Volatility and Chain Length Interplay of Primary ... - ACS Publications

Subscriber access provided by READING UNIV

Article

Volatility and Chain Length Interplay of Primary Amines: Mechanistic Investigation on the Stability and Reversibility of Ammonia Responsive Hybrid Perovskites. Sayantan Sasmal, Arup Sinha, Bruno Donnadieu, Raj Ganesh S Pala, Sri Sivakumar, and Suresh Valiyaveettil ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17971 • Publication Date (Web): 26 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Volatility and Chain Length Interplay of Primary Amines: Mechanistic Investigation on the Stability and Reversibility of Ammonia Responsive Hybrid Perovskites Sayantan Sasmal,1, 2 Arup Sinha,1 Bruno Donnadieu,1 Raj Ganesh S. Pala,3* Sri Sivakumar,2, 3, 4* Suresh Valiyaveettil1* 1

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 2 Materials Science Programme, Indian Institute of Technology Kanpur, UP-208016, India 3 Department of Chemical Engineering, Indian Institute of Technology Kanpur, UP-208016, India 4 Centre for Environmental Science & Engineering, Thematic Unit of Excellence on Soft Nanofabrication, Indian Institute of Technology Kanpur, UP-208016, India

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 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, the correlation between instability and properties of alkylamines, and the relation between packing of alkyl chains inside the crystal lattice with the response time towards NH3 gas. The current investigation highlights that vapour pressure of alkylamine formed in presence of NH3 determine the reversibility and stability of original perovskite lattice. In addition, close packing of alkyl chains inside the perovskite crystal lattice reduces the response towards NH3 gas. The mechanistic study addresses three important factors such as quick response, reversibility, and stability of perovskite materials in presence of NH3 gas, could lead to the design of stable and sensitive 2D hybrid perovskite materials for developing sensors. Keywords: Hybrid Perovskite, NH3 response, reversibility, volatility, alkyl chain packing, alkylammonium lead(II) iodide.

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 22

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 detectable signal is generated due to modulation of the electronic structure upon exposure to the NH3 gas.4,5 However, such sensors suffer severe reduction in sensitivity over time due to irreversible restructuring of the material upon exposure to the analyte.5 Organic-inorganic halide perovskites, mainly established as photovoltaic material,

6-11

generates a distinguishable optical signal owing to switching of crystalline phases upon exposure to different polar molecules.12 This property has been gainfully utilized in sensing of ammonia molecules which interacts with methylammonium lead iodide (MAPbI3) film upon a short exposure (for a few seconds).12,13 The material undergoes a severe degradation due to irreversible reaction between MAPbI3 and NH3 gas.14 Such irreversible structural 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 act 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 the exposure to ammonia, methylammonium cation packed inside the MAPbI3 lattice transforms into methylamine, which is volatile and evaporate from the material. Moreover, using extensive investigation with other alkylammonium lead iodide perovskites, we demonstrate that three distinct factors govern the interaction between hybrid perovskite and NH3 gas - (i) packing of alkyl


Page 3 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ammonium chain inside the perovskite crystal provides the accessible voids which ultimately influences the rapidity of interaction with NH3 gas, (ii) in situ formation of alkyl amine due to rapid proton exchange between alkylammonium cations and excess NH3 to form nonfluorescent material and reconversion to original fluorescent alkylamine perovskite after removal of NH3 determines rapid response and successive reversibility and lastly, (iii) volatility of the amines formed in situ determines the overall stability and reversibility of the hybrid perovskites. Hence, our strategy towards NH3 sensitive and stable hybrid perovskites involve suppression of volatility of the amines, by choosing alkylamines with low vapour pressure. Most importantly, such alkylamine produces 2D hybrid perovskite 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 NH3 sensor. The observed stability and sensitivity towards NH3 along with in-depth novel mechanistic study encourage the development of hybrid perovskite sensors in the future.



Page 4 of 22

Scheme 1. Proposed reversible interaction of NH3 with alkylammonium lead(II) iodide.

EXPERIMENTAL SECTION: Materials and Characterization: Methylamine (40% in methanol) and Lead Iodide (99.99%) were purchased from TCI Chemicals. Hydrogen iodide (57% in water), Butylamine (99.5%), Cyclohexylamine (98%), Octylamine (99%) and anhydrous Dimethyl formamide (DMF, 99.8%) were purchased from Sigma Aldrich. All exposure studies were done with the vapours coming out from 28% aqueous NH3 solution purchased from VWR chemicals. Vapour pressure data was 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 X-ray diffraction was done by Bruker D8 advance powder X-ray diffractometer with Anton Paar HTK 1200 high temperature chamber. Nuclear magnetic resonance spectra of the samples were recorded by Bruker Avance I 300. Fluorescence images were recorded using 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 (Carry Eclipse Fluorescence Spectrophotometer-Agilent Technology). The fast response and the nonresponsive behaviour 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 analysed using 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 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.


Page 5 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NH3 Exposure Study: Stability test of all the 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) with the NH3 gas. To check the rapid response in presence and absence of NH3, fluorescence arising from the films of different perovskites were checked with Olympus DP 71 microscope by flooding the film with NH3 gas (1 mL) using a syringe. Rapid response was also checked using a Carry Eclipse Fluorescence Spectrophotometer by purging the same amount of NH3 gas on the film, followed by N2 gas. To examine the sensitivity, octylammonium lead(II) iodide ((OA)2PbI4) film was cast on a glass slide and placed inside a capped fluorescence cuvette and monitored the changes in fluorescence of the film 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 hours, the excess solvent was evaporated at 50 °C and a white precipitate of CH3NH3I (MAI) was obtained. The precipitate was washed with anhydrous diethyl ether thrice and dried under vacuum. In order 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 x 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 was 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 x 10 mL) and dissolved in methanol (1 mL), recrystallized and dried in vacuum. In order 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 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.

RESULTS AND DISCUSSION It is already known that conventional MAPbI3 undergoes complete restructuring due to the irreversible reaction with NH3, impeding its use in gas sensing application.14 In order to design a stable hybrid perovskite with continuous reversibility for gas sensing application 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 MAPbI3 film was examined using X-ray diffraction after exposing the film to NH3 gas with 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 seconds of exposure proved the fragility of MAPbI3 towards NH3 (Figure 1A). The new material formed was identified as NH4PbI3.2H2O based on published data18,19 indicate a complete breakdown of perovskite crystal leading towards degradation of MAPbI3. The observed gradual decrease in intensity of 101 plane of NH4PbI3.2H2O with continued exposure of NH3 (15 – 60 seconds)


Page 6 of 22

Page 7 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 seconds of exposure to NH3 gas (Figure 1A), which is accounted as the consequence of a proton exchange reaction between methyl ammonium cation and adsorbed ammonia in the film (equation 1). (CH3NH3+I-)PbI2(s) + NH3(g)

CH3NH2(g) + NH4+I-PbI2(s)

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

Figure 1: (A) XRD pattern of the MAPbI3 film after exposure to NH3. Dotted lines indicate the dominant peaks of MAPbI3 phase which is completely disappeared after exposure to NH3 gas for 15 seconds. The new phase formed was indexed as NH4PbI3.2H2O.19 (B) 1H NMR spectra recorded in 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 CH3- peak after exposing the solid MAPbI3 film to NH3 gas for 60 seconds (iii) corresponds to the evaporation of CH3NH2 from the solid film. The peak d in NMR (iii) corresponds to NH4PbI3.2H2O.



To confirm the formation and evaporation of methylamine through proton exchange, 1

H 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 CH3-peak 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 presence of excess ammonia. In addition, we also exposed the solid MAPbI3 film to NH3 gas for 60 seconds, 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 (Figure 1B, iii), which confirm the disappearance of CH3NH2 from solid MAPbI3 film after exposure to the NH3 gas. Moreover, the new broad peak 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 towards complete degradation of MAPbI3, now the correlation between the volatility (based on vapour pressure) of precursor amine with degradation of corresponding hybrid perovskite needs to be verified with different alkyl hybrid perovskites. Relationship Between Vapour Pressure of In Situ Formed Free Amine and Stability of Hybrid Perovskite In order to confirm the correlation between the vapour pressure of in situ formed free alkylamine and changes in structure and properties observed in presence of NH3 gas, hybrid perovskites from a series of amines such as butylamine (BA, vapour pressure = 68 mmHg at 20 °C), octylamine (OA, vapour pressure = 0.60 mmHg at 20 °C) were prepared and fully characterised (see details in 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 *) were visible after NH3 exposure for 15 to 30


Page 8 of 22

Page 9 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


seconds (Figure 2; B, C). The presence of XRD pattern of (BA)2PbI4 phase after 30 seconds of NH3 exposure indicates the higher stability of (BA)2PbI4 as compared to MAPbI3, however longer exposure (60 seconds) results into irreversibility (confirmed from XRD data, Figure 2D) due to moderate vapour pressure of butylamine (68 mmHg at 20 °C). The irreversible nature of (BA)2PbI4 led to the use of extremely less volatile amine i.e., octylamine (0.60 mmHg at 20 °C) for the preparation of stable hybrid perovskites.

Figure 2: XRD pattern of (BA)2PbI4 film before (A), after 15 seconds (B), 30 seconds (C), 60 seconds (D) exposure to NH3. Dotted lines indicate the gradual diminishing of (BA)2PbI4 phase with increasing the time of NH3 exposure. * represent the presence of NH4PbI3.2H2O. The decrease in intensity of 101 plane and increase in the intensity of 002 planes of NH4PbI3.2H2O is characteristics of an oriented film. The (OA)2PbI4 film was indexed using previously reported data23 and no significant amount of NH4PbI3.2H2O phase was observed after exposure to NH3 gas for 60 seconds, which indicates 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 seconds exposure to NH3 gas indicate the same proton transfer reaction. Proton exchange occurs between



octylammonium salt and NH3 molecules during the exposure of NH3 gas to (OA)2PbI4, which then leads to the formation of free octylamine. To confirm such acid-base reaction, 1H NMR spectra of pure (OA)2PbI4 in DMSO-d6 in 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, -CH2CH2-NH3+) in the absence of NH3 (Figure 3B, i) were shifted upfield to 2.57 ppm (peak d', CH2-NH2) and 1.34 ppm (peak c', -CH2-CH2-NH2, Figure 3B, ii) in presence of NH3, suggesting the formation of octylamine through proton exchange.

Figure 3: (A) Effect of NH3 on (OA)2PbI4; before and after 30 seconds, 60 seconds, 75 seconds of NH3 exposure. The dotted line shows the presence of (OA)2PbI4 phase indexed by previous literature23 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') due to the formation of free amine in presence of NH3. Similarly, overlapping 1H NMR spectra of (OA)2PbI4 film in DMSO-d6 before (Figure 4, i) and after (Figure 4, ii) exposure to NH3 gas for 60 seconds indicate the presence of octylammonium salt in both. Such enhanced stability arises due to the formation of less volatile octylamine during the exposure (forward reaction) and reconverted to octylammonium salt in the absence of the NH3 gas (reverse reaction). (see equation 2)


Page 10 of 22

Page 11 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CH3(CH2)7NH3+ I- (s) + NH3 (g)

CH3(CH2)7NH2 (l) + NH4+I-……. (2)

Figure 4: 1H NMR spectra of (OA)2PbI4 in DMSO-d6 before (i) and after 60 seconds (ii) of NH3 exposure. It is evident that methylamine formed in presence of NH3 on MAPbI3 evaporates quickly from the perovskite film owing to high vapour pressure (1395.9 mmHg at 20 °C), which is responsible for the observed high instability of MAPbI3 film in presence of NH3. On the other hand, the low vapour pressure of octylamine (0.60 mmHg at 20 °C) limits the evaporation from the film, thus the observed high stability of (OA)2PbI4 during NH3 exposure. This enhanced stability of (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 Since, 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 the (OA)2PbI4 changed drastically during the exposure of NH3 gas represented in Figure 5. The fluorescence microscopy images of (OA)2PbI4 film, where original green fluorescence (Figure 5A) rapidly disappears to non-fluorescent (NH4PbI3 mixed with Octylamine) (Figure 5B) in 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 MAPbI3 film, where almost no recovery of fluorescence was observed (Figure 5D-F) based on the time of exposure. The instability of MAPbI3 in presence of NH3 gas is due to the evaporation of volatile methylamine from the film, which significantly affects the properties.

Figure 5: Changes in fluorescence of (OA)2PbI4 (A-C) and MAPbI3 (D-F) films before (A, D), during (B, E) and after (C, F) exposure to the NH3 gas. To further validate the observed emission properties from microscopic studies, fluorescence spectra of the solid film of (OA)2PbI4 were recorded in presence and absence of NH3 gas. Figure 6A shows fluorescence spectra before and after exposure to NH3 gas, which indicates the reversibility in emission behaviour of (OA)2PbI4. The drastic decrease in fluorescence intensity at 519 nm in presence of NH3 indicates the conversion of (OA)2PbI4 to


Page 12 of 22

Page 13 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a mixture of octylamine 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 presence of NH3 (Figure S1, Supporting Information (SI)). This indicates that the NH3 gas induced structure-property reversibility is a universal characteristic of alkylammonium lead(II) iodide perovskites.

Figure 6: Fluorescence spectra (A) of (OA)2PbI4 before exposure, in presence and absence of NH3. Emission spectra were obtained using 380 nm excitation wavelength. The florescence response traces (B) of (OA)2PbI4 (i) and (CHA)2PbI4 (ii) film in response to the introduction (on) and removal of NH3 (off). Fluorescence response traces was performed at 519 nm emission wavelength using an excitation at 380 nm. Drastic changes in fluorescence and retention of the flipping (on and off) behaviour with the continuous switching of the 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 the structural reversibility during the continuous on and off switching of exposure to NH3 gas. As discussed above, the presence of excess NH3 led to the release of free amines and based on the volatility of the amines determine the stability towards 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



behaviour of (OA)2PbI4. Formation of volatile methylamine makes conventional MAPbI3 unstable to withstand continuous exposure cycles of NH3 and changes its property irreversibly after first pulse of NH3,13 whereas formation of low volatile octylamine in presence and reconversion to octylammonium in the absence of NH3 makes (OA)2PbI4 as 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 presence of water vapour as established by fluorescence spectroscopy and optical microscopy (Figure 7A and 7B). The green fluorescence of (OA)2PbI4 film showed no changes during the exposure or removal of water vapor (Figure 7B) proved the sensitivity of (OA)2PbI4 specific towards NH3, not towards atmospheric moisture.

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


Page 14 of 22

Page 15 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Above mentioned acid-base reaction and volatility of precursor linear amine explain the response and reversibility of alkylammonium lead iodide perovskites in presence of NH3 gas. The cyclohexylammonium lead(II) iodide perovskite (CHA)2PbI4,26 (XRD, Figure S2, SI), showed no significant changes in optical properties during or after the exposure of NH3 gas as investigated by fluorescence microscopy (Figure S3, SI) 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 towards 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 In order to investigate the differences in crystal packing, we analysed previously reported single crystal data22-23,26-27 of a series of alkylammonium lead(II) iodides using 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). From the lattice structure and the observed behaviour 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 towards NH3 molecules, while the relatively close-packed crystal lattice of (CHA)2PbI4 reduce the penetration towards NH3 molecules. Presence of these voids renders the crystal lattice vulnerable towards the incoming ammonia and hence easily gets ruptured. Interestingly, the room temperature monoclinic structure of (OA)2PbI4 undergoes a transition to orthorhombic crystal structure at 45 °C (Figure S4, A, SI) which is also reported by others.23 This orthorhombic structure of (OA)2PbI4 is nonresponsive towards NH3. No



significant changes were observed in fluorescence emission spectra (Figure S4, B, SI, excitation wavelength 380 nm) and fluorescence microscope images (Figure S4, C-E, SI) in presence of NH3 show the nonresponsive behaviour 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 behaviour of the thermally treated (OA)2PbI4 (Figure S4, F, SI). Such response was justified by the close-packed crystal structure23 (Figure S4, G, SI), 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 towards NH3. We believe the presence of accessible voids facilitates NH3 to diffuse into the lattice to facilitate proton exchange with alkylammonium cation, leading to the formation of free amine inside the lattice, which then changes the optical properties of the perovskite material.


Page 16 of 22

Page 17 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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. To check the sensitivity, we kept (OA)2PbI4 film inside a fluorescence cuvette and introduced NH3 gas at different concentrations (Figure 9, see experimental section for detail). The fluorescence emission from (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 sensor. We believe rapidity in response due to the facile conversion of octylamine from octylammonium cation in presence of NH3 gas, continuous reversibility and stability due to less volatility of in situ formed octylamine along with high sensitivity may establish (OA)2PbI4 as an optimum material for the future NH3 sensor.



Figure 9: Fluorescence spectra of (OA)2PbI4 film in presence of different concentration of NH3. The limit of detection for NH3 is around 30 ppm.

CONCLUSION In summary, we explored three major aspects of signal transduction of organic-inorganic hybrid perovskite in presence of NH3, namely, stability, the origin of fascinating reversibility and rapidity coupled with sensitivity. Our results reveal a rapid proton switching between alkylammonium cation packed inside the perovskite crystal lattice with NH3 gas and this is analysed using 1H NMR and XRD. In addition, the volatility of alkylamine formed in situ determines the 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 structure and properties. The analyses of crystal structures of a series of linear alkyl ammonium lead(II) iodide perovskites


Page 18 of 22

Page 19 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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, extremely less volatility of free octyl amine 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 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.

ASSOCIATED CONTENT Supporting Information The supporting Information is available free of charge on the ACS Publication website at DOI: Fluorescence microscopy images and XRD spectra of alkylammonium lead(II) iodide.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]; [email protected] ORCID Suresh Valiyaveettil: 0000-0001-6990-660X Raj Ganesh S. Pala: 0000-0001-5243-487X Sri Sivakumar: 0000-0002-6472-2702

Notes



The authors declare no competing financial interest. ACKNOWLEDGEMENTS S. S. and S. V. gratefully acknowledge the funding support from the National University of Singapore under the joint Ph.D. program between NUS and IITK. We are thankful for Dr. Nancy Singhal to show the operation principle of Fluorescence Spectrophotometer.

REFERENCES 1. 2. 3. 4. 5. 6.

7.

8. 9.

10.

11.

12.

13.

Timmer, B.; Olthuis, W.; Berg, A. V. D. Ammonia sensors and their applications—a review. Sensors and Actuators B. 2005, 107, 666–677. Close, L. G.; Catlin, F. I.; Cohn, A. M. Acute and chronic effects of ammonia burns on the respiratory tract. Arch. Otolaryngol. 1980, 106, 151–158. Hoz, R. E. de la; Schueter, D. P.; Rom, W. N. Chronic lung disease secondary to ammonia inhalation injury: a report on three cases. Am. J. Ind. Med. 1996, 29. 209–214. Rout, C. S.; Hegde, M.; Govindaraj, A.; Rao, C. N. R. Ammonia sensors based on metal oxide nanostructures. Nanotechnology. 2007, 18, 205504 Joshi, A.; Gangal, S. A.; Gupta, S. K. Ammonia sensing properties of polypyrrole thin films at room temperature. Sensors and Actuators B. 2011, 156, 938-942. Kojima, A.; Teshima, K.; Shirai Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 60506051. Jana, A.; Mittal, M.; Singla, A.; Sapra, S. Solvent-free, mechanochemical syntheses of bulk trihalide perovskites and their nanoparticles. Chem. Commun. 2017, 53, 30463049. Manser, J. S.; Christians, J. A.; Kamat, P. V.; Intriguing Optoelectronic Properties of Metal Halide Perovskites. Chem. Rev. 2016, 116, 12956-13008. Safdari, M.; Svensson, P. H.; Hoang, M. T.; Oh, I.; Kloo, L.; Gardner, J. M. Layered 2D alkyldiammonium lead iodide perovskites: synthesis, characterization, and use in solar cells. J. Mater. Chem. A. 2016, 4, 15638–15646. Cho, J.; Choi, Y.-H.; O'Loughlin, T. E.; Jesus, L. D.; Banerjee, S. Ligand-Mediated Modulation of Layer Thicknesses of Perovskite Methylammonium Lead Bromide Nanoplatelets. Chem. Mater. 2016, 28, 6909−6916. Safdari, M.; Fischer, A.; Xu, B.; Kloo, L.; Gardner, J. M. Structure and function relationships in alkylammonium lead(II) iodide solar cells. J. Mater. Chem. A. 2015, 3, 9201–9207. Zhao, Y.; Zhu, K. Optical bleaching of perovskite (CH3NH3)PbI3 through roomtemperature phase transformation induced by ammonia. Chem. Commun. 2014, 50, 1605-1607. Bao, C.; Yang, J.; Zhu, W.; Zhou, X.; Gao, H.; Li, F.; Fu, G.; Yu, T.; Zou, Z. A resistance change effect in perovskite CH3NH3PbI3 films induced by ammonia. Chem. Commun. 2015, 51, 15426-15429.


Page 20 of 22

Page 21 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14.

15. 16.

17.

18. 19. 20.

21. 22.

23.

24.

25. 26. 27.

Huang, W.; Manser, J. S.; Sadhu, S.; Kamat, P. V.; Ptasinska, S. Direct Observation of Reversible Transformation of CH3NH3PbI3 and NH4PbI3 Induced by Polar Gaseous Molecules. J. Phys. Chem. Lett. 2016, 7, 5068-5073. Spek, A. L. Single-crystal structure validation with the program PLATON. J. Appl. Cryst. 2003, 36, 7-13. Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; Grätzel, M.; Park, N.-G. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. Lin, H.; Zhu, L.; Huang, H.; Reckmeier, C. J.; Liang, C.; Rogach, A. L; Choy, W. C. H. Efficient near-infrared light-emitting diodes based on organometallic halide perovskite– poly(2-ethyl-2-oxazoline) nanocomposite thin films. Nanoscale. 2016, 8, 19846-19852. Fan, L. Q.; Wu, J. H. NH4PbI3 Structure reports online. Acta Cryst. E63, 2007, i 189. Bedlivy, D.; Mereiter, K. The Structures of Potassium Lead Triiodide Dihydrate and Ammonium Lead Triiodide Dihydrate. Acta Cryst. B36, 1980, 782-785. Liang, K.; Mitzi, D. B.; Prikas, M. T. Synthesis and Characterization of OrganicInorganic Perovskite Thin Films Prepared Using a Versatile Two-Step Dipping Technique. Chem. Mater. 1998, 10, 403-411. Miller, A. J.; Le Châtelier's principle and the equilibrium constant. J. Chem. Educ. 1954, 9, 455. Billing, D. G.; Lemmerer, A. Synthesis, characterization and phase transitions in the inorganic–organic layered perovskite-type hybrids [(CnH2n + 1NH3)2PbI4], n = 4, 5 and 6. Acta Cryst. B63, 2007, 735-747. Lemmerer, A.; Billing, D. G. Synthesis, characterization and phase transitions of the inorganic–organic layered perovskite-type hybrids [(CnH2n+1NH3)2PbI4], n = 7, 8, 9 and 10. Dalton Trans. 2012, 41, 1146-1157. Tian, X.; Wang, Q.; Chen, X.; Yang, W.; Wu, Z.; Xu, X.; Jiang, M.; Zhou, Z. Enhanced performance of core-shell structured polyaniline at helical carbon nanotube hybrids for ammonia gas sensor. Appl. Phys. Lett. 2014, 105, 203109. Aziza, Z. B.; Zhang, Q.; Baillargeat, D. Graphene/mica based ammonia gas sensors. Appl. Phys. Lett., 2014, 105, 254102. Billing, D. G.; Lemmerer, A. Inorganic–organic hybrid materials incorporating primary cyclic ammonium cations: The lead iodide series. CrystEngComm 2007, 9, 236-244. 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.



Graphic for manuscript


Page 22 of 22

Volatility and Chain Length Interplay of Primary ... - ACS Publications

Recommend Documents