Metal nanoparticles and their environment can be locally heated on an ultrafast time scale using femtosecond pulsed illumination of their plasmon resonance, making them of interest for spatiotemporal temperature control. Here, we propose experimental approaches to obtain time-resolved particle and medium temperatures using gold nanoparticles. 23.5 and 39 nm nanoparticles dispersed in water and DMF:water mixture were heated and probed using transient absorption spectroscopy. Simulations indicate that the change in absorbance >10 ps after excitation arises from temperature-induced alterations in the dielectric functions of the particle and the medium. Thus, we measured the temperature-dependent absorbance spectra of nanoparticles, where the signal reflects the combined response of the particle and the medium to heating for a known temperature. We then disentangled the spectra obtaining the particle (Method 1) and the medium contributions (Method 2) to heating independently, followed by a consistency check between the two approaches (Method 3). Accordingly, the transient absorbance spectrum was resolved to extract particle and medium temperatures at each time delay. The resulting profiles are in line with each other, revealing temperature increases of ∼80 K for the particle and 5–15 K for the medium when excited at 400 nm with ∼4 J/m² fluence. A faster particle temperature decay was observed with decreasing particle size and a faster medium temperature decay with increasing medium thermal diffusivity, in agreement with expectations. Overall, we demonstrate an experimental methodology for simultaneous determination of particle and medium temperatures under a spatiotemporal gradient which is relevant for studies with transient heating and nanoparticles as sensors.

Carbon monoxide separation from industrial waste gases could contribute largely to carbon circularity. Traditional separation technologies are unable to separate CO from N₂ selectively. Instead, electroactive carriers show promise in selective separation of CO from N₂, where CO binds a complex in one oxidation state and releases in another oxidation state. We study Cu(i)/Cu(ii)-chloride complexes as potential carrier materials with high binding affinity to CO, good solubility and low energy consumption of the process. We show that the electrolyte composition of a copper chloride system affects the binding affinity and stability of the copper carrier (Cu⁺). Cyclic voltammetry measurements reveal that the CO binding constant increase from the previously reported 1600 M⁻¹ for 1 M KCl to 5500 M⁻¹ for 0.5 M CaCl₂. However, this increase in binding constant is not reflected to the same extent in the CO capture capacity, showing a smaller increase in CO capture. In general, the binding constant decreases with chloride concentration, while the Cu⁺ stability window increases. This highlights a trade-off that needs to be considered for electrolyte selection in electrochemical CO separation with copper chlorides.

Piezoelectric energy harvesting is a process in which energy in the form of kinetic movements can be harvested and converted into useful electrical energy using piezoelectric materials. Metal-organic frameworks (MOFs) have a huge potential for piezoelectric energy harvesting owing to their high flexibility, structural tunability, and very low dielectric constants due to their high porosity. The piezoelectric constant d relevant for piezoelectric energy harvesting depends on the piezoelectric constant e and the flexibility of the structure (i.e. mechanical properties). The mechanical properties of MOFs have previously been extensively studied but the piezoelectric constant e was never explored for MOFs. In this work, we generate a database of piezoelectric properties, specifically e for around ∼1608 previously synthesized non-centrosymmetric MOF structures. The calculations were performed using the density functional perturbation theory (DFPT) method. The highest piezoelectric constant e obtained in this work is approximately ∼2.76 C m⁻², which is significantly higher than that of the flexible organic piezoelectric polymer polyvinylidene fluoride (PVDF) and its copolymers. In this work, we analyze and identify structural factors that influence the values of the piezoelectric constant for high-performing MOFs. Based on that, a series of guidelines for the design of MOF structures that can lead to a high piezoelectric constant e are presented. One class of high-performing piezoelectric MOFs is based on polar patterns of O—(short)—Mo—(long)—O unequal bond length, reminiscent of ferroelectric inorganic oxides. This class could have potential for ferroelectricity, meaning that the bond length pattern could be reversed by external electrical field. We substantiate this by showing experimentally via SHG-microscopy that the O—(short)—Mo—(long)—O unequal bond lengths are indeed malleable by external conditions.

As molecular catalysts are increasingly employed in heterogenized systems such as CO₂ electroreduction, a need arises for more systematic approaches to characterize their preparation, distribution, and activity. Current means of classifying electroactive versus spectator molecules are insufficient, while improvement of ink formulations, depositions, and distributions relies primarily on indirect links to electrochemical performance. In this study, we expand the common utilization of Cyclic Voltammetry (CV) in homogeneous systems toward heterogenized molecular catalyst architectures. We illustrate how, even with porous catalyst layers containing carbon, ionomer, and molecules, a combined redox wave integration and UV-vis analysis can be used as a tool for designing a reproducible deposition procedure. An in-depth CV analysis is then used to study the effect of catalyst aggregation and quantify the number of electroactive sites on carbon supports. We show that FeTPP (Iron(III)meso-tetraphenylporphyrin chloride) gives a non-linear electroactive response when loading is varied, allowing for the identification of distinct loading regions of insufficient, optimal, and excessive coverage. A FeTPP to Vulcan carbon mass ratio of 0.1 provides the highest number of electroactive species, giving the lowest expected aggregation. Overall, the CV approaches are extendable to any redox-active catalysts, providing a versatile means of characterizing porous heterogeneous molecular catalyst systems.

Nucleation, the pivotal first step of crystallization, governs the essential characteristics of crystallization products, including size distribution, morphology, and polymorphism. While understanding this process is paramount to the design of chemical, pharmaceutical, and industrial production processes, major knowledge gaps remain, especially with respect to the crystallization of porous solids. Also for nanocrystalline ZIF-8, one of the most widely studied metal-organic frameworks, questions regarding the species involved in the nucleation pathway and their structural and chemical transformations remain unanswered. By combining harmonic light scattering, inherently sensitive to structural changes, with NMR spectroscopy, which reveals molecular exchanges between particles and solution, we were able to capture the crystallization mechanism of ZIF-8 in unprecedented detail. This dual approach provides concurrent structural and chemical insights, revealing key processes not previously observed in ZIF crystallization. Upon mixing, small charged prenucleation clusters (PNCs) are formed, exhibiting an excess of ligands and net positive charge. We show that nucleation is initiated by aggregation of PNCs, through the release of ligands and associated protons to the liquid. This leads to the formation of charge neutral amorphous precursor particles (APPs), which incorporate neutral monomers from the solution and crystallized ZIF-8. Our work highlights chemical dynamics as a vital, yet often overlooked, dimension in the multistage structural evolution of MOFs. By establishing the critical role of PNCs in the nucleation of ZIF-8, new pathways open up for controlling crystallization of metal-organic frameworks through targeted chemical interactions with these species.

Due to the irreversible tautomerization of imine linkages to their corresponding ketoenamines, β-ketoenamine-linked Covalent Organic Frameworks (COFs) are a stable type of COF that displays high surface areas. In the solvothermal synthesis of such COFs, the use of (acetic) acid is ubiquitous. However, the effect of the added acid on the COF properties (notably their surface area) has never been investigated. Building on an extensive literature overview, we systematically studied the effect of the pK_a of several added acids on COF performance characteristics and extended the investigation by including a series of (organo-)bases with varying pK_a. Interestingly, the highest BET surface areas, above 1400 m²/g, were found in the alkaline region of the pK_a window, with a maximum near pK_a ∼10.8 for triethylamine (TEA) and N,N-diisopropylethylamine (DIPEA). Considering the pK_a values related to the three phenolic hydroxyl groups of 2,4,6-triformylphloroglucinol, one of the COF building blocks, these organobases fully deprotonate two of these hydroxyl groups and partly deprotonate the third one, which optimizes the reaction rate of the β-ketoenamine bond formation, explaining the improved COF crystallinity and associated microporosity. The largely overlooked use of organobases in the synthesis of β-ketoenamine-linked COFs thus offers a promising approach to improve the COF performance.

A key challenge in capturing CO2 from postcombustion gases is humidity due to competitive adsorption between CO2 and H2O. Multivariate (MTV) metal-organic frameworks (MOFs) have been considered a promising option to address this problem, e.g., combining CO2-affinitive and hydrophobic groups. Here, we synthesized a series of amine and methyl cofunctionalized MTV MIL-53(Al)-xNH2(1 - x)CH3 and their parent materials. All the mixed linker MIL-53(Al)-xNH2(1 - x)CH3 showed amino linker enrichment compared to the synthesis ratio, yet the linkers were distributed relatively homogeneously from the bulk to the surface. Material hydrophobicity or hydrophilicity varied with methyl or amino group content, respectively. The single-component adsorption indicated that certain mixed linker MIL-53(Al)-xNH2(1 - x)CH3 might outcompete the parent materials. In CO2-H2O competitive adsorption, however, the hydrophobic parental MIL-53(Al)-CH3 outperformed the mixed linker MOFs. CO2 adsorption capacities of 5.4, 4.9, and 3.6 wt % were found for 0.3 bar of CO2 under 0, 5, and 10% RH, respectively. The results highlight that materials with enhanced hydrophobicity and tight-fitting pores can outperform groups with high CO2 affinity in the CO2 capture under humid conditions.

Metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) are highly versatile materials based on inorganic modes connected via organic linkers or purely via the connection of organic building blocks, respectively. This results in 3-D nanoporous frameworks, which, due to their combination of high porosity and variability of building blocks, can exhibit exceptional properties that make them attractive. Certain applications (e.g., in electronics and as membranes) require a thin film or even a patterned morphology on various substrates. Inkjet printing of MOFs has emerged as a simple and effective technique for the scalable production of a wide range of MOF (gradient) films and patterns on a wide range of substrates according to specific requirements. This review comprehensively reviews the achievements in inkjet printing of both MOFs and COFs. We discuss the different substrates, ink formulation, and hardware intertwined requirements needed to achieve high-resolution printing and obtain desired properties such as porosity, physical-mechanical characteristics, and uniform thickness. Crucial aspects related to ink formulation, such as colloidal stability and size control of MOFs and COFs, are discussed. Additionally, we highlight potential opportunities for furthering the development of inkjet printing of MOFs/COFs and critically assess the reporting of the printing procedures and characterization of the resultant materials. In this manner, this review aims to contribute to the advancements in understanding and optimization of inkjet printing of MOFs and COFs, as this technique holds great potential for diverse applications and functionalization of MOF/COF films and patterns.

By tuning the steric environment and free pore space in metal-organic frameworks, a large variety of rotor dynamics of the organic linkers can appear. Nitrofunctionalized MIL-53 is a terephthalate-linker-based MOF that shows coupled rotor dynamics between the neighboring linkers along the pore direction. Here, we use classical molecular dynamics up to 6 × 2 × 2 supercells to investigate the range of the correlated linker dynamics. Interestingly, we observe an PNPNPNPN... conformational arrangement (P = nearly planar and N = nonplanar) for the conformations of the linkers in a row along the pore direction in the MOF. We identified correlated linker dynamics emerging among the direct and next nearest neighboring linkers along the pore. Due to 180° rotational flips of the planar linkers along the pore, (1) a change in the width of librations in their direct neighbors (PN) is observed; (2) intriguingly, their next nearest planar neighbors (PP) rotate between 0° and ±180° to reattain aligned (0°, 0°) or (±180°, ±180°) conformations. The presence of correlated dynamics in such linkers over long-length scales occurring at nanoseconds time scales is desirable for applications like ferroelectric switching or diffusion control via geared linker rotation, and this work provides insight into the design for such applications.

Advancing the field of photocatalysis requires the elucidation of structural properties that underpin the photocatalytic properties of promising materials. The focus of the present study is layered, Bi-rich bismuth oxyhalides, which are widely studied for photocatalytic applications yet poorly structurally understood, due to high levels of disorder, nano-sized domains, and the large number of structurally similar compounds. By connecting insights from multiple scattering techniques, utilizing electron-, X-ray- and neutron probes, the crystal phase of the synthesized materials is allocated as layered Bi₂₄O₃₁X₁₀ (X = Cl, Br), albeit with significant deviation from the reported 3D crystalline model. The materials comprise anisotropic platelet-shaped crystalline domains, exhibiting significant in-plane ordering in two dimensions but disorder and an ultra-thin morphology in the layer stacking direction. Increased synthesis pH tailored larger, more ordered crystalline domains, leading to longer excited state lifetimes determined via femtosecond transient absorption spectroscopy (fs-TAS). Although this likely contributes to improved photocatalytic properties, assessed via the photooxidation of benzylamine, increasing the overall surface area facilitated the most significant improvement in photocatalytic performance. This study, therefore, enabled both phase allocation and a nuanced discussion of the structure-property relationship for complicated, ultra-thin photocatalysts.

Titanium-based metal-organic framework, NH2-MIL-125(Ti), has been widely investigated for photocatalytic applications but has low activity in the hydrogen evolution reaction (HER). In this work, we show a one-step low-cost postmodification of NH2-MIL-125(Ti) via impregnation of Co(NO3)2. The resulting Co@NH2-MIL-125(Ti) with embedded single-site CoII species, confirmed by XPS and XAS measurements, shows enhanced activity under visible light exposure. The increased H2 production is likely triggered by the presence of active CoI transient sites detected upon collection of pump-flow-probe XANES spectra. Furthermore, both photocatalysts demonstrated a drastic increase in HER performance after consecutive reuse while maintaining their structural integrity and consistent H2 production. Via thorough characterization, we revealed two mechanisms for the formation of highly active proton reduction sites: nondestructive linker elimination resulting in coordinatively unsaturated Ti sites and restructuring of single CoII sites. Overall, this straightforward manner of confinement of CoII cocatalysts within NH2-MIL-125(Ti) offers a highly stable visible-light-responsive photocatalyst.

Large amounts of carbon monoxide are produced by industrial processes such as biomass gasification and steel manufacturing. The CO present in vent streams is often burnt, this produces a large amount of CO₂, e.g., oxidation of CO from metallurgic flue gasses is solely responsible for 2.7% of manmade CO₂ emissions. The separation of N₂ from CO due to their very similar physical properties is very challenging, meaning that numerous energy-intensive steps are required for CO separation, making the CO separation from many process streams uneconomical in spite of CO being a valuable building block in the production of major chemicals through C1 chemistry and the production of linear hydrocarbons by the Fischer-Tropsch process. The development of suitable processes for the separation of carbon monoxide has both industrial and environmental significance. Especially since CO is a main product of electrocatalytic CO₂ reduction, an emerging sustainable technology to enable carbon neutrality. This technology also requires an energy-efficient separation process. Therefore, there is a great need to develop energy efficient CO separation processes adequate for these different process streams. As such the urgency of separating carbon monoxide is gaining greater recognition, with research in the field becoming more and more crucial. This review details the principles on which CO separation is based and provides an overview of currently commercialised CO separation processes and their limitations. Adsorption is identified as a technology with the potential for CO separation with high selectivity and energy efficiency. We review the research efforts, mainly seen in the last decades, in developing new materials for CO separation via ad/bsorption and membrane technology. We have geared our review to both traditional CO sources and emerging CO sources, including CO production from CO₂ conversion. To that end, a variety of emerging processes as potential CO₂-to-CO technologies are discussed and, specifically, the need for CO capture after electrochemical CO₂ reduction is highlighted, which is still underexposed in the available literature. Altogether, we aim to highlight the knowledge gaps that could guide future research to improve CO separation performance for industrial implementation.

Several metal–organic frameworks (MOFs) excel in harvesting water from the air or as heat pumps as they show a steep increase in water uptake at 10–30 % relative humidity (RH%). A precise understanding of which structural characteristics govern such behavior is lacking. Herein, CAU-10-H and CAU-10-CH₃ are studied with -H, -CH₃ corresponding to the functions grafted to the organic linker. CAU-10-H shows a steep water uptake ≈18 RH% of interest for water harvesting, yet the subtle replacement of -H by -CH₃ in the organic linker drastically changes the water adsorption behavior to less steep water uptake at much higher humidity values. The materials’ structural deformation and water ordering during adsorption with in situ sum-frequency generation, in situ X-ray diffraction, and molecular simulations are unraveled. In CAU-10-H, an energetically favorable water cluster is formed in the hydrophobic pore, tethered via H-bonds to the framework μ-OH groups, while for CAU-10-CH₃, such a favorable cluster cannot form. By relating the findings to the features of water adsorption isotherms of a series of MOFs, it is concluded that favorable water adsorption occurs when sites of intermediate hydrophilicity are present in a hydrophobic structure, and the formation of energetically favorable water clusters is possible.

In order to capture and separate CO2 from the air or flue gas streams through nanoporous adsorbents, the influence of the humidity in these streams has to be taken into account as it hampers the capture process in two main ways: (1) water preferentially binds to CO2 adsorption sites and lowers the overall capacity, and (2) water causes hydrolytic degradation and pore collapse of the porous framework. Here, we have used a water-stable polyimide covalent organic framework (COF) in N2/CO2/H2O breakthrough studies and assessed its performance under varying levels of relative humidity (RH). We discovered that at limited relative humidity, the competitive binding of H2O over CO2 is replaced by cooperative adsorption. For some conditions, the CO2 capacity was significantly higher under humid versus dry conditions (e.g., a 25% capacity increase at 343 K and 10% RH). These results in combination with FT-IR studies on equilibrated COFs at controlled RH values allowed us to assign the effect of cooperative adsorption to CO2 being adsorbed on single-site adsorbed water. Additionally, once water cluster formation sets in, loss of CO2 capacity is inevitable. Finally, the polyimide COF used in this research retained performance after a total exposure time of >75 h and temperatures up to 403 K. This research provides insight in how cooperative CO2-H2O can be achieved and as such provides directions for the development of CO2 physisorbents that can function in humid streams.