Nanostructure-dependent indentation fracture toughness of metal-organic framework monoliths

Monolithic metal-organic frameworks (MOFs) represent a promising solution for the industrial implementation of this emerging class of multifunctional materials, due to their structural stability. When compared to MOF powders, monoliths exhibit other intriguing properties like hierarchical porosity, that significantly improves volumetric adsorption capacity. The mechanical characterization of MOF monoliths plays a pivotal role in their industrial expansion, but so far, several key aspects remain unclear. In particular, the fracture behavior of MOF monoliths has not been explored. In this work, we studied the initiation and propagation of cracks in four prototypical MOF monoliths, namely ZIF-8, HKUST-1, MIL-68 and MOF-808. We observed that shear faults inside the contact area represent the main failure mechanism of MOF monoliths and are the source of radial cracks. MIL-68 and MOF-808 showed a remarkably high resistance to cracking, which can be ascribed to their consolidated nanostructure

Triboelectric behaviour of selected zeolitic-imidazolate frameworks: exploring chemical, morphological and topological influences †

Tribo- and contact electrification remain poorly understood, baffling and discombobulating scientists for millennia. Despite the technology needed to harvest mechanical energy with triboelectric generators being incredibly rudimentary and the fact that a triboelectric output can be obtained from almost any two material combinations, research into triboelectric generator materials typically focuses on achieving the highest possible output; meanwhile, understanding trends and triboelectric behaviours of related but lower performing materials is often overlooked or not studied. Metal–organic frameworks, a class of typically highly porous and crystalline coordination polymers are excellent media to study to fill this knowledge gap. Their chemistry, topology and morphology can be individually varied while keeping other material properties constant. Here we study 5 closely related zeolitic-imidazolate type metal–organic frameworks for their triboelectric performance and behaviour by contact-separating each one with five counter materials. We elucidate the triboelectric electron transfer behaviour of each material, develop a triboelectric series and characterise the surface potential by Kelvin-probe force microscopy. From our results we draw conclusions on how the chemistry, morphology and topology affect the triboelectric output by testing and characterising our series of frameworks to help better understand triboelectric phenomena

High-performance triboelectric nanogenerators incorporating chlorinated zeolitic imidazolate frameworks with topologically tunable dielectric and surface adhesion properties

Triboelectric nanogenerator (TENG), a device that can convert mechanical energy into electricity based on the principle of triboelectrification, has gained tremendous attention since its first discovery in 2012. Although TENG has versatile applications in energy harvesting and self-powered sensing, its commercialization is still limited by the low power output. Recently, metal-organic frameworks (MOFs), with their large surface area and excellent tunability, have been explored to enhance the electrical performance of TENG. Herein, we synthesized nanoparticles of hydrophobic zeolitic imidazolate framework ZIF-71 (RHO topology) and its non-porous counterpart ZIF-72 (LCS topology), which were subsequently incorporated in a polydimethylsiloxane (PDMS) matrix as filler materials. By modifying the topology of ZIF nanofillers, we found the dielectric constant and surface adhesion of composites are both enhanced, thereby generating significantly higher triboelectric output. Moreover, we show the resultant ZIF/PDMS nanocomposite films exhibit enhanced triboelectric properties and long-term stability under cyclic mechanical loading. After integrating the prepared nanocomposite films into TENG devices, we accomplished the peak output voltage and current of 578 V and 19 μA for thin films (3 ×3 cm2, thickness ∼0.33 mm), respectively, by embedding 1 wt % of ZIF-72 nanoparticles into PDMS matrix, with an instantaneous maximum power density of ∼5 W m−2. In this study, the mechanism of improved TENG performance by incorporating MOF nanoparticles has, for the first time, been revealed through nanoscale-resolved mechanical and chemical studies. Furthermore, the practicality of MOF-based TENG was demonstrated by harvesting energy from oscillatory motions, for powering up commercial microelectronics, transmitting electrical signals remotely, and functioning as a self-powered Morse code generator

Fracture behavior of MOF monoliths revealed by nanoindentation and nanoscratch

Monolithic metal-organic frameworks (MOFs) represent a promising solution for the industrial implementation of this emerging class of multifunctional materials, due to their structural stability. When compared to MOF powders, monoliths exhibit other intriguing properties like hierarchical porosity, that significantly improves volumetric adsorption capacity. The mechanical characterization of MOF monoliths plays a pivotal role in their industrial expansion, but so far, several key aspects remain unclear. In particular, the fracture behavior of MOF monoliths has not been explored. In this work, we studied the initiation and propagation of cracks in four prototypical MOF monoliths, namely ZIF-8, HKUST-1, MIL-68 and MOF-808. We observed that shear faults inside the contact area represent the main failure mechanism of MOF monoliths and are the source of radial cracks. MIL-68 and MOF-808 showed a remarkably high resistance to cracking, which can be ascribed to their consolidated nanostructure.Comment: 15 pages, 5 figures, Supporting Informatio

Unravelling the ageing effects of PDMS‐based triboelectric nanogenerators

Ageing of elastomeric materials in triboelectric nanogenerators (TENGs) often leads to compromised electrical performance and can greatly affect their real-world application as next-generation energy harvesters and self-powered sensors. Herein, the ageing behavior of PDMS-based TENGs is investigated by probing the dielectric and mechanical properties of the membrane materials. Over time, ageing of PDMS after 17 months is evinced by a decline by 71%, 68% and 52% in open-circuit voltage, short-circuit current and charge transfer, respectively, and an increase by 6.8 times in surface charge decay rate. The reduced electrical performance can be attributed to a decrease in work function, dielectric constant, surface adhesion and heterogeneity in stiffness, as well as an increase in loss tangent. The effect of chemical chain scission on the PDMS surface is confirmed through nearfield infrared nanospectroscopy. This study gives insights into the underlying mechanism behind the ageing of PDMS-based TENGs, paving the way to future work for ameliorating these ageing-related issues with the aim to ensure long-term stability of practical triboelectric devices

Tailored broad-spectrum emission in hybrid aggregation induced emission (AIE)-MOFs: boosting white light efficiency in electrospun Janus microfibers

Advances in energy-efficient lighting and display technologies demand innovative materials with tailored broad-spectrum emission properties. Hybrid aggregation-induced emission metal-organic frameworks (AIE-MOFs) offer a promising avenue, combining unique characteristics of organic and inorganic components to yield enhanced luminescence efficiency and robust material stability. The study introduces a spectrum of D (donor)-A (acceptor) type AIE-active ligands into MOFs, enabling tunable emission across the visible spectrum, thus underscoring the versatility of these hybridized MOF materials. The emission properties of AIE-MOFs are further harnessed by integrating them into polymer matrices, resulting in high-performance electrospun fibers with tunable emission. A significant achievement involves the fabrication of Janus-type white light-emitting AIE-MOF fiber composites via side-by-side electrospinning, accomplishing a high quantum yield of 58%, which doubled the performance of homogeneous fibers. Complementing the experimental findings, micro-Raman and nano-Fourier transform infrared spectroscopy are employed as local spectroscopic probes, affording a deeper understanding of the material properties and the mechanisms contributing to enhance light emission. In the understanding, this study presents an unconventional implementation of hybrid AIE-MOFs in Janus-type structures for white light emission. It significantly improves the efficiency of white light sources in optoelectronics, charting a promising direction for future research in the emergent AIE-MOF field

Optochemically Responsive 2D Nanosheets of a 3D Metal-Organic Framework Material

Outstanding functional tunability underpinning metal-organic framework (MOF) confers a versatile platform to contrive next-generation chemical sensors, optoelectronics, energy harvesters and converters. We report a rare exemplar of a porous 2D nanosheet material, constructed from an extended 3D MOF structure. We develop a rapid supramolecular self-assembly methodology at ambient conditions, to synthesize readily-exfoliatable MOF nanosheets, functionalized in situ by adopting the Guest@MOF (Host) strategy. Nanoscale confinement of light-emitting molecules (as functional guest) inside the MOF pores generates unusual combination of optical, electronic, and chemical properties, arising from the strong host-guest coupling effects. We show highly promising photonics based chemical sensing opened up by the new Guest@MOF composite systems. By harnessing host-guest optochemical interactions of functionalized MOF nanosheets, we have accomplished detection of an extensive range of volatile organic compounds (VOCs) and small molecules important for many practical applications.Comment: 21 pages, 5 figures, 1 schem

Accelerating Extreme Learning Machine on FPGA by Hardware Implementation of Given Rotation - QRD

Currently, Extreme Learning Machine (ELM) is one of the research trends in the machine learning field due to its remarkable performances in terms of complexity and computational speed. However, the big data era and the limitations of general-purpose processor cause the increasing of interest in hardware implementation of ELM in order to reduce the computational time. Hence, this work presents the hardware-software co-design of ELM to improve the overall performances. In the co-design paradigm, one of the important components of ELM, namely Given Rotation-QRD (GR-QRD) is developed as a hardware core. Field Programmable Gate Array (FPGA) is chosen as the platform for ELM implementation due to its reconfigurable capability and high parallelism. Moreover, the learning accuracy and computational time would be used to evaluate the performances of the proposed ELM design. Our experiment has shown that GR-QRD accelerator helps to reduce the computational time of ELM training by 41.75% while maintaining the same training accuracy in comparison to pure software of ELM