Adsorption of Pollutants by CsPbBr3 Perovskite

Introduction: Climate change and environmental sustainability ask for a more efficient and circular waste management. In parallel, water and air purification is another major goal that challenges the scientific community to develop new materials, techniques and strategies. Moreover, there is a growing interest to produce green energy with low-cost materials. In particular, in the last decade, halide perovskites have received considerable interest from the communities of physicists of matter, chemists and materials scientists for the many possible applications to replace the classic semiconductors for photovoltaics, optoelectronics and sensors [1, 2]. Perovskites are a large class of compounds characterized by the formula ABX3, having the same structure as CaTiO3. The inorganic halide perovskites (HP) are constituted by a halogen (X), a divalent metal (B), and a monovalent metal (A). The main physical properties that make both organic and inorganic halide perovskites excellent materials for devices are: the high mobility of the charge carriers, the high absorption coefficient being direct band-gap semiconductors and the small cross section of the defects (mainly halogen vacancies). In particular, CsPbX3 (X=Cl, Br, I) is deeply investigated because the bandgap energy Eg can be finely tuned to cover the whole visible range, when alloying different halides. Typically, CsPbBr3 is studied for optoelectronic devices like lasers, light amplification system (resonators and waveguides) and other photonic devices. This research will show how halide perovskites can also be used as substrates for the absorption and catalysts for the photodegradation of pollutants. As an example, Methylene Blue (MB), a typical pollutant that can be found in water as waste of textile industry, has been considered. The final idea is the possibility to circularly reuse perovskites recovered from other applications, such as photovoltaics [3], and exploit their pollutant absorption properties. In this way it is possible to create a double advantage for the environment: reduce the amount of solid waste and improve water quality. Materials and methods: This research is focused on the use of CsPbBr3 as absorbers of 10^-5M aqueous solutions of MB, which simulates a typical range of concentration of organic dye pollutants in textile industry effluents. The CsPbBr3 was synthesized in powder form according to the following procedure: the precursor salts PbBr2 (0.17M) and CsBr (0.035M) were dissolved in DMF (Dymethylformamide) and Methanol (MeOH), respectively. Under magnetic stirring, the PbBr2 was mixed with CsBr and the solvents were allowed to evaporate. Once the powdered perovskite was obtained, it was cleaned in 2-propanol, in order to eliminate any solvent residue. Four different molar ratios of PbBr2/CsBr salt precursors, namely 1; 1.9; 2.9 and 3.9, were investigated, leading to the systems called PVK 1, PVK 1.9, PVK 2.9 and PVK 3.9. The XRD analysis of the obtained powders confirmed the presence in all cases of the CsPbBr3 phase, with a significant percentage of spurious phases for higher molar ratios. The analysis for the absorption of the MB and its photodegradation was carried out using a UV-VIS spectrometer and illumination was performed by a solar simulator. The study was carried out analyzing the percentage of MB absorbed by the CsPbBr3 nanopowders (NPs) as a function of time. In addition, to facilitate the manipulation of the CsPbBr3 NPs and reduce any eventual Pb release in water during their employment as adsorbents/catalysts, they were incorporated into millimiter-sized hydrogel bubbles made of chitosan (CH), a biopolymer extracted from the shell of waste shrimps. MB absorption experiments were also conducted for this hybrid hydrogel composites, and compared with the absorption of analogous systems made of chitosan alone. Results and discussions: In order to investigate the capabilities of the different CsPbBr 3 samples (PVK 1, PVK 1.9, PVK 2.9 and PVK 3.9) concerning the pollutant absorption, the synthesized powders were simply put in contact with a MB solution (10 M). Tw o different adsorbent loadings were investigated: 1% and 10%. As shown in Figure 15 , a very reduced contact time between the MB and perovskite NPs is needed to obtain significant MB absorption. In general, higher absorption is obtained when the perovskite powder loading is 10%. The absorption was confirmed both by the visual analysis, and by the UVVIS analysis conducted through the spectrophotometer. The samples prepared with a molar ratio of 1:1 between the precursor salts show a greater MB absorption capability than the other ratios. In particular, a complete absorption of the MB can be obtained in only five minute when the PVK system is composed of pure CsPbBr 3 When CsPbBr 3 is incorporated inside chitosan bubbles (CHB), the adsorption capability of the system is reduced, as enlightened by the fact that complete MB removal cannot be obtained even after 5 hours. Anyway, it is noteworthy that the incorporation of the perovskite NPs enables to enhance the absorption capabilities of bubbles made of pure chitosan. Figure 2 shows MB absorption data related to various chitosan perovskite bubbles, obtained by incorporating the different perovskite powders at 1%. In contrast to what occurs in the case of powders, the hydrogel systems that show best adsorption capabilities (remo val of 70% of MB after overnight contact) are those containing perovskite systems 1.9 and 2.9 ( CsPbBr Fig. 2 ). Furthermore, unlike the absorption of MB by bare NPs, the absorption is greater and faster with a reduced percentage of perovskite inside the C 3 results in the case of PVK loading equal to 1%). HB (best These results open the way to the use of halide perovskite material as adsorbers for different pollutants in water. Interestingly, the use of chitosan allows to solve the already known stabilization problems of these materials in water [4] .