Anion exchange in inorganic perovskite nanocrystal polymer composites

We demonstrate a facile, low-cost and room-temperature method of anion exchange in cesium lead bromide nanocrystals (CsPbBr3 NCs), embedded into a polymer matrix.

Solution-processed all-inorganic cesium lead halide perovskite (CsPbX 3 , X ¼ Cl, Br, I) nanocrystals (NCs) have drawn a lot of attention lately, due to their exceptional optical properties, including medium optical bandgaps, strong absorption coefficients, high luminescence quantum yields and narrow emission bandwidths. [1][2][3][4][5] Owing to these properties, they have been introduced as a new class of photoactive materials for nextgeneration, low-cost, high-performance exible optoelectronics, 6,7 including perovskite-based solar cells, 8 lasing sources, 9,10 photodetectors 11 and light-emitting diodes [12][13][14][15] with high brightness and tunable emission. At the same time, allinorganic perovskites exhibit higher thermal and chemical stability, 16 as well as higher resistance to humidity 17 than their organic-inorganic counterparts, such as MAPbX 3 . The stability of halide perovskite NCs still remains a research topic of great interest. 18 It has been reported that the robustness of CsPbX 3 NCs can be improved by the addition of a small amount of polymer (poly(maleic anhydride-alt-1-octadecene)-PMA into the precursor solutions, which creates an additional ligand coating around each individual NC, or via encapsulation into PMMA or polyethylene oxide). 12,[19][20][21] Furthermore, a silica-coating process has been reported to enhance the stability of inorganic perovskite NC-based LEDs. 22,23 A prominent property of perovskite NCs is their ability to undergo a post-synthesis anion exchange, in solution, using chemical precursors or photo-induced processes. [24][25][26][27] Despite the numerous studies on anion exchange reactions in the liquid phase, only a few reports have demonstrated such reactions in solid state, either in the bulk or in the form of NCs. In particular, Hoffman et al. 28 reported the conversion of CsPbBr 3 to CsPbI 3 lms following heat treatment with a PbI 2 solution. While, Guhrenz et al. 27,29 reported a method of anion exchange via the direct incorporation of CsPbX 3 NCs into ion-rich matrices. In parallel, there have been reports of post-synthetic halide exchange reactions in organicinorganic metal-halide bulk perovskites (OIHPS) upon exposure to halogen (X 2 ) 30-32 and hydrogen halide (HX) gases. 33 Gasinduced formation/transformation (GIFT) of OIHPS has shown tremendous promise in various applications, including solar cells, optoelectronics, sensors, and beyond, however, a detailed understanding of the mechanisms underlying the GIFT phenomena is still lacking. 31 In this communication, we introduce for the rst time a GIFT process in perovskite NCs in solid state. In particular, we present a simple, post-synthesis and room temperature, solidstate anion exchange method to tune the emission properties of inorganic perovskite NCs, hosted into a polymer matrix. We demonstrate anion exchange in nanocomposite layers, comprising of CsPbBr 3 NCs dispersed in polydimethylsiloxane (PDMS), upon their exposure to a halide precursor gas atmosphere at room temperature. Fig. 1 represents a schematic illustration of the schematic route followed for the transformation of CsPbBr 3 to CsPbCl 3 . It is shown that the extent of the anion exchange reaction and therefore the NCs' emission properties can be nely tuned by adjusting the exposure time and concentration of Cl 2 gas; the iodine anion exchange process is also demonstrated. Apart from the tunability of nanoparticle emission, it is shown that the PDMS matrix protects the NCs against adverse humidity effects, giving rise to stable optical properties. These properties can open up new avenues for the in situ and low-cost optical modulation of perovskite polymernanocomposites, useful in various optoelectronic applications.
Following synthesis, the NC colloids in hexane showed a characteristic uorescence peak at 521 nm, with a full width half maximum (FWHM) of $25 nm (Fig. S2 †). The incorporation of NCs into PDMS 34 gave rise to a nanocomposite with a characteristic yellowish color under ambient light (Fig. 2, inset) and a pronounced green emission upon excitation with UV light (Fig. 3b). As shown in Fig. 2, the NCs' absorption maximum was slightly red-shied from 495 nm in solution to 510 nm in the nanocomposite, while the uorescence maximum was slightly blue-shied from 521 nm in solution to 515 nm in the nanocomposite (Fig. 2). This is mainly due to the increase in the dielectric properties of the surrounding medium, from hexane with n hexane ¼ 2.06, to PDMS with n PDMS ¼ 2.3-2.8. Furthermore, a slight broadening of the respective emission peak was observed due to the formation of NC clusters, which was by Two-Photon Excited Fluorescence (TPEF) Microscopy ( Fig. S4 †).
More importantly, the emission of NCs hosted into PDMS was observed to be remarkably stable over time, upon exposure to ambient conditions. This is in contrast to the widely reported sensitivity of CsPbBr 3 NCs to ambient air and/or moisture. [35][36][37] To further explore such emission stability and robustness against humidity, we investigated the emission spectra evolution of the PDMS:CsPbBr 3 NC layers, following their immersion into water. It was observed that the prolonged (24 h) interaction of the nanocomposites with water caused no signicant effect on their respective emission spectra (Fig. S5 †). Furthermore the uorescence spectra of the nanocomposites remain practically unchanged upon storage of the nanocomposites for 30 days in ambient conditions (Fig. S5 †). Both of the above observations are strong indications that the polymer matrix successfully protects the NCs against the effects of humidity.
We also observed that the optical absorption and uorescence spectra of the PDMS:CsPbBr 3 NC layers progressively blue-shied upon their exposure to Cl 2 gas, indicating the anionic exchange of the participating halides. The solid-state   chlorination process is presented in the ESI. † Representative results are shown in Fig. 3. In particular, exposure to Cl 2 gas, of 70 mbar-partial pressure, for 100 s gave rise to a blue-shi of both the absorption and emission peaks from $510 nm to $410 nm. This shi is reasonable, considering that the emission peak of CsPbBr 3 NCs is around 510 nm while that of CsPbCl 3 NCs is observed at $390 nm. 24 At the same time, quenching of the uorescence quantum yield was observed. Both phenomena, i.e. the partial replacement of Br ions with Cl ions and the uorescence quenching are in accordance with former ndings 24,25 regarding NC colloids. It should be noted that the FWHM of the blue-emitting composite layers attained is comparable to that of the initial layers. In addition, an incomplete exchange reaction took place for the thickest ($2 mm) samples tested. This is presented in Fig. S6, † showing that two characteristic absorption peaks, at $409 nm and $465 nm, arise upon exposure of the sample to a chlorine environment (Fig. S6 †). The corresponding uorescence spectra conrm the emission from two peaks, at $411 nm and $475 nm, with the latter being the most pronounced (Fig. S7 †). This is possibly due to the formation of mixed halide CsPb(Br/Cl) NCs with different Cl : Br ratios. On the contrary, in the case of a thinner layer ($500 mm), a single absorption peak at 409 nm is observed (Fig. 3a), while at the same time the emission peak shis from 515 nm (Fig. 3d, black line) to 411 nm within 100 s of exposure to chlorine (Fig. 3d, violet line), indicating the formation of CsPb(Br/Cl) NCs with a Br : Cl ratio of 2 : 3. 24 Following the exposure for 100 s, the phenomenon is partially reversible (Fig. S15 †), i.e. the uorescence spectrum slowly red-shis with time and saturates to a peak emission value of 475 nm, attributed to the chemical composition of CsPbBr 3 Cl 2 NCs (Br : Cl ratio of 3 : 2). In Fig. 3b and c typical images of a nanocomposite layer under UV light excitation, before and aer exposure to Cl 2 , are presented, respectively. It can be clearly seen that, the emitted green color of the pristine sample changes to blue upon chlorine treatment. Also, as shown in Fig. S8, † the color of the respective sample changes from yellow to light grey. In literature, anion conversion reactions have already been interpreted in terms of halogen reduction potentials, at least in the case of OIHPs. 30 These studies showed that exposure of OIHPs to a halogen gas, X 2 , can displace the crystal halide anions, Y À , at room temperature, provided that X features a higher standard reduction potential than the displaced halide, Y. 30,31 Our results indicate that this could also occur in the all-inorganic lead halide perovskites as well. Considering the higher reduction potential of Cl 2 compared to Br 2 , Cl 2 can oxidize Br À and convert CsPbBr 3 to CsPbCl 3 with solely gas-phase by-products. In the case of PDMS:CsPbBr 3 NCs, this process is facilitated by the high permeability and diffusivity of Cl 2 gas in PDMS, 38 enabling chlorine atoms to interact with the embedded perovskite NCs. Based also on the relevant literature, the ow rate of Cl 2 gas across a PDMS membrane is proportional to the difference in partial pressure and inversely proportional to the membrane thickness; 39 this could account for the decient anion exchange process taking place in the thicker nanocomposite layers.
To further shed light on the anion exchange process, the exposure of the nanocomposite layers to different Cl 2 gas partial pressures was investigated. The corresponding results are presented in Fig. 4 and S9; † in these gures I 1 is the intensity of the initial emission peak ($515 nm) and I 2 is the intensity of the emission peak that emerges upon exposure to Cl 2 (i.e. at $435 nm). It can be observed (Fig. 4a) that, as the Cl 2 gas pressure is increased from 0 to 70 mbar, the initial emission peak progressively blue-shis and I 1 decreases, while, on the other hand, I 2 gradually increases. It is also shown in Fig. 4b and c that both the I 2 /I 1 intensity ratio and the 1 st emission peak shi tend to saturate at a similar Cl 2 gas critical partial pressure ($20 mbar). These observations indicate the potential of the perovskite nanocomposite layers to operate as halide gas sensing elements. It is notable that the uorescence signal of these nanocomposites is preserved, even aer 24 h of treatment with chlorine.
The photoluminescence quantum yield (PLQY) of the initial CsPbBr 3 nanocrystals in hexane, measured via the comparative method, 40 was equal to 48%. Compared to the nanocrystals in solution, it is observed that when an equal vol% of CsPbBr 3 nanocrystals is embedded into PDMS, the photoluminescence intensity decreases (Fig. S16 †). Accordingly, the corresponding PLQY measured for the PDMS:CsPbBr 3 NC layers was dropped to 36%. Following chlorine treatment, the PLQY of the nanocomposites was decreased by almost 10 times, i.e. to 4%, which is in accordance to previous reports on the anion exchange effect on the PLQY. 24 Experiments in the presence of an iodine precursor gas were also performed, 32 as schematically shown in Fig. 1. The solid- Fig. 4 (a) Normalized fluorescence spectra of, 0.5 mm thick, PDMS:CsPbBr 3 NCs nanocomposite layers upon exposure to different partial Cl 2 pressures. The corresponding evolution of the initial peak with intensity I 1 (top) and of the peak that emerges after chlorine treatment with intensity I 2 , (bottom) are shown on the right. (b) Fluorescence intensity ratio, I 2 /I 1 , and (c) spectral shift of the first emission peak, as a function of the partial pressure of Cl 2 gas. state iodination process is presented in the ESI. † Fig. 5a and b present the absorption and uorescence spectra of the PDMS:CsPbBr 3 NC nanocomposite layers following sequential treatment, rst with I 2 gas under ambient conditions, followed by Cl 2 gas. Following exposure to I 2 gas under ambient conditions for 10 minutes, the nanocomposites showed a red-shied emission peak at $660 nm ($1.87 eV) that complies with that reported for CsPbI 3 NCs. 24 Subsequently, these nanocomposites were placed in a chlorine environment and their emission peak was observed to blue-shi to $410 nm (3.02 eV), i.e. close to that observed upon direct chlorination of the pristine PDMS:CsPbBr 3 NC layers. Considering the lower reduction potential of I 2 compared to that of Cl 2 , a redox-type conversion reaction, i.e. oxidation of Br À by I 2 and subsequent conversion of CsPbBr 3 to CsPbI 3 , could not account for the observed displacement of the emission peak. However, it is well known that the ambient humidity remarkably affects the I 2 gas stability, leading to the formation of HI and HIO. 41 It has also been reported that mutual anion conversions in perovskite NCs can be alternatively realised upon exposure to gaseous HX, via ion-exchange reactions. 33 Based on this, the possibility of HI formation due to ambient humidity may account for the observed red shi in the UV-Vis and uorescence spectra. Experiments involving exposure of PDMS:CsPbBr 3 NC nanocomposites to HX gases are currently in progress to clarify this issue.
It can be concluded that the anion exchange process can only proceed along a single direction, that is Br À > Cl À , Br À > I À , I À > Cl À . This is further conrmed by experiments with PDMS:CsPbI 3 NCs nanocomposite layers showing a characteristic shi of the initial uorescence peak to lower wavelengths upon exposure to Cl 2 gas ( Fig. S17 and S18 †). Our ndings comply with the reduction potential relationship of the three, considering that Cl 2 exhibits higher reduction potential compared to Br 2 and I 2 exhibits higher reduction potential compared to Br 2 .
To further account for the microscopic mechanism behind the anion exchange process, FTIR, XPS and XRD spectra of the PDMS:CsPbBr 3 NC layers, prior and aer chlorine treatment, were recorded. The corresponding FTIR spectra, presented in Fig. S10 and S11, † reveal no signicant change in the chemical structure of the nanocomposites following halogen gas treatment. The survey XPS scans (Fig. S12 †), recorded from the samples before and aer chlorine treatment, show mainly the presence of O, C and Si, attributed to the PDMS matrix.
Fig. S13 † shows the respective high-resolution XPS spectra of Cs 3d, Pb 4f and Br 3d peaks. Prior to Cl 2 exposure, traces of Cs, Pb and a small amount of Br were detected. While, aer exposure to Cl 2 , traces of Cs, Pb and a small amount of Cl were detected, indicating the replacement of Br with Cl. Finally, the corresponding XRD spectra are presented in Fig. S14, † showing a shi of the characteristic peaks of CsPbBr 3 NCs from 29.05 to 29.15 and from 38.2 to 39.2 , aer chlorine treatment. On the contrary, exposure to HI gives rise to a shi of the NCs' XRD peaks to lower diffraction angles (Fig. S14 †). Both of the above ndings are in accordance to previous literature observations on Br-Cl anion exchange reactions in perovskite NCs. 24,25,42,43 In accordance to the emission spectra, the corresponding XRD spectra remain practically unaffected upon storage of the nanocomposites for 30 days in ambient conditions (Fig. S14 †).

Conclusions
In summary, we have demonstrated a straightforward route to realize a, solid-state, anion exchange process in cesium lead halide perovskite NCs hosted into a polymer matrix. It is based on the exposure of perovskite NC:PDMS nanocomposite layers to a controlled halogen gas atmosphere. Using this method the nanocomposite absorption and emission properties can be spectrally tuned from the visible to ultraviolet, upon varying the exposure time to the respective halogen gas partial pressure. It is important to note here that the PDMS matrix constitutes a robust environment for the embedded perovskite NCs and secures their stability against humidity. The tunable optical characteristics, adjustable NC loadings and the ease of handling make the resulting nanocomposites attractive for applications in optoelectronics, e.g., as color conversion materials for solid-state lighting, laser gain media, and solar light concentrators. Most importantly, all inorganic cesium lead halide perovskite NC-based nanocomposites are presented as suitable candidates for halogen gas sensing applications. Presumably, the solid-state anion exchange strategy presented here can be practically applied to other inorganic as well as organic-inorganic perovskite polymer nanocomposites.

Funding sources
This work was supported by the State Scholarship Foundation (IKY) within the framework of the Action "Postdoctoral Researchers Support" (MIS: 5001552) from the resources of the OP "Human Resources Development, Education and Lifelong Learning" -ESPA 2014-2020 Program, contract number: 2016-050-0503-8904.

Conflicts of interest
There are no conicts to declare.