Elucidating the Structural Evolution of a Highy Porous Responsive Metal-Organic Framework (DUT-49(M)) upon Guests Desorption by Time-Resolved In-Situ Powder X-Ray Diffraction

Variation in the metal centres of M-M paddle-wheel SBU results in the formation of isostructural DUT-49(M) frameworks. However, the porosity of the framework was found to be different for each of the structures. While a high and moderate porosity was obtained for DUT-49(Cu) and DUT-49(Ni), respectively, other members of the series [DUT-49(M); M= Mn, Fe, Co, Zn, Cd] show very low porosity and shapes of the adsorption isotherms which is not expected for op phases of these MOFs. Investigation on those MOFs revealed that those frameworks undergo structural collapse during the solvent removal at the activation step. Thus, herein, we aimed to study the detailed structural transformations that are possibly occurring during the removal of the subcritical fluid from the framework

Elucidating the Structural Evolution of a Highly Porous Responsive Metal–Organic Framework (DUT-49(M)) upon Guest Desorption by Time-Resolved in Situ Powder X-ray Diffraction

Removal of the guest molecules from the pores of metal–organic frameworks (MOFs) is one of the critical steps in particular for highly porous frameworks associated with high internal stress. In the case of isostructural mesoporous DUT-49(M) (M = Cu, Ni, Mn, Fe, Co, Zn, Cd) frameworks, only DUT-49(Cu) and DUT-49(Ni) could be successfully desolvated so far and only by using supercritical activation. To get a deeper insight into the processes occurring upon the desorption of the solvent from the pores of DUT-49(M), the desolvation was monitored in situ by synchrotron powder X-ray diffraction (PXRD). Analysis of the time-resolved PXRD data shows the full structural transformation pathway of the solid, which involves continuous and discontinuous phase transitions from the open pore (op) to the intermediate pore (ip) phase and from the ip to the contracted pore (cp) phase for DUT-49(Cu) and DUT-49(Ni). For DUT-49(Zn), the op to ip transition is directly followed by amorphization of the framework. All other frameworks show direct amorphization of the op phase

In\mathrm{In} situ\mathrm{situ} investigation of the formation mechanism of αα-Bi2_2Rh nanoparticles in polyol reductions

The synthesis of intermetallic phases formed from elements with very different melting points is often time and energy consuming, and in extreme cases the evaporation of a reactant may even prevent formation completely. An alternative, facile synthesis approach is the reduction of metal salts in the polyol process, which requires only moderate temperatures and short reaction times. In addition, the starting materials for this procedure are readily available and do not require any special treatment to remove or prevent passivation layers, for example. Although the formation of intermetallic particles via the polyol process is an established method, little attention has been paid to the mechanism behind it. However, it is precisely a deeper understanding of the underlying mechanisms that would enable better and more targeted synthesis planning and product design. Taking the well-known formation of Bi$_2$Rh particles from Bi(NO$_3$)$_3$ and various rhodium salts in ethylene glycol as an example, we studied the chemical process in detail. We investigated the effects of anion type and pH on the polyol reaction. The reaction was also probed by in situ X-ray diffraction using synchrotron radiation. Products, intermediates and solutions were characterized by X-ray and electron diffraction, electron microscopy and optical spectroscopy. In the first step, co-reduction of the metal cations leads to BiRh. Only with increasing reaction temperature, the remaining bismuth cations in the solution are reduced and incorporated into the BiRh particles, leading to a gradual transition from BiRh to $α$-Bi$_2$Rh

The importance of crystal size for breathing kinetics in MIL-53(Al)

Herein we analyze the switching kinetics of a breathing framework MIL-53(Al) with respect to different crystallite size regimes. Synchrotron time-resolved powder X-ray diffraction (PXRD) and adsorption rate analysis of n-butane physisorption at 298 K demonstrate the decisive role of crystal size affecting the time domain of breathing transitions in MIL-53(Al)

Epitaxy and Shape Heterogeneity of a Nanoparticle Ensemble during Redox Cycles

The role of metal–support epitaxy on shape and size heterogeneity of nanoparticles and their response to gas atmospheres is not very well explored. Here we show that an ensemble of Pd nanoparticles, grown on MgO(001) by deposition under ultrahigh vacuum, mostly consists of two distinctly epitaxially oriented particles, each having a different structural response to redox cycles. X-ray reciprocal space patterns were acquired in situ under oxidizing and reducing environments. Each type of nanoparticle has a truncated octahedral shape, whereby the majority grows with a cube-on-cube epitaxy on the substrate. Less frequently occurring and larger particles have their principal crystal axes rotated ±3.7° with respect to the substrate’s. Upon oxidation, the top (001) facets of both types of particles shrink. The relative change of the rotated particles’ top facets is much more pronounced. This finding indicates that a larger mass transfer is involved for the rotated particles and that a larger portion of high-index facets forms. On the main facets of the cube-on-cube particles, the oxidation process results in a considerable strain, as concluded from the evolution to largely asymmetric facet scattering signals. The shape and strain responses are reversible upon reduction, either by annealing to 973 K in vacuum or by reducing with hydrogen. The presented results are important for unraveling different elements of heterogeneity and their effect on the performance of real polycrystalline catalysts. It is shown that a correlation can exist between the particle-support epitaxy and redox-cycling-induced shape changes

A Scandium MOF with an Unprecedented Inorganic Building Unit, Delimiting the Micropore Windows

A new scandium metal–organic framework (Sc-MOF) with the composition of [Sc(OH)(OBA)], denoted as Sc-CAU-21, was prepared under solvothermal reaction conditions using 4,4′-oxidibenzoic acid (H$_2$OBA) as the ligand. Single-crystal structure determination revealed the presence of the new inorganic building unit (IBU) {Sc$_8$(μ-OH)$_8$(O$_2$C)$_{16}$}. It is composed of cis-connected ScO$_6$ polyhedra forming an eight-membered ring through bridging μ-OH groups. The connection of the IBUs leads to a 3D framework, containing 1D pores with a diameter between 4.2 and 5.6 Å. Pore access is limited by the size of the IBU, and in contrast to the isoreticular aluminum compound Al-CAU-21 [Al(OH)(OBA)], which is nonporous toward nitrogen at 77 K, Sc-CAU-21 exhibits a specific surface area of 610 m$^2$ g$^{–1}$. The title compound is thermally stable in air up to 350°C and can be employed as a host for photoluminescent ions. Sc-CAU-21 exhibits a ligand-based blue emission, and (co)substituting Sc$^{3+}$ ions with Ln$^{3+}$ ions (Eu$^{3+}$, Tb$^{3+}$, and Dy$^{3+}$) allows the tuning of the emitting color of the phosphor from red to green. Single-phase white-light emission with CIE color coordinates close to the ideal for white-light emission was also achieved. The luminescence property was utilized in combination with powder X-ray diffraction to study in situ the crystallization process of Sc-CAU-21:Tb and Sc-CAU-21:Eu. Both studies indicate a two-step crystallization process, with a crystalline intermediate, prior to the formation of Sc-CAU-21:Ln

Spatiotemporal Design of the Metal-Organic Framework DUT-8(M)

Switchable metal-organic frameworks change their structure in time and selectively open their pores adsorbing guest molecules, leading to highly selective separation, pressure amplification, sensing and actuation applications. The three-dimensional engineering of metal-organic frameworks has reached a high level of maturity, but spatiotemporal evolution opens a new perspective towards engineering materials in the 4th dimension (time) by t-axis design, in essence exploiting the deliberate tuning of activation barriers. This work demonstrates the first example in which an explicit temporal engineering of a switchable metal-organic framework (DUT-8, M1M2(ndc)2dabco, ndc = 2,6,-naphthalenedicarboxylate, dabco = 1,4 diazabicyclo[2.2.2]octane, M1 = Ni, M2 = Co) is presented. The temporal response is deliberately tuned by variation of cobalt content. We present a spectrum of advanced analytical methods for analyzing the switching kinetics stimulated by vapor adsorption using in situ time resolved techniques ranging from ensemble adsorption and advanced synchrotron X-ray diffraction experiments to individual crystal analysis. A novel analysis technique based on microscopic observation of individual crystals in a microfluidic channel reveals the lowest limit for adsorption switching reported so far. The time constants for the bulk ensembles range from 2 - 300 s. Differences in spatiotemporal response of crystal ensembles originate from a delay (induction) time that varies statistically and widens characteristically with increasing cobalt content reflecting increasing activation barriers

Nanotubes from the Misfit Layered Compound (SmS)1.19TaS2(SmS)_{1.19}TaS_2 : Atomic Structure, Charge Transfer, and Electrical Properties

Misfit layered compounds (MLCs) MX-TX$_2$, where M, T = metal atoms and X = S, Se, or Te, and their nanotubes are of significant interest due to their rich chemistry and unique quasi-1D structure. In particular, LnX-TX$_2$ (Ln = rare-earth atom) constitute a relatively large family of MLCs, from which nanotubes have been synthesized. The properties of MLCs can be tuned by the chemical and structural interplay between LnX and TX$_2$ sublayers and alloying of each of the Ln, T, and X elements. In order to engineer them to gain desirable performance, a detailed understanding of their complex structure is indispensable. MLC nanotubes are a relative newcomer and offer new opportunities. In particular, like WS$_2$ nanotubes before, the confinement of the free carriers in these quasi-1D nanostructures and their chiral nature offer intriguing physical behavior. High-resolution transmission electron microscopy in conjunction with a focused ion beam are engaged to study SmS-TaS$_2$ nanotubes and their cross-sections at the atomic scale. The atomic resolution images distinctly reveal that Ta is in trigonal prismatic coordination with S atoms in a hexagonal structure. Furthermore, the position of the sulfur atoms in both the SmS and the TaS$_2$ sublattices is revealed. X-ray photoelectron spectroscopy, electron energy loss spectroscopy, and X-ray absorption spectroscopy are carried out. These analyses conclude that charge transfer from the Sm to the Ta atoms leads to filling of the Ta 5d$_z$$^2$ level, which is confirmed by density functional theory (DFT) calculations. Transport measurements show that the nanotubes are semimetallic with resistivities in the range of 10$^{–4}$ Ω·cm at room temperature, and magnetic susceptibility measurements show a superconducting transition at 4 K