Asymmetric Low-Frequency Pulsed Strategy Enables Ultralong CO2 Reduction Stability and Controllable Product Selectivity.

Copper-based catalysts are widely explored in electrochemical CO2 reduction (CO2RR) because of their ability to convert CO2 into high-value-added multicarbon products. However, the poor stability and low selectivity limit the practical applications of these catalysts. Here, we proposed a simple and efficient asymmetric low-frequency pulsed strategy (ALPS) to significantly enhance the stability and the selectivity of the Cu-dimethylpyrazole complex Cu3(DMPz)3 catalyst in CO2RR. Under traditional potentiostatic conditions, Cu3(DMPz)3 exhibited poor CO2RR performance with the Faradaic efficiency (FE) of 34.5% for C2H4 and FE of 5.9% for CH4 as well as the low stability for less than 1 h. We optimized two distinguished ALPS methods toward CH4 and C2H4, correspondingly. The high selectivities of catalytic product CH4 (FECH4 = 80.3% and above 76.6% within 24 h) and C2H4 (FEC2H4 = 70.7% and above 66.8% within 24 h) can be obtained, respectively. The ultralong stability for 300 h (FECH4 > 60%) and 145 h (FEC2H4 > 50%) was also recorded with the ALPS method. Microscopy (HRTEM, SAED, and HAADF) measurements revealed that the ALPS method in situ generated and stabilized extremely dispersive and active Cu-based clusters (∼2.7 nm) from Cu3(DMPz)3. Meanwhile, ex situ spectroscopies (XPS, AES, and XANES) and in situ XANES indicated that this ALPS method modulated the Cu oxidation states, such as Cu(0 and I) with C2H4 selectivity and Cu(I and II) with CH4 selectivity. The mechanism under the ALPS methods was explored by in situ ATR-FTIR, in situ Raman, and DFT computation. The ALPS methods provide a new opportunity to boost the selectivity and stability of CO2RR.

Linker Scissoring Strategy Enables Precise Shaping of Metal-Organic Frameworks for Chromatographic Separation.

Precise shaping of metal-organic frameworks (MOFs) is significant in both fundamental coordination chemistry and practical applications, such as catalysis, separation, and biomedicine. Herein, we demonstrated a linker scissoring strategy for precisely shaping MOFs through surface conformational pairing. In this strategy, the bidentate linkers which were designed according to the original tetratopic ligands and the coordination environment of MOF surfaces, were utilized as the covering agents. The shape of these covering agents and the surface conformation of metals onto MOFs restricted them to coordinate on specific MOF facets thus precisely controlling the shape of the MOFs. Different shapes of PCN-608 from nanoplate (PCN-NP) to nanorod (PCN-NR) have been targeted by adding different bidentate linkers. The universality of this strategy was demonstrated by controlling the shapes of the NU-MOFs from nanoplate to nanorod. This strategy provides a new guiding principle to synthesize MOF nanocrystals with controlled shapes.

[Preparation and application of chromatographic stationary phase based on two-dimensional materials].

The stationary phase is the heart of chromatographic separation technology and a critical contributor to the overall separation performance of a chromatographic separation technique. However, traditional silicon-based materials designed for this purpose usually feature complex preparation processes, suboptimal permeability, pronounced mass-transfer resistance, and limited pH-range compatibility. These limitations have spurred ongoing research efforts aimed at developing new chromatographic stationary phases characterized by higher separation efficiency, adaptable selectivity, and a broader scope of applicability. In this context, the scientific community has made significant strides toward the development of new-generation materials suitable for use as chromatographic stationary phases. These materials include carbon-based nanomaterial arrays, carbon quantum dots, and two-dimensional (2D) materials. 2D-materials are characterized by nanometer-scale thicknesses, extensive specific surface areas, distinctive layered structures, and outstanding mechanical properties under standard conditions. Thus, these materials demonstrate excellent utility in various applications, such as electrical and thermal conductivity enhancements, gas storage and separation solutions, membrane separation technologies, and catalysis. Graphene, which is arguably the most popular 2D-material used for chromatographic separation, consists of a 2D-lattice of carbon atoms arranged in a single layer, with a large specific surface area and efficient adsorption properties. Its widespread adoption in research and various industries is a testament to its versatility and effectiveness. In addition to graphene, the scientific community has developed various 2D-materials that mirror the layered structures of graphene, such as boron nitride, transition-metal sulfides, and 2D porous organic frameworks, all of which offer unique advantages. 2D porous organic frameworks, in particular, have received attention because of their nanosheet morphology, one-dimensional pores, and special interlayer forces; thus, these frameworks are considered promising candidate chromatographic stationary phase materials. Such recognition is especially true for 2D-metal organic frameworks (MOFs) and 2D-covalent organic frameworks (COFs), which exhibit low densities, high porosities, and substantial specific surface areas. The modifiability of these materials, in terms of pore size, shape, functional groups, and layer-stacking arrangements allows for excellent separation selectivity, highlighting their promising potential in chromatographic separation. Compared with their three-dimensional counterparts, 2D-MOFs feature a simple pore structure that offers reduced mass-transfer resistance and enhanced column efficiency. These attributes highlight the advantages of 2D-MOF nanosheets as chromatographic stationary phases. Similarly, 2D-COFs, given their high specific surface area and porosity, not only exhibit great thermal stability and chemical tolerance but also support a wide selection of solvents and operational conditions. Therefore, their role in the preparation of chromatographic stationary phases is considered highly promising. This review discusses the latest research developments in 2D porous organic framework materials in the context of gas- and liquid-chromatographic stationary phases. It introduces the synthesis methods for these novel materials, elucidates their retention mechanisms, and describes the applications of other 2D-materials, such as graphene, its derivatives, graphitic carbon nitride, and boron nitride, in chromatography. This review aims to shed light on the promising development prospects and future directions of 2D-materials in the field of chromatographic separation, offering valuable insights into the rational design and application of new 2D-materials in chromatography.

Preparation and second-harmonic generation properties of a self-assembled multilayer film based on nanoporous isopolyoxomolybdate and bipolar hemicyanine.

An electrostatic self-assembled multilayer film has successfully been prepared by alternating adsorption of a wheel-shaped nanoporous isopolyoxomolybdate and a bipolar hemicyanine derivative, as monitored by absorption spectroscopy. The square roots of second-harmonic intensities of the film exhibit quadratic dependence on film thickness up to 10 layers. The average titled angle of the nonlinear optical chromophores to the substrate normal is derived to be 34 degrees by polarized second-harmonic generation measurements.

Water penetration/accommodation and phase behaviour of the neutral Langmuir monolayer at the air/water interface probed with sum frequency generation vibrational spectroscopy (SFG-VS).

A strong and broad hydrogen bonded O-H band around 3520 cm(-1) is observed in the insoluble monolayer of the neutral liquid crystal molecules of 4''-n-pentyl-4-p-cyanobiphenyl (5CB) and 4''-n-octyl-4-p-cyanobiphenyl (8CB) throughout the whole surface density range, but not in the 4-pentyl-4'-cyanoterphenyl (5CT) monolayer, at the air/water interface. This novel spectral feature suggests the existence of an oriented water cluster species which has penetrated or accommodated into the Langmuir monolayer of the 8CB and 5CB molecules. This finding provided a molecular level mechanism for the stark difference in the phase behaviour between the CB and CT insoluble Langmuir monolayers at the air/water interface. It also calls for attention to the details of the specific water-surface interaction in mediating the structure and the phase behaviour of the molecular assemblies at the heterogeneous aqueous interfaces.

Quantitative measurement and interpretation of optical second harmonic generation from molecular interfaces.

Second harmonic generation (SHG) has been proven a uniquely effective technique in the investigation of molecular structure and conformations, as well as dynamics of molecular interfaces. The ability to apply SHG to molecular interface studies depends on the ability to abstract quantitative information from the measurable quantities in the actual SHG experiments. In this review, we try to assess recent developments in the SHG experimental methodologies towards quantitative analysis of the nonlinear optical properties of the achiral molecular interfaces with rotational isotropy along the interface normal. These developments include the methodology for orientational analysis of the SHG experimental data, the experimental approaches for more accurate SHG measurements, and a novel treatment of the symmetry properties of the molecular polarizability tensors in association with the experimentally measurable quantities. In the end, the recent developments on the problem of surface versus bulk contribution in SHG surface studies is discussed. These developments can put SHG on a more solid foundation for molecular interface studies, and to pave the way for better understanding and application of SHG surface studies in general.

Reconsideration of second-harmonic generation from isotropic liquid interface: broken Kleinman symmetry of neat air/water interface from dipolar contribution.

It has been generally accepted that there are significant quadrupolar and bulk contributions to the second-harmonic generation (SHG) reflected from the neat air/water interface, as well as common liquid interfaces. Because there has been no general methodology to determine the quadrupolar and bulk contributions to the SHG signal from a liquid interface, this conclusion was reached based on the following two experimental phenomena: the breaking of the macroscopic Kleinman symmetry and the significant temperature dependence of the SHG signal from the neat air/water interface. However, because the sum frequency generation vibrational spectroscopy (SFG-VS) measurement of the neat air/water interface observed no apparent temperature dependence, the temperature dependence in the SHG measurement has been reexamined and proven to be an experimental artifact. Here we present a complete microscopic analysis of the susceptibility tensors of the air/water interface, and show that dipolar contribution alone can be used to address the issue of the breaking of the macroscopic Kleinman symmetry at the neat air/water interface. Using this analysis, the orientation of the water molecules at the interface can be obtained, and it is consistent with the measurement from SFG-VS. Therefore, the key rationales to conclude significantly quadrupolar and bulk contributions to the SHG signal of the neat air/water interface can no longer be considered as valid as before. This new understanding of the air/water interface can shed light on our understanding of the nonlinear optical responses from other molecular interfaces as well.