsPbBr3 NCs (l).
NSs and vice versa.20 To the best of the authors’ knowledge there are no reports where the compositions as well as morphology are tailored simultaneously in an anion exchange reaction. Here, in this report, we present the compositional variation and morphological evolution of CsPbX3 perovskites by varying the halide precursors in an anion exchange reaction, which have resulted in tunable optical properties. CsPbI3 NCs of uniform size have been synthesized using edible olive oil as a solvent as well as a ligand, by replacing oleic acid. Thereafter, postsynthetic anion exchange reaction of CsPbI3 has been performed using cetyl trimethyl ammonium bromide (CTAB) as a bromide ion precursor. With the variation of anion exchange reaction temperature, the reaction rate drastically increases accompanied with a morphological change of perovskite NCs to NWs and NSs. The optical characteristics and photoconducting properties of solution-processed CsPbBr3 NWs are presented to evaluate their potential applications in optoelectronic devices.
principle, a hard−hard and soft−soft combination is favorable for the formation of a stable adduct. Therefore, the iodide ion of perovskite prefers to attach with the CTA+ to form a comparatively stable adduct. Simultaneously, the vacant position of iodide ion is occupied by the bromide ion. To get an insight into the anion exchange reaction, the reaction temperature has been varied using 45 mg of CTAB and the reaction kinetics has been studied by UV−visible (Figure 2) and photoluminescence (PL) spectroscopy (Figure 3). Figure 2 shows the evolution of absorption spectra describing the formation CsPbBr3 from CsPbI3 sequentially at three different reaction temperatures (60, 80, and 100 °C). CsPbI3 exhibits absorption in the broad region of the visible wavelengths. The PL spectrum of CsPbI3 exhibits an asymmetric emission peak, which converts to a symmetric one after incorporation of Br into the host crystal. This indicates that, initially, compositional defects are present in CsPbI3.22 A similar asymmetric PL spectrum was reported by Lee et al., because of the presence of both cation and anion vacancies in perovskite NCs.23 It is observed that with the increase in temperature, the rate of reaction increases drastically (Figure 2d). Similar results are also observed in PL spectra acquired at 3 min intervals while monitoring the anion exchange reactions. Initially, CsPbI3 shows a sharp emission peak at 680 nm, which is gradually shifted to 545 nm for CsPbBr3, indicating full conversion from iodide to bromide. Figure 3d shows the sequential digital images as a function of time for CsPbBr3 formation from CsPbI3 at 60 °C. Two control experiments at room temperature and 0 °C have been carried out to confirm the role of temperature. At room temperature it takes almost 4−5 days for partial anion exchange before degradation of the perovskite structure (Figure S2a, Supporting Information). However, at 0 °C, the reaction does not occur at all and the solution remains almost unchanged after 15 days (Figure S2b, Supporting Information). Almost 40 min is required to complete the anion
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RESULTS AND DISCUSSIONS A colloidal solution of CsPbX3 (X = Br, I) NCs in hexane has been synthesized using edible olive oil. In the process, oleic acid present in olive oil forms caesium-oleate as well as leadoleate. Oleylamine used in the synthesis process helps in the formation of an OLAm−X complex and simultaneously stabilizes the NCs in the colloidal solution.9 Anion exchange reaction has been carried out using CTAB as the surfactant and a bromide ion source, to achieve tunable absorption and emission properties. The schematic of the CsPbBr3 NW formation during anion exchange reaction is shown in Figure 1. The quaternary cetyl tertiary ammonium cation (CTA+) of CTAB is a soft acid because of the absence of the hydrogen atom.21 On the other hand, iodide anion is the softest base among the halide group because of its highly polarizable nature. According to the hard soft acid base 12950
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exchange at 60 °C (Figure 2a), whereas 30 min is enough at 80 °C (Figure 2b) for the completion of anion exchange reaction. The completion time further decreases to 15 min at 100 °C (Figure 2c). The rate of the reaction can be monitored indirectly through the change of the band gap with time, as presented in Figure 2d. The rate of the change of band gap is calculated to be 0.0186, 0.0297, and 0.032 eV/min at 60, 80, and 100 °C, respectively. This clearly indicates that the rate of anion exchange reaction with CTAB is strongly temperaturedependent. There are two reasons for the increase of reaction rate with temperature. The rate of formation of the reverse micelle mainly depends on the solubility of the surfactant as well as the Kraft temperature, the temperature above which micelle formation occurs at a certain concentration of the surfactant. This Kraft temperature depends on the polarity of the solvent as well as the nonpolar chain of the surfactant.24 With increase in reaction temperature, the solubility of CTAB increases, resulting in enhanced probability of micelle formation. The second reason for the increase in the rate of reaction with temperature is the activation energy. With the increase in temperature, CTAB molecules are activated for the anion exchange reaction, which leads to the enhancement of the reaction rate at higher temperatures. We have also performed a time-resolved PL decay study of the anion exchange product using 45 mg of CTAB at 60 °C (Figure S2c, Supporting Information). The PL decay lifetime of the nanostructures depends on the compositions, size, morphology, and surface state present in the system. Sun et al. reported the average PL decay lifetime for a CsPbBr3 nanocube of 1−22 ns.25 Initially, the PL decay lifetime for CsPbI3 NCs is found to be 43.2 ns and after full transformation (40 min) to CsPbBr3 the PL decay lifetime decreases to 12.1 ns, clearly supporting the anion exchange process. The decay time of CsPbI3 NCs is found to be longer than that of CsPbBr3 NWs, indicating the presence of a defect state in CsPbI3 NCs. Similarly, a longer decay lifetime was observed by Ding et al. because of the presence of a defect state in MAPbI3 crystals.26 Simultaneously, the evolution of morphology of the samples has been monitored through the transmission electron microscopy (TEM) analysis, as displayed in Figure 4. Interestingly, the reaction temperature influences the morphology of the lead halide perovskite. Figure 4b shows the TEM micrograph of few-micron-long CsPbBr3 NWs with a diameter of ∼200 nm synthesized at 60 °C. Rhombus-shaped 2D flakes of size ∼150 × 150 nm are obtained at 80 °C (Figure 4c). The selected area electron diffraction (SAED) pattern and high-resolution lattice fringe patterns of the sample grown at different temperatures are shown in Figure 4e−h,i−l, respectively. From the SAED pattern, high crystallinity of the NCs, NWs, and NSs are found. The high-resolution transmission electron microscopy (HRTEM) lattice fringe pattern shows an interplanar spacing of 0.62 (Figure 4i) and 0.29 nm (Figure 4j−l) corresponding to the (100) plane of CsPbI3 and (110) plane of CsPbBr3, respectively. The shapes of the NCs are distorted at 100 °C, because of the degradation of the perovskite NCs. Seth and Samanta27 reported the synthesis of the CsPbBr6 nanostructure of different morphologies by a surfactantmediated antisolvent precipitation method. CTAB preferentially binds to the (110) plane as compared to the (300) one, which leads to the formation of NWs as CTAB concentration increases to a saturation level. In microemulsion technique,
surfactants result in thermodynamically stable reverse micelles in a nonpolar solvent.28 Similarly, the polar head group of CTAB and perovskite NCs forms the inner core of a cylindrical/lamellar reverse micelle in nonpolar hexane during anion exchange reaction. As a result, CsPbBr3 NWs are obtained as shown schematically in Figure 1 at 60 °C. To get an insight into the formation of micelle and particle size distribution, we have performed dynamic light scattering (DLS) measurements for the product of anion exchange reaction at 60, 80, and 100 °C (Figure S8). We have also performed DLS measurement during the anion exchange reaction at different times at 60 °C. The average micelle size for initial (at 0 min) CsPbI3 NCs with CTAB is found to be 20−25 nm. After 15 min, the micelle size increases to 1−2 μm possibly due to the formation CsPbBrxI3−x NWs by partial halide exchange (Figure S9). The average particle size after anion exchange at 60 °C is found to be above 3−4 μm after completion of the reaction. The average size at 80 and 100 °C is found to be ∼1−1.5 μm and 30−40 nm, respectively. The results agree well with the particle size extracted from TEM analysis. The size and shape of the reverse micelle strongly depend upon the temperature. A rise in temperature causes an increase of micelle aggregation number. Simultaneously, with the increase in temperature, the radii of the micelle also increase with a broader size distribution, leading to the formation of rod- or plate-shaped micelles.29 Similarly, a morphology change from small cubic NCs to long NWs and larger NSs is also observed in this study. However, any further increase in temperature causes the degradation of perovskites (CsPbI3 NC) before completion of the anion exchange reaction. Pradhan et al. have also observed spontaneous transformation of CsPb(BrxI1−x)3 nanocubes to NWs because of partial removal of oleic acid and oleylamine ligands, which facilitates the growth along the ⟨002⟩ direction.30 Further, three different amounts of CTAB (30, 45, and, 60 mg) have been employed for anion exchange and the process was monitored by UV−visible and PL spectroscopy. It is observed that the concentration of CTAB does not affect the rate of the reaction (Figure S3a,b, Supporting Information). Almost the same time duration is required for the completion of anion exchange reaction at 60 °C. As CTAB is sparingly soluble in hexane, at different CTAB concentrations, the solubility remains almost the same at 60 °C. That is why for all the cases, almost the same duration is required for the completion of anion exchange reaction. However, with the increase in amount of CTAB from 30 to 60 mg, the density of CsPbBr3 NWs increases, as evident from the TEM image (Figures S4 and S7, Supporting Information). The crystalline quality of CsPbBr3 NWs has been studied through X-ray diffraction (XRD) analysis. XRD patterns of CsPbI3 and CsPbBr3 synthesized at different temperatures are shown in Figure 5. The characteristics peak for the (100) plane of CsPbI3 is observed at 14.1°. The above peak is shifted to 14.8° for CsPbBr3 NWs because of the smaller atomic size of Br compared to I. During anion exchange at different temperatures, the peak position for CsPbBr3 (100) remains almost unchanged but the full width at half-maximum of the peak for the (100) plane changes largely. The main difference between the orthorhombic and cubic phases is the appearance of an additional shoulder peak at ∼30°.31 The XRD results clearly indicate the formation of an orthorhombic phase of CsPbBr3 NWs at 60 °C. The shoulder peak is found to be 12951
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W at 520 nm. The peak at 520 nm is attributed to the direct band edge absorption in CsPbBr3 NWs. The external quantum efficiency (EQE) is a very important parameter to evaluate the performance of a photoconductive device. EQE is defined as the ratio of numbers of charge carriers generated compared to the numbers of photons of particular incident energy on the device at a particular wavelength. The EQE spectrum of the fabricated device is shown in the Figure S6 (Supporting Information). The fabricated device shows a high EQE of ∼70% at an applied bias of 2 V at 520 nm.
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Figure 5. Typical XRD patterns of CsPbI3 NCs and CsPbBr3 NWs and NSs after anion exchange reaction at 60 and 80 °C (asterisk peaks are due to the presence of CTAB shown in Figure S10).
CONCLUSIONS In this article, a monodispersed CsPbI3 NC of an average size of ∼20 nm is synthesized using edible olive oil. We have performed the bromide exchange reaction using CTAB, as a bromide source to obtain CsPbBr3, from CsPbI3 NCs. A sequential transformation to CsPbBr3 from CsPbI3 is observed through UV−visible and PL spectroscopy. Interestingly, a simultaneous evolution of nanostructures with different morphologies is observed because of the formation of different shaped micelles by varying the anion exchange temperature. TEM and XRD analyses reveal the formation of high-quality CsPbI3 NCs, long CsPbBr3 NWs, and 2D NSs, resulting in tunable optical band gap and emission characteristics. Finally, we have successfully fabricated a CsPbBr3 NW-based (ITO/ CsPbBr3NWs/spiro-OMeTAD/Al) photoconductive detector through solution processing, which exhibits optical switching and moderate responsivity in the visible region, paving the way for fabricating future optoelectronic devices.
almost vanished in the case of CsPbBr3 at 80 and 100 °C, confirming the cubic phase of CsPbBr3. However, XRD patterns at different temperatures indicate that the crystallinity of the sample is degraded with the increase in anion exchange reaction temperature from 60 to 100 °C. This observation agrees well with the SAED patterns. To demonstrate potential application of CsPbBr3 NWs, a photoconductive device on indium tin oxide (ITO)-coated glass has been fabricated. The schematic of the fabricated device is shown in Figure 6a. The cross-sectional field emission scanning electron microscopy (FESEM) image (inset of Figure 6a) of the device shows the thickness of each layer. The photoresponse of the fabricated device is studied under white light illumination using a solar simulator at different voltages, as shown in Figure 6. The fabricated device shows almost a 2 order change in current under the illumination, as shown in Figure 6b. The device shows a moderately high on−off ratio (46) at 2 V in Figure 6c. Responsivity is a useful parameter to describe the efficiency of a photoconductive device. The responsivity curve in Figure 6d shows a broad visible response from 400 to 550 nm, with a maximum responsivity of 0.28 A/
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EXPERIMENTAL SECTION Materials and Chemicals. All the chemicals of analytical grade, including caesium carbonate (Cs2CO3, 99.9%), lead iodide (PbI2, 99.9%), olive oil (90%), oleylamine (OAm, 70%), n-hexane (99%), and CTAB were purchased from Sigma-Aldrich and used without further purification.
Figure 6. (a) Schematic diagram of the fabricated photoconductive device structure and cross-sectional FESEM image in the inset, (b) current− voltage characteristics of the fabricated device under dark and white light illumination, (c) switching characteristics of the device at 0 and 2 V, and (d) responsivity curve of the CsPbBr3-based photoconductive detector device. 12952
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Preparation of Caesium-Oleate. Cs2CO3 (0.814 g, 0.002 mol) was dispersed with 40 mL of olive oil in a two-necked round-bottom flask (rbf). The rbf was degassed under vacuum at 120 °C for 15 min and filled with nitrogen. Following this, the dispersion was heated to 150 °C to form a semitransparent solution under N2. The Cs-oleate solution was kept at 100 °C before injection into the lead-oleate solution in the next step to avoid precipitation. Synthesis of CsPbX3 (X = Br, I) NCs. Olive oil (5 mL) and PbI2 (870 mg, 0.188 mol) or PbBr2 (690 mg) or PbCl2 (520 mg) were taken in a 25 mL two-necked rbf and evacuated under vacuum at 120 °C for 1 h. Oleyl amine (0.5 mL) was injected swiftly under nitrogen at 120 °C. The temperature of the reaction mixture was raised to 150 °C for solubilization of PbI2 to form lead-oleate. Finally, pre-heated (at 100 °C) Csoleate (4 mL) solution was injected into the semitransparent lead-oleate solution. After 5 s, the reaction mixture was cooled in an ice bath to stop the reaction. The resultant crude solution was centrifuged at 20 000 rpm to get the precipitate as CsPbI3 NCs. A clear solution of CsPbI3 NCs was obtained after adding 20 mL of hexane to the crude NCs. Material Characterization. The surface morphology of CsPbBr3 and CsPbI3 perovskite was investigated using HRTEM (model no. JEM-2100, detector CCD camera from Gatan, Inc. USA, operating voltage 200 kV from JEOL, Japan). The UV−visible absorption spectrum of the as-synthesized CsPbCl3, CsPbBr3 perovskite NCs (Figure S1, Supporting Information) and CsPbI3 NCs by hot injection approach was recorded using a fiber-probe-based UV−vis−NIR CCD spectrometer. The crystallinity of as-synthesized perovskite NCs, NWs, and NSs was studied using a high-resolution X-ray diffractometer (model No Philips X’Pert MRD). PL and PL decay of all perovskite samples were obtained by using a 376 nm excitation wavelength. Anion Exchange Reaction. Anion exchange reaction of CsPbI3 in hexane was performed at different temperatures. Of the as-prepared CsPbI3 solution in hexane, 10 mL, and known amounts (30, 45, 60 mg) of CTAB were taken in a 25 mL rbf. The reaction mixture was kept in a closed vessel by sealing and stirred at desired temperatures (0, 60, 80, and 100 °C and room temperature). After 3 min intervals, 0.1 mL of the reaction mixture was taken out using a syringe to study the changes of the optical property at different durations. Device Fabrication. Photoconductive devices were fabricated using as-grown CsPbBr3 NWs on an ITO-coated glass substrate. CsPbBr3 NWs were deposited on precleaned ITO-coated glass substrates by spin-coating (2000 rpm for 15 s) to obtain a thickness of ∼250 nm. The perovskite-coated ITO substrates were dried at 60 °C for 1 h in an oven. A mixture of spiro-OMeTD/Li-TFSi (70 mg spiro-OMeTD in 1 mL of chlorobenzene, 45 μL of LiTFSi in acetonitrile (170 mg/mL), and 10 μL of tBP) solution was spin-coated on top of the perovskite layer at 3000 rpm for 15 s. Bilayer-coated ITO substrates were dried at 60 °C for 30 min. The Al electrode was deposited (dia ≈ 0.1 mm) as a top contact by thermal evaporation with a base pressure of 2 × 10−6 Torr. Current− voltage (I−V) measurements were performed using a Keithley 4200-A semiconductor parameter analyzer and a solar simulator as the light source.
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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/acsomega.9b00829.
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Evolution of PL spectra during anion exchange reaction at room temperature and 0 °C using 45 mg of CTAB; PL-decay lifetime study of the anion exchange reaction of CsPbI3 in the presence of CTAB at 60 °C; evolution of UV−visible absorption spectra during anion exchange reaction in varying amounts of CTAB; morphological evolution with the variation of CTAB amounts by TEM; and EQE spectrum of the fabricated photoconductive device (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected],
[email protected] (A.M.). *E-mail:
[email protected] (S.K.R.). ORCID
Anupam Midya: 0000-0003-0498-6017 Samit K. Ray: 0000-0002-8099-6690 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by DST grant no. IFA 13-MS-09 (INSPIRE) and DST/TM/CERI/C74(G) project. A.G. acknowledges the CSIR, Government of India, for awarding the senior research fellowship.
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DOI: 10.1021/acsomega.9b00829 ACS Omega 2019, 4, 12948−12954
