Acoustic Properties of Metal-Organic Frameworks

Chiral Two-Dimensional Hybrid Organic-Inorganic Perovskites for Piezoelectric Ultrasound Detection

Hybrid organic-inorganic perovskites (HOIPs) have exhibited striking application potential in piezoelectric energy harvesting and sensing due to their high piezoelectricity, light weight, and solution processability. However, to date, the application of piezoelectric HOIPs in ultrasound detection has not yet been explored. Here, we report the synthesis of a pair of chiral two-dimensional piezoelectric HOIPs, R-(4-bromo-2-butylammonium)2PbBr4 and S-(4-bromo-2-butylammonium)2PbBr4 [R-(BrBA)2PbBr4 and S-(BrBA)2PbBr4], which show low mechanical strength and significant piezoelectric strain coefficients that are advantageous for mechanoelectrical energy conversion. Benefiting from these virtues, the R-(BrBA)2PbBr4@PBAT and S-(BrBA)2PbBr4@PBAT [PBAT = poly(butyleneadipate-co-terephthalate)] composite films show prominent underwater ultrasound detection performance with a transmission effectivity of 12.0% using a 10.0 MHz probe, comparable with that of a polyvinylidene fluoride (PVDF) device fabricated in the same conditions. Density functional theory calculations reveal that R-(BrBA)2PbBr4 and S-(BrBA)2PbBr4 have a beneficial acoustic impedance (5.07-6.76 MRayl) compatible with that of water (1.5 MRayl), which is responsible for the facile ultrasound-induced electricity generation. These encouraging results open up new possibilities for applying piezoelectric HOIPs in underwater ultrasound detection and imaging technologies.

Structure-Property Relationship of Piezoelectric Properties in Zeolitic Imidazolate Frameworks: A Computational Study

Metal-organic frameworks (MOFs) are a class of nanoporous crystalline materials with very high structural tunability. They possess a very low dielectric permittivity ε_r due to their porosity and hence are favorable for piezoelectric energy harvesting. Even though they have huge potential as piezoelectric materials, a detailed analysis and structure-property relationship of the piezoelectric properties in MOFs are lacking so far. This work focuses on a class of cubic non-centrosymmetric MOFs, namely, zeolitic imidazolate frameworks (ZIFs) to rationalize how the variation of different building blocks of the structure, that is, metal node and linker substituents affect the piezoelectric constants. The piezoelectric tensor for the ZIFs is computed from ab initio theoretical methods. From the calculations, we analyze the different contributions to the final piezoelectric constant d₁₄, namely, the clamped ion (e₁₄⁰) and the internal strain (e₁₄^int) contributions and the mechanical properties. For the studied ZIFs, even though e₁₄ (e₁₄⁰ + e₁₄^int) is similar for all ZIFs, the resultant piezoelectric coefficient d₁₄ calculated from piezoelectric constant e₁₄ and elastic compliance constant s₄₄ varies significantly among the different structures. It is the largest for CdIF-1 (Cd²⁺ and -CH₃ linker substituent). This is mainly due to the higher elasticity or flexibility of the framework. Interestingly, the magnitude of d₁₄ for CdIF-1 is higher than II-VI inorganic piezoelectrics and of a similar magnitude as the quintessential piezoelectric polymer polyvinylidene fluoride.

Insights into Solid-To-Solid Transformation of MOF Amorphous Phases

Metal-organic frameworks (MOFs) have been recently explored as crystalline solids for conversion into amorphous phases demonstrating non-specific mechanical, catalytic, and optical properties. The real-time control of such structural transformations and their outcomes still remain a challenge. Here, we use in situ high-resolution transmission electron microscopy with 0.01 s time resolution to explore non-thermal (electron induced) amorphization of a MOF single crystal, followed by transformation into an amorphous nanomaterial. By comparing a series of M-BTC (M: Fe³⁺, Co³⁺, Co²⁺, Ni²⁺, and Cu²⁺; BTC: 1,3,5-benzentricarboxylic acid), we demonstrate that the topology of a metal cluster of the parent MOFs determines the rate of formation and the chemistry of the resulting phases containing an intact ligand and metal or metal oxide nanoparticles. Confocal Raman and photoluminescence spectroscopies further confirm the integrity of the BTC ligand and coordination bond breaking, while high-resolution imaging with chemical and structural analysis over time allows for tracking the dynamics of solid-to-solid transformations. The revealed relationship between the initial and resulting structures and the stability of the obtained phase and its photoluminescence over time contribute to the design of new amorphous MOF-based optical nanomaterials.

Recent Progress in the Removal of Legacy and Emerging Organic Contaminants from Wastewater Using Metal-Organic Frameworks: An Overview on Adsorption and Catalysis Processes

Water covers about 70% of the Earth’s surface, but the amount of freshwater available for human use is only 2.5% and, although it is continuously replenished via the water cycle, freshwater is a finite and limited resource. The Earth’s water is affected by pollution and while water quality is an issue of global concern, the specific regulations on contaminants of emerging concern (CECs) are limited. In order to achieve the goals set by EU regulations, the treatment of wastewater is a scientifically and technologically challenging issue. Metal–organic frameworks (MOFs) are promising materials used for the removal of priority and emerging contaminants from wastewater, since they can mitigate those contaminants via both adsorption as well as catalysis processes. MOFs can offer selective adsorption of CECs by various adsorption mechanisms. The catalytic removal of priority and emerging organic contaminants from wastewater using MOFs implies Fenton, electro-Fenton, and photo-Fenton processes. Overall, MOFs can be considered as promising materials for the elimination of priority and emerging organic contaminants from various wastewater types, but the involved processes must be studied in detail for a larger number of compounds.

Harnessing Electrostatic Interactions for Enhanced Conductivity in Metal-Organic Frameworks

The poor electrical conductivity of metal-organic frameworks (MOFs) has been a stumbling block for its applications in many important fields. Therefore, exploring a simple and effective strategy to regulate the conductivity of MOFs is highly desired. Herein, anionic guest molecules are incorporated inside the pores of a cationic MOF (PFC-8), which increases its conductivity by five orders of magnitude while maintaining the original porosity. In contrast, the same operation in an isoreticular neutral framework (PFC-9) does not bring such a significant change. Theoretical studies reveal that the guest molecules, stabilized inside pores through electrostatic interaction, play the role of electron donors as do in semiconductors, bringing in an analogous n-type semiconductor mechanism for electron conduction. Therefore, we demonstrate that harnessing electrostatic interaction provides a new way to regulate the conductivity of MOFs without necessarily altering the original porous structure. This strategy would greatly broaden MOFs' application potential in electronic and optoelectronic technologies.