Volatility and Chain Length Interplay of Primary Amines: Mechanistic

Jan 26, 2018 - ... irreversible changes in the chemical and physical structure, which is one of the major lacuna preventing its utility in commercial ...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 6711−6718

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Volatility and Chain Length Interplay of Primary Amines: Mechanistic Investigation on the Stability and Reversibility of AmmoniaResponsive Hybrid Perovskites Sayantan Sasmal,†,‡ Arup Sinha,† Bruno Donnadieu,† Raj Ganesh S. Pala,*,§ Sri Sivakumar,*,‡,§,∥ and Suresh Valiyaveettil*,† †

Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore Materials Science Programme, §Department of Chemical Engineering, and ∥Centre for Environmental Science & Engineering, Thematic Unit of Excellence on Soft Nanofabrication, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh 208016, India

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‡

S Supporting Information *

ABSTRACT: Hybrid organic−inorganic perovskites possess promising signal transduction properties, which can be exploited in a variety of sensing applications. Interestingly, the highly polar nature of these materials, while being a bane in terms of stability, can be a boon for sensitivity when they are exposed to polar gases in a controlled atmosphere. However, signal transduction during sensing induces irreversible changes in the chemical and physical structure, which is one of the major lacuna preventing its utility in commercial applications. In the context of developing alkylammonium lead(II) iodide perovskite materials for sensing, here we address major issues such as reversibility of structure and properties, correlation between instability and properties of alkylamines, and relation between packing of alkyl chains inside the crystal lattice and the response time toward NH3 gas. The current investigation highlights that the vapor pressure of alkylamine formed in the presence of NH3 determines the reversibility and stability of the original perovskite lattice. In addition, close packing of alkyl chains inside the perovskite crystal lattice reduces the response toward NH3 gas. The mechanistic study addresses three important factors such as quick response, reversibility, and stability of perovskite materials in the presence of NH3 gas, which could lead to the design of stable and sensitive two-dimensional hybrid perovskite materials for developing sensors. KEYWORDS: hybrid perovskite, NH3 response, reversibility, volatility, alkyl chain packing, alkylammonium lead(II) iodide

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INTRODUCTION Ammonia gas is used in many industries such as manufacturing of fertilizers, refrigeration, and in explosives.1 However, continuous exposure to NH3 gas has a severe impact on human health, which includes intense eye and throat irritation and deadly pulmonary disorder.2,3 Conventional NH3-sensing material involves metal oxides and conducting polymers, wherein a detectable signal is generated due to modulation of the electronic structure upon exposure to NH3 gas.4,5 However, such sensors suffer severe reduction in sensitivity over time because of irreversible restructuring of the material upon exposure to the analyte.5 Organic−inorganic halide perovskites, mainly established as photovoltaic materials,6−11 generate a distinguishable optical signal owing to switching of crystalline phases upon exposure to different polar molecules.12 This property has been gainfully utilized in the sensing of ammonia molecules, which interact with the methylammonium lead iodide (MAPbI3) film upon a short exposure (for a few seconds).12,13 The material undergoes severe degradation due to irreversible reaction between MAPbI3 and NH3 gas.14 Such an irreversible structural © 2018 American Chemical Society

transformation is a major impediment for using hybrid perovskites in sensing applications. Moreover, limited understanding of the mechanistic pathway in which hybrid perovskites interact with NH3 also restricts the necessary improvements and acts as a major lacuna in utilizing these hybrid perovskite materials in sensing applications. To this end, we first elucidate the reasons behind the irreversibility of structural transformations (Scheme 1). We demonstrate that upon exposure to ammonia, the methylammonium cation packed inside the MAPbI3 lattice transforms into methylamine, which is volatile and evaporates from the material. Moreover, using extensive investigation with other alkylammonium lead iodide perovskites, we demonstrate that three distinct factors govern the interaction between the hybrid perovskite and NH3 gas(i) packing of alkyl ammonium chain inside the perovskite crystal provides accessible voids, which ultimately influences the rapidity of interaction with NH3 gas; Received: November 25, 2017 Accepted: January 26, 2018 Published: January 26, 2018 6711

DOI: 10.1021/acsami.7b17971 ACS Appl. Mater. Interfaces 2018, 10, 6711−6718

ACS Applied Materials & Interfaces

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Scheme 1. Proposed Reversible Interaction of NH3 with Alkylammonium Lead(II) Iodide

Research Article

EXPERIMENTAL SECTION

Materials and Characterization. Methylamine (40% in methanol) and lead iodide (99.99%) were purchased from TCI Chemicals. Hydrogen iodide (HI, 57% in water), butylamine (BA, 99.5%), cyclohexylamine (98%), octylamine (OA, 99%), and anhydrous dimethyl formamide (DMF, 99.8%) were purchased from SigmaAldrich. All exposure studies were done with the vapors coming out from 28% aqueous NH3 solution purchased from VWR chemicals. Vapor pressure data were collected from Sigma-Aldrich website. Powder X-ray diffraction (XRD) analyses of all samples were done at room temperature using a Bruker D8 ADVANCE powder X-ray diffractometer with Cu Kα (1.54 Å) radiation. High-temperature powder XRD was done by a Bruker D8 ADVANCE powder X-ray diffractometer with an Anton Paar HTK 1200 high-temperature chamber. Nuclear magnetic resonance (NMR) spectra of the samples were recorded by Bruker AVANCE I 300. Fluorescence images were recorded using an Olympus DP 71 microscope. The rapid response with and without the presence of NH3 was checked using real-time kinetic study with the help of fluorescence spectroscopy (Cary Eclipse fluorescence spectrophotometer-Agilent Technology). The fast response and the nonresponsive behavior of the hybrid perovskites were explained by the lattice structure of the crystals. Sensitivity and selectivity were measured by keeping the solid film inside a cuvette and recording the fluorescence spectrum. The reported single-crystal data of perovskites were analyzed using the crystallographic package, PLATON,15 and the refined structures were compared on the basis of the available void spaces, which act as possible defects in the crystals in the presence of NH3. Voids inside the crystal structures are located and represented by orange spheres with radii equal to the contact radius to the nearest van der Waals surface. Schemes were drawn with the help of VESTA software. NH3 Exposure Study. Stability test of all samples were performed by exposing films of perovskites (10 μL of respective DMF solution was drop-cast on a 1 cm2 glass slide and dried at 80 °C) to NH3 gas. To check the rapid response in the presence and absence of NH3, the fluorescence arising from the films of different perovskites were checked with an Olympus DP 71 microscope by flooding the film with NH3 gas (1 mL) using a syringe. Rapid response was also checked using a Cary Eclipse fluorescence spectrophotometer by purging the same amount of NH3 gas on the film, followed by N2 gas. To examine the sensitivity, an octylammonium lead(II) iodide [(OA)2PbI4] film was cast on a glass slide and placed inside a capped fluorescence cuvette, and the changes in the fluorescence of the film were

(ii) in situ formation of alkyl amine because of rapid proton exchange between alkylammonium cations and excess NH3 to form a nonfluorescent material and reconversion to the original fluorescent alkylamine perovskite after removal of NH3 determines the rapid response and successive reversibility; and (iii) volatility of the amines formed in situ determines the overall stability and reversibility of the hybrid perovskites. Hence, our strategy toward NH3-sensitive and stable hybrid perovskites involve suppression of volatility of the amines, by choosing alkylamines with a low vapor pressure. Most importantly, such alkylamine produces two-dimensional hybrid perovskites capable of switching (on and off) fluorescence without any reduction in efficiency during several cycles of NH3 exposure. Such fast response and high reversibility of structure and properties make our perovskite materials potential candidates for an NH3 sensor. The observed stability and sensitivity toward NH3 along with the in-depth novel mechanistic study encourage the development of hybrid perovskite sensors in the future.

Figure 1. (A) XRD pattern of the MAPbI3 film after exposure to NH3. Dotted lines indicate the dominant peaks of the MAPbI3 phase, which completely disappears after exposure to NH3 gas for 15 s. The new phase formed was indexed as NH4PbI3·2H2O.19 (B) 1H NMR spectra recorded in dimethyl sulfoxide (DMSO)-d6 of virgin MAPbI3 (i), where a, b peaks represent the CH3− and −NH3+ protons, respectively. After addition of one drop of NH3 solution (ii) and D2O in MAPbI3 solution, CH3NH3+ converted to CH3NH2, indicated by peak c. The disappearance of the CH3− peak after exposing the solid MAPbI3 film to NH3 gas for 60 s (iii) corresponds to the evaporation of CH3NH2 from the solid film. The peak d in NMR (iii) corresponds to NH4PbI3·2H2O. 6712

DOI: 10.1021/acsami.7b17971 ACS Appl. Mater. Interfaces 2018, 10, 6711−6718

Research Article

ACS Applied Materials & Interfaces monitored at different concentrations of NH3 gas introduced using a syringe. All fluorescence spectra were recorded using an excitation wavelength of 380 nm. The concentration of NH3 was measured separately by dissolving the same volume of NH3 gas in water and titrating with a standard oxalic acid solution. Synthesis of MAPbI3. MAPbI3 was prepared by adapting a previously reported literature.16 First, methylamine (1 mL, 40% in methanol) solution was stirred with HI (1.1 mL, 57% in water) in an ice bath for 2 h; the excess solvent was evaporated at 50 °C; and a white precipitate of CH3NH3I [methyl ammonium iodide (MAI)] was obtained. The precipitate was washed with anhydrous diethyl ether thrice and dried under vacuum. To prepare CH3NH3PbI3 (MAPbI3), MAI (30.2 mg, 0.19 mmol) was mixed with PbI2 (87.59 mg, 0.19 mmol) in anhydrous DMF (3 mL) at 60 °C for 12 h. The resulting solution (10 μL) was drop-cast on a glass slide (1 × 1 cm2) and dried at 80 °C to form a black film, which was used for further characterization. Synthesis of (CnH2n+1NH3)2PbI4, n = 4, 8. To prepare (CnH2n+1 NH3)2PbI4, appropriate amounts of alkylamine were reacted with excess HI in an ice bath for 2 h and concentrated at 70 °C. The obtained white precipitate was washed with anhydrous diethyl ether (3 × 10 mL), dissolved in methanol (1 mL), recrystallized, and dried in vacuum. To prepare (CnH2n+1NH3)2PbI4, PbI2 (0.19 mmol) was stirred with alkylammonium iodide (0.38 mmol) in anhydrous DMF (3 mL) at 60 °C for 12 h, and the solution (10 μL) was drop-cast on a glass slide (1 cm2) and dried at 80 °C, resulting in a film, which was used for further characterization. A similar procedure was also used for the synthesis of perovskite from cyclohexylamine, (C6H11NH3)2PbI4 or (CHA)2PbI4.

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To confirm the formation and evaporation of methylamine through proton exchange, 1H NMR spectra of the MAPbI3 film were recorded before and after adding NH3 in solution. Figure 1B(i) represents the 1H NMR spectra of MAPbI3 in DMSO-d6, where the peak at 2.37 ppm is assigned for CH3NH3+I−. When a drop of aqueous NH3 was added into the solution, the CH3peak was upfield-shifted to 2.21 ppm (Figure 1B(ii)). Such changes in chemical shifts indicate the formation of unprotonated CH3NH2 via proton exchange in the presence of excess ammonia. In addition, we also exposed the solid MAPbI3 film to NH3 gas for 60 s, dissolved the exposed materials immediately in DMSO-d6, and recorded the 1H NMR spectrum. No peaks from CH3-protons were seen in the 1H NMR spectra (Figure 1B(iii)), which confirm the disappearance of CH3NH2 from the solid MAPbI3 film after exposure to NH3 gas. Moreover, the new broad peak that appeared at 6.49 ppm in Figure 1B(iii) corresponds to the formation of NH4PbI3·2H2O. XRD and 1H NMR studies proved that in situ formation of free methylamine and its high volatility are responsible for the irreversible interaction leading toward complete degradation of MAPbI3; now, the correlation between the volatility (based on vapor pressure) of the precursor amine and degradation of the corresponding hybrid perovskite needs to be verified with different alkyl hybrid perovskites. Relationship between Vapor Pressure of in Situ Formed Free Amine and Stability of Hybrid Perovskite. To confirm the correlation between the vapor pressure of in situ formed free alkylamine and changes in the structure and properties observed in the presence of NH3 gas, hybrid perovskites from a series of amines such as BA (vapor pressure = 68 mm Hg at 20 °C) and OA (vapor pressure = 0.60 mm Hg at 20 °C) were prepared and fully characterized (see details in the Experimental Section). The (BA)2PbI4 film was indexed with the help of previously reported literature (Figure 2A).22 A mixed phase of (BA)2PbI4 (marked as dotted lines) and NH4PbI3·2H2O (marked as *) was visible after NH3 exposure for 15 to 30 s (Figure 2B,C). The presence of XRD pattern of the (BA)2PbI4 phase after 30 s of NH3 exposure indicates the higher stability of (BA)2PbI4 as compared to MAPbI3, however,

RESULTS AND DISCUSSION

It is already known that conventional MAPbI3 undergoes complete restructuring because of the irreversible reaction with NH3, impeding its use in gas-sensing applications.14 To design a stable hybrid perovskite with continuous reversibility for gassensing applications, the mechanistic pathway of complete degradation associated with the irreversible reactivity of MAPbI3 needs to be investigated in-depth. Mechanism of Instability of MAPbI3 in NH3. To investigate the degradation pathway of conventional MAPbI3, the phase purity of the MAPbI3 film was examined using XRD after exposing the film to NH3 gas for different time intervals (Figure 1A). This resulted in the disappearance of all characteristic XRD peaks of MAPbI3,17 and appearance of completely new peaks even after 15 s of exposure proved the fragility of MAPbI3 toward NH3 (Figure 1A). The new material formed was identified as NH4PbI3·2H2O based on published data,18,19which indicates a complete breakdown of the perovskite crystal, leading toward degradation of MAPbI3. The observed gradual decrease in the intensity of the 101 plane of NH4PbI3·2H2O with continued exposure of NH3 (15−60 s) is due to restructuring of the crystal lattice along the 002 plane, which is characteristic of an oriented film.20 XRD data show that conventional MAPbI3 is not stable for 15 s of exposure to NH3 gas (Figure 1A), which is accounted as the consequence of a proton exchange reaction between the methyl ammonium cation and adsorbed ammonia in the film (eq 1). (CH3NH3+I−)PbI 2(s) + NH3(g) ⇌ CH3NH 2(g) + NH4 +I−PbI 2(s)

Figure 2. XRD pattern of (BA)2PbI4 film before (A), after 15 s (B), after 30 s (C), and after 60 s (D) of exposure to NH3. Dotted lines indicate the gradual diminishing of the (BA)2PbI4 phase with increasing time of NH3 exposure. * represents the presence of NH4PbI3·2H2O. The decrease in the intensity of the 101 plane and the increase in the intensity of 002 planes of NH4PbI3·2H2O are characteristics of an oriented film.

(1)

Owing to the high vapor pressure (1395.9 mm Hg at 20 °C), the newly formed methylamine evaporates from the film, and the equilibrium shifts toward the formation of NH4PbI3, based on Le Chatelier’s principle.21 6713

DOI: 10.1021/acsami.7b17971 ACS Appl. Mater. Interfaces 2018, 10, 6711−6718

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ACS Applied Materials & Interfaces

Figure 3. (A) Effect of NH3 on (OA)2PbI4 before and after 30, 60, and 75 s of NH3 exposure. The dotted line shows the presence of the (OA)2PbI4 phase indexed by previous literature,23 and *marks represent the appearance of NH4PbI3·2H2O. (B) 1H NMR spectra of (OA)2PbI4 before addition (i) and in the presence (ii) of NH3 along with one drop of D2O. The position of peaks corresponding to CH2 groups adjacent to NH3+ (marked as c and d) were upfield-shifted significantly (c′ and d′) because of the formation of free amine in the presence of NH3.

longer exposure (60 s) results into irreversibility (confirmed from XRD data, Figure 2D) due to the moderate vapor pressure of BA (68 mm Hg at 20 °C). The irreversible nature of (BA)2PbI4 led to the use of extremely less volatile amine i.e., OA (0.60 mm Hg at 20 °C) for the preparation of stable hybrid perovskites. The (OA)2PbI4 film was indexed using previously reported data,23 and no significant amount of NH4PbI3·2H2O phase was observed after exposure to NH3 gas for 60 s, which indicates the high stability of (OA)2PbI4 (Figure 3A). It is noted that diffraction patterns of NH4PbI3·2H2O phase started appearing along with (OA)2PbI4 after 75 s exposure to NH3 gas, indicating the same proton transfer reaction. Proton exchange occurs between the octylammonium salt and NH3 molecules during the exposure of NH3 gas to (OA)2PbI4, which then leads to the formation of free OA. To confirm such acid−base reaction, 1H NMR spectra of pure (OA)2PbI4 in DMSO-d6 in the presence and absence of NH3 were compared (Figure 3B). The chemical shifts of peaks d (2.76 ppm, −CH2−NH3+) and c (1.51 ppm, −CH2−CH2−NH3+) in the absence of NH3 (Figure 3B(i)) were upfield- shifted to 2.57 ppm (peak d′, −CH2−NH2) and 1.34 ppm (peak c′, −CH2−CH2−NH2, Figure 3B(ii)) in the presence of NH3, suggesting the formation of OA through proton exchange. Similarly, overlapping 1H NMR spectra of the (OA)2PbI4 film in DMSO-d6 before (Figure 4(i)) and after (Figure 4(ii)) exposure to NH3 gas for 60 s indicate the presence of octylammonium salt in both. Such enhanced stability arises due to the formation of less volatile OA during the exposure (forward reaction), which is reconverted to octylammonium salt in the absence of NH3 gas (reverse reaction) (see eq 2)

Figure 4. 1H NMR spectra of (OA)2PbI4 in DMSO-d6 before (i) and after 60 s (ii) of NH3 exposure.

during NH3 exposure. This enhanced stability of the (OA)2PbI4 film in NH3 prompted us to discover the signal transduction, utilizing its fluorescence during NH3 exposure. Rapid Changes in Fluorescence with Structural Reversibility of (OA)2PbI4. Because signal transduction in presence of the analyte is one of the major criteria for sensing, the fluorescence property of hybrid perovskites is exploited in the current case. The original optical properties of (OA)2PbI4 changed drastically during the exposure of NH3 gas, as represented in Figure 5. The fluorescence microscopy images of the (OA)2PbI4 film with the original green fluorescence can be seen in Figure 5A, which rapidly disappears to nonfluorescent (NH4PbI3 mixed with OA) (Figure 5B) in the presence of excess NH3 gas. After the removal of NH3 gas, the material again shows a green fluorescence, which indicates the formation of (OA)2PbI4 (Figure 5C). This was not the case with the MAPbI3 film, where almost no recovery of fluorescence was observed (Figure 5D−F) based on the time of exposure. The instability of MAPbI3 in the presence of NH3 gas is due to the evaporation of volatile methylamine from the film, which significantly affects the properties. To further validate the observed emission properties from microscopic studies, the fluorescence spectra of the solid film of (OA)2PbI4 were recorded in the presence and absence of NH3

CH3(CH 2)7 NH3+I−(s) + NH3(g) ⇌ CH3(CH 2)7 NH 2(l) + NH4 +I−

(2)

It is evident that methylamine formed in the presence of NH3 on MAPbI3 evaporates quickly from the perovskite film owing to high vapor pressure (1395.9 mm Hg at 20 °C), which is responsible for the observed high instability of the MAPbI3 film in the presence of NH3. On the other hand, the low vapor pressure of OA (0.60 mm Hg at 20 °C) limits the evaporation from the film, and hence high stability of (OA)2PbI4 is observed 6714

DOI: 10.1021/acsami.7b17971 ACS Appl. Mater. Interfaces 2018, 10, 6711−6718

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ACS Applied Materials & Interfaces

Figure 5. Changes in the fluorescence of (OA)2PbI4 (A−C) and MAPbI3 (D−F) films before (A,D), during (B,E), and after (C,F) exposure to NH3 gas.

Figure 6. Fluorescence spectra (A) of (OA)2PbI4 before exposure in the presence and absence of NH3. Emission spectra were obtained using 380 nm excitation wavelength. Fluorescence response traces (B) of (OA)2PbI4 (i) and (CHA)2PbI4 (ii) films in response to the introduction (on) and removal of NH3 (off). Fluorescence response traces were performed at 519 nm emission wavelength using an excitation at 380 nm.

gas. Figure 6A shows the fluorescence spectra before and after exposure to NH3 gas, which indicates the reversibility in the emission behavior of (OA)2PbI4. The drastic decrease in the fluorescence intensity at 519 nm in the presence of NH3 indicates the conversion of (OA)2PbI4 to a mixture of OA and NH4PbI3·2H2O, proved previously in Figure 3. Similar reversibility in optical properties associated with structural changes was also observed for (BA)2PbI4 perovskite films in the presence of NH3 (Figure S1, Supporting Information). This indicates that the NH3 gas-induced structure-property reversibility is a universal characteristic of alkylammonium lead(II) iodide perovskites. Drastic changes in the fluorescence and retention of the flipping (on and off) behavior with the continuous switching of NH3 gas are the important criteria to prove (OA)2PbI4 film as a stable sensing material. The exposure of (OA)2PbI4 to NH3 (Figure 6B(i)) showed rapid changes in optical properties with structural reversibility during the continuous on and off switching of NH3 gas. As discussed above, the presence of excess NH3 led to the release of free amines, and the volatility of the amines determines the stability toward repeated multiple (e.g., 12) cycles of NH3 exposure and removal without a reduction in optical properties. Fluorescence with no significant loss in intensity during cyclical switching (on and off) of NH3 gas indicated the stability and fast responsive behavior of (OA)2PbI4. Formation of volatile methylamine makes conventional MAPbI3 unstable to withstand continuous exposure cycles of NH3 and changes its property irreversibly after the first pulse of NH3,13 whereas formation of low volatile OA in

the presence of NH3 and reconversion to octylammonium in the absence of NH3 makes (OA)2PbI4 an interesting room temperature sensory material for NH3 (Figure 6B(i)).24,25 Interestingly, (OA)2PbI4 did not show response or changes in properties in the presence of water vapor, as established by fluorescence spectroscopy and optical microscopy (Figure 7A,B). The green fluorescence of the (OA)2PbI4 film showed no changes during the exposure or removal of water vapor

Figure 7. (A) Fluorescence spectra of (OA)2PbI4 before (i), in the presence of NH3 (ii), and after removal (iii) of water vapor. Emission spectra were obtained using an excitation wavelength at 380 nm. (B) Green fluorescence of the (OA)2PbI4 film was retained during and after exposure to water vapor. 6715

DOI: 10.1021/acsami.7b17971 ACS Appl. Mater. Interfaces 2018, 10, 6711−6718

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significant changes were observed in the fluorescence emission spectra (Figure S4B, Supporting Information, excitation wavelength 380 nm), and fluorescence microscope images (Figure S4C−E, Supporting Information) in the presence of NH3 show the nonresponsive behavior of orthorhombic (OA)2PbI4. In addition, no significant drop in fluorescence emission intensity was observed before and after three successive cycles of exposure and removal of NH3, which indicate the nonresponsive behavior of the thermally treated (OA)2PbI4 (Figure S4F, Supporting Information). Such response was justified by the closely packed crystal structure23 (Figure S4G, Supporting Information), refined using PLATON. The absence of voids in the lattice of both orthorhombic (OA)2PbI4 and (CHA)2PbI4 is responsible for the observed lack of response toward NH3. We believe the presence of accessible voids facilitates NH3 to diffuse into the lattice to facilitate proton exchange with the alkylammonium cation, leading to the formation of free amine inside the lattice, which then changes the optical properties of the perovskite material. To check the sensitivity, we kept the (OA)2PbI4 film inside a fluorescence cuvette and introduced NH3 gas at different concentrations (Figure 9, see Experimental Section for details).

(Figure 7B), which proved the sensitivity of (OA)2PbI4 specifically toward NH3 and not toward atmospheric moisture. The abovementioned acid−base reaction and volatility of the precursor linear amine explain the response and reversibility of alkylammonium lead iodide perovskites in the presence of NH3 gas. The cyclohexylammonium lead(II) iodide perovskite (CHA)2PbI426 (XRD, Figure S2, Supporting Information) showed no significant changes in optical properties during or after exposure of NH3 gas, as investigated by fluorescence microscopy (Figure S3, Supporting Information) and real-time fluorescence studies (Figure 6B(ii)). It is conceivable that cyclohexyl ammonium cations should undergo proton exchange with NH3 molecules, similar to other alkyl ammonium cations; thus, the observed nonresponsive nature of (CHA)2PbI4 toward NH3 gas prompted us to examine the differences in the lattice structures of these perovskites. Correlation between Packing of Alkylammonium Cation inside the Crystal and Response to NH3 Gas. To investigate the differences in crystal packing, we analyzed previously reported single-crystal data22,23,26,27 of a series of alkylammonium lead(II) iodides using the crystallographic package, PLATON.15 The refined structures clearly indicate the presence of numerous voids in MAPbI3, (BA)2PbI4, and (OA)2PbI4, but not in the lattice of (CHA)2PbI4 (Figure 8).

Figure 9. Fluorescence spectra of (OA)2PbI4 film in the presence of different concentrations of NH3. The limit of detection for NH3 is around 30 ppm. Figure 8. Refined structures of MAPbI3 (A), (BA)2PbI4 (B), (OA)2PbI4 (C), and (CHA)2PbI4 (D) using crystallographic package tool, PLATON, where orange spheres represent potential void spaces. (CHA)2PbI4 (D) does not show significant void spaces inside the lattice.

The fluorescence emission from the (OA)2PbI4 film was suppressed up to 30 ppm of NH3, and further optimization is needed to develop (OA)2PbI4 as an interesting material for fabricating NH3 sensors. We believe rapidity in response due to the facile conversion of OA from the octylammonium cation in the presence of NH3 gas and continuous reversibility and stability due to less volatility of in situ formed OA along with high sensitivity may establish (OA)2PbI4 as an optimum material for future NH3 sensors.

From the lattice structure and the observed behavior of these alkylammonium lead(II) iodides with NH3, we conclude that the voids in the crystal lattice of the linear alkylammonium hybrid perovskites make the structures more vulnerable toward NH3 molecules, whereas the relatively closely packed crystal lattice of (CHA)2PbI4 reduces the penetration toward NH3 molecules. Presence of these voids renders the crystal lattice vulnerable toward the incoming ammonia and hence easily gets ruptured. Interestingly, the room temperature monoclinic structure of (OA)2PbI4 undergoes a transition to an orthorhombic crystal structure at 45 °C (Figure S4A, Supporting Information), which has also been reported by others.23 This orthorhombic structure of (OA)2PbI4 is nonresponsive toward NH3. No

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CONCLUSIONS In summary, we explored three major aspects of signal transduction of organic−inorganic hybrid perovskites in the presence of NH3, namely, stability, origin of fascinating reversibility, and rapidity coupled with sensitivity. Our results reveal rapid proton switching between the alkylammonium cation packed inside the perovskite crystal lattice and NH3 gas, and this is analyzed using 1H NMR and XRD. In addition, the volatility of alkylamine formed in situ determines the 6716

DOI: 10.1021/acsami.7b17971 ACS Appl. Mater. Interfaces 2018, 10, 6711−6718

Research Article

ACS Applied Materials & Interfaces

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regeneration of the original hybrid perovskite after removal of NH3. Owing to this reason, MAPbI3 is less stable than (OA)2PbI4 during the cyclical exposure of NH3 gas. The fast and reversible exchange of protons between the alkylammonium cation and NH3 inside the lattice is responsible for the observed changes in the structure and properties. The analyses of crystal structures of a series of linear alkyl ammonium lead(II) iodide perovskites also showed that packing of flexible alkyl chains provides significant void spaces for the percolation of NH3 inside the lattice, leading to rapid interaction with high sensitivity. Presence of accessible voids and extremely less volatility of free OA formed in situ along with facile proton switching makes (OA)2PbI4 stable, sensitive, and reversible enough to withstand this fascinating switching with many cycles (on and off) of NH3 exposure. Our results highlight the mechanism and structure-property correlations of a series of alkylammonium lead(II) iodide perovskites in the presence and absence of NH3 gas. Such results could pave the way for the design of new organic−inorganic hybrid perovskites for developing gas sensors in the near future.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b17971. Fluorescence microscopy images and XRD spectra of alkylammonium lead(II) iodide (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R.G.S.P.). *E-mail: [email protected] (S.S.). *E-mail: [email protected] (S.V.). ORCID

Sri Sivakumar: 0000-0002-6472-2702 Suresh Valiyaveettil: 0000-0001-6990-660X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Sayantan Sasmal and Suresh Valiyaveettil gratefully acknowledge the funding support from the National University of Singapore under the joint Ph.D. program between NUS and IITK. We are thankful to Dr. Nancy Singhal for showing the operation principle of the fluorescence spectrophotometer. Sri Sivakumar acknowledge DST for funding.

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REFERENCES

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DOI: 10.1021/acsami.7b17971 ACS Appl. Mater. Interfaces 2018, 10, 6711−6718

Research Article

ACS Applied Materials & Interfaces (27) Dang, Y.; Liu, Y.; Sun, Y.; Yuan, D.; Liu, X.; Lu, W.; Liu, G.; Xia, H.; Tao, X. Bulk crystal growth of hybrid perovskite material CH3NH3PbI3. CrystEngComm 2015, 17, 665−670.

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DOI: 10.1021/acsami.7b17971 ACS Appl. Mater. Interfaces 2018, 10, 6711−6718