Graphene-oxide-supported covalent organic polymers based on zinc phthalocyanine for efficient optical limiting and hydrogen evolution

Octupolar Cyclotriphosphazene-Cored Self-Standing Covalent Organic Framework Membranes as Nonlinear Optical Materials: Impact of Linkage Types and Material Forms

Conjugated and processable self-standing vinylene-linked covalent organic framework membranes (COFMs) are highly demanding for photonics and optoelectronics. In this work, we have fabricated the first cyclotriphosphazene (CTP) cored vinylene-linked self-standing COFM (CTP-PDAN). For comparison purposes, we have successfully fabricated the imine-linked congener (CTP-PDA). Leveraging the inherent nonlinear optical (NLO) response of the CTP core, both membranes were directly mounted to evaluate NLO parameters using the open-aperture (OA) Z-scan technique. Direct measurement of NLO responses on membranes is advantageous and free from solvent and scattering effects, making it a more practical approach compared to the conventional dispersion mode. The OA Z-scan transmission yields a reverse saturable absorption signature exhibiting a higher NLO absorption coefficient (β) of 58.37 cm/GW for CTP-PDAN, compared to that of the imine-linked CTP-PDA COFM (β = 8.5 cm/GW). These results can be correlated to the efficient conjugation through the vinylene linkage in CTP-PDAN compared to the imine linked congener.

Covalent Organic Frameworks as Emerging Nonlinear Optical Materials

The vastness of organic synthetic strategies and knowledge of reticular chemistry have made covalent organic frameworks (COFs) one of the most chemically and structurally diverse class of materials with potential applications ranging from gas storage, molecular separation, and catalysis to energy storage and magnetism. Recently, this class of porous materials has garnered increasing interest as potential nonlinear optical (NLO) materials. Traditionally, inorganic crystals, small-molecule organic chromophores, and oligomers have been studied for their NLO response. Nevertheless, COFs offer significant advantages over existing NLO materials in terms of higher mechanical strength, thermochemical stability, and extended conjugation. Herein, we discuss crucial aspects, terminology, and measurement techniques related to NLO, followed by a critical analysis of the design principles for COFs with NLO response. Furthermore, we touch on selected potential applications of these NLO materials. Finally, future prospects and challenges of COFs as NLO materials are discussed.

Two-photon absorption in halide perovskites and their applications

Active research on halide perovskites has given us a deep understanding of this family of materials and their potential for applications in advanced optoelectronic devices. One of the prominent outcomes is the use of perovskite materials for nonlinear optical applications. Two-photon absorption in perovskites, in particular their nanostructures, has been extensively studied and shows huge promise for many applications. However, we are still far from a thorough understanding of two-photon absorption in halide perovskites from a micro to macro perspective. Here we summarize different techniques for studying the two-photon absorption in nonlinear optical materials. We discuss the in-depth photophysics in two-photon absorption in halide perovskites. A comprehensive summary about the factors which influence two-photon absorption provides the direction to improve the two-photon absorption properties of halide perovskites. A summary of the recent applications of two-photon absorption in halide perovskites provides inspirations for engineers to utilize halide perovskites in two-photon absorption device development. This review will help readers to have a comprehensive and in-depth understanding of the research field of two-photon absorption of halide perovskites from microscopic mechanisms to applications. The article can serve as a manual and give inspiration for future researchers.

Strongly Coupled Nitrogen-Doped Mo2C@CoNi Alloy Hybrid Architecture toward Efficient Hydrogen Evolution Reaction

Development of high-efficiency electrocatalysts for water splitting is a promising channel to produce clean hydrogen energy. Herein, we demonstrate that the combination of nitrogen-doped Mo₂C and CoNi alloy to form a hybrid architecture is an effective way to produce hydrogen from electrochemical water splitting. Benefiting from a combination of mechanisms, the optimized N-Mo₂C@CoNi-650 shows remarkable hydrogen evolution reaction (HER) activity with small overpotentials of 35, 123, and 220 mV to reach the current density of 10, 50, and 100 mA cm^-2 in alkaline media, respectively, outperforming most previously reported HER electrocatalysts. The efficient electrocatalytic performance is ascribed to the highly exposed active sites, fast reaction kinetics, and improved charge-transfer steaming from the synergistic effect between each component. This work presents a new insight into designing and preparing highly efficient electrocatalysts toward the HER.

Progress and Perspectives on Covalent-organic Frameworks (COFs) and Composites for Various Energy Applications

Covalent-organic frameworks (COFs), being a new member of the crystalline porous materials family, have emerged as important materials for energy storage/conversion/generation devices. They possess high surface areas, ordered micro/mesopores, designable structures and an ability to precisely control electro-active groups in their pores, which broaden their application window. Thanks to their low weight density, long range crystallinity, reticular nature and tunable synthesis approach towards two and three dimensional (2D and 3D) networks, they have been found suitable for a range of challenging electrochemical applications. Our review focuses on the progress made on the design, synthesis and structure of COFs and their composites for various energy applications, such as metal-ion batteries, supercapacitors, water-splitting and solar cells. Additionally, attempts have been made to correlate the structural and mechanistic characteristics of COFs with their applications.

Rational design of FeOx-MoP@MWCNT composite electrocatalysts toward efficient overall water splitting

Herein, a series of FeOx-MoP@MWCNT composite electrocatalysts was designed and prepared to investigate the influence of the content of FeOx on the water splitting performance. The optimized FeOx-MoP@MWCNTs-2 exhibits excellent hydrogen and oxygen evolution reaction activity while a cell voltage of 1.51 V with outstanding stability is attained, attributed to the synergistic effect of each component, as evidenced by the experimental and density functional theory results. The observed electrocatalytic activity outperforms current state-of-the-art non-precious metal electrocatalysts.

Enhanced optical limiting and hydrogen evolution of graphene oxide nanohybrids covalently functionalized by covalent organic polymer based on porphyrin

Herein, we report a novel graphene oxide (GO) nanohybrid covalently functionalized by covalent organic polymer (COP) based on porphyrin (GO-TPPCOP), as the optical limiter and hydrogen evolution reaction (HER) electrocatalyst. The GO-TPPCOP nanohybrid exhibits markedly enhanced optical limiting and HER activity over that of TPP, GO and TPPCOP alone. More importantly, the optical limiting property and HER activity of GO-TPPCOP nanohybrid are comparable to the state-of-the-art activity of materials from some previous reports. The possible mechanisms of optical limiting and HER are explored by various means, including UV-Vis absorption, fluorescence, photocurrent, electrochemical impedance spectra and Raman spectroscopic techniques. It is demonstrated that the synergistic effect and charge transfer between GO and TPPCOP are important factors in determining its optical limiting and HER performances. These results demonstrate a new strategy to design and develop functional nanohybrids for efficient optical limiting and HER activity by the covalent linkage of GO with COPs.

Similarities and differences between Mn(II) and Zn(II) coordination polymers supported by porphyrin-based ligands: synthesis, structures and nonlinear optical properties

Four coordination polymers (CPs) Mn-TMPP (1), Zn-TMPP (2), Mn-THPP (3), and Zn-THPP (4) have been synthesized and characterized (H2TMPP = meso-tetrakis (6-methylpyridin-3-yl) porphyrin; H2THPP = meso-tetrakis (6-(hydroxymethyl) pyridin-3-yl) porphyrin). The one-dimensional (1D) chain compound 1 is formed via a head-to-tail connection of the Mn-TMPP unit, wherein the central Mn2+ features a square pyramidal geometry coordinated by four N atoms from the porphyrin skeleton and one additional N atom from an adjacent Mn-TMPP unit. Compound 2 features an octahedral Zn2+ center associated with four N atoms from the porphyrin skeleton to define the equatorial plane and two additional N donors at the axial positions to give a two-dimensional (2D) CP. The 1D chain of 1 and the 2D layer of 2 possess distinctive molecular structures but nearly identical molecular arrangements in their unit cells viewed along all three crystallographic axes. By contrast, Mn- and Zn-based CPs 3 and 4 supported by the THPP ligand share both identical molecular connectivities and crystal packing. In 3/4, each Mn/Zn center is chelated by four N donors of the porphyrin interior to define the equatorial plane of an octahedron, whose axial sites are occupied by two alcoholic OH groups from a pair of trans-located pyridinemethanol moieties. The third-order nonlinear optical properties of 1-4 investigated using the Z-scan technique at 532 nm revealed reverse saturable absorption and self-focusing effects for all four CPs, with hyperpolarizability values (γ) in the range 1.42 × 10-28 esu to 7.64 × 10-28 esu. These high γ values are comparable to the best porphyrin-based molecular assemblies, demonstrating potential for these materials in optical limiting applications.

Electrochemical hydrogen and oxygen evolution reactions from a cobalt-porphyrin-based covalent organic polymer

Covalent organic polymers have attracted much attention due to their high specific surface area, superlative porosity, and diversity in electronic structure. Herein, a novel porous cobalt-porphyrin-based covalent organic polymer (CoCOP) is fabricated through the Schiff-base condensation reaction, which is used as a difunctional electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The CoCOP possesses a high surface area and strong synergistic effect between the cobalt-porphyrins and the CN groups, resulting in efficient HER and OER performances. The CoCOP required relatively low overpotentials for both HER (121 mV to reach 1.0 mA cm^-2 and 310 mV to reach 10 mA cm^-2) and OER (166 mV to reach 1.0 mA cm^-2 and 350 mV to reach 10 mA cm^-2) in alkaline media. This work may provide a new idea for the design of non-noble metal-based coordination polymers with excellent structure and high electrocatalytic performance.

Hydrogen Evolution Reaction-From Single Crystal to Single Atom Catalysts

Hydrogen evolution reaction (HER) is one of the most important reactions in electrochemistry. This is not only because it is the simplest way to produce high purity hydrogen and the fact that it is the side reaction in many other technologies. HER actually shaped current electrochemistry because it was in focus of active research for so many years (and it still is). The number of catalysts investigated for HER is immense, and it is not possible to overview them all. In fact, it seems that the complexity of the field overcomes the complexity of HER. The aim of this review is to point out some of the latest developments in HER catalysis, current directions and some of the missing links between a single crystal, nanosized supported catalysts and recently emerging, single-atom catalysts for HER.

Nonlinear optical performances of graphene oxide ternary nanohybrids functionalized by axially coordinated gallium porphyrins

A GaTPP-GO-GaTTP ternary nanohybrid possesses enhanced optical nonlinearities compared to other samples due to the efficient charge transfer effect.