Exclusive Coordination between Melem and Silver(I) Ions: From Irregular Aggregates to Nanofibers to Crystal Cubes

There is growing focus on metal-free molecules and polymers owing to their potential applications in various energy and catalysis-related applications. Melem (2,5,8-triamino-s-heptazine, C6H6N10) has emerged as a metal-free material for solar-to-fuel conversion. However, its reactivity with metal ions or organic molecules has never been reported although it possesses multiple supramolecular interaction sites. In this work, we report on the synthesis of a novel metal–organic coordination framework (melem-Ag) by simply introducing Ag+ into the aqueous suspension of aggregated melem particles. Notably, as the reaction progresses, the melem disappears, and the morphology of the newly formed complex spontaneously evolves from nanofibers to single-crystalline blocks, which possess the same chemical structure, indicating that the morphology evolution is driven by Ostwald ripening. The structure of melem-Ag displays infinite nanocages of triangular pyramids consisting of melem molecules and Ag+, linked via Ag–N coordinate bonding and Ag–Ag argentophilic interactions. It is noteworthy that Ag+ is the only transition-metal cation that reacts with melem suspensions, even in the presence of other transition-metal cations (Co2+, Ni2+, Cu2+, and Zn2+). The coordination of Ag+ to melem results in metal-to-ligand charge transfer (MLCT), resulting in a quenched photoluminescence and enhanced light absorption. Exposing the melem-Ag crystals to UV light for varying time intervals results in the formation of colorful powders, which may be used for Ag-decorated photocatalysts

Competition of Hydrogen Bonds and Coordinate Bonds Induces a Reversible Crystal Transformation

Achieving reversible molecular crystal transformation between coordinate aggregates and hydrogen bonded assemblies has been a challenging task because coordinate bonds are generally much stronger than hydrogen bonds. Recently, we have reported the incorporation of silver ions into the cyanuric acid–melamine (CAM) network, resulting in the formation of a 1D coordination polymer (crystal 1) through forming the κ1N–Ag−κ2N coordination bonds. In this work, we find crystal 1 will undergo reversible transformation to hydrogen bonded coordinate units (crystal 2) through the breaking of coordinate chains and then the addition of CAM hydrogen bonding motifs into the framework. Crystal 2 presents a pseudohexagonal arrangement comprised of the κ1N–Ag−κ2N units connected by two sets of the triple hydrogen bonds, which extends two-dimensionally and stacks into a layer-structured crystal. Light was shed on the tautomerization of CA and M ligands associated with the crystal transformations using single crystal X-ray diffraction and infrared spectroscopy by analyzing the bond lengths and vibrations. We also highlight that photoluminescence can be a useful tool to probe the tautomer conversions of conjugated molecules. Furthermore, crystal 1 demonstrates high flexibility and can be bent over 180° and recover to its original shape after stress release. Crystal 2, on the contrary, is brittle and shows distinct mechanical anisotropy along different crystal orientations, as unveiled by nanoindentation measurements. The elastic modulus is well correlated with the chemical bonding strength along each orientation, and it is noteworthy that the contribution of the triple hydrogen bonds is comparable to that of the coordination bonds

Tuning the Morphology of g‑C₃N₄ for Improvement of Z‑Scheme Photocatalytic Water Oxidation

Solar-driven water oxidation is the key step for overall water splitting that efficiently harvests and converts solar energy into fuels; the development of a highly efficient photocatalyst that can mediate water oxidation has become an appealing challenge. Herein, we report a facile two-step process to decorate silver phosphate (Ag₃PO₄) particles on different types of graphitic carbon nitrides (g-C₃N₄) as composite photocatalysts for water oxidation. For all the Ag₃PO₄/g-C₃N₄ materials, an in situ Z-scheme is created by the generation of Ag nanoparticles which act as a cross-linking bridge between Ag₃PO₄ and g-C₃N₄ in the composite, resulting in better charge separation and higher catalytic performance. A detailed analysis emphasizes the importance of the g-C₃N₄ on the chemical, photophysical, and catalytic properties of the composite materials. Our results show that the alteration of the morphology dominates the performance of the composite materials

Step-by-Step Mechanism Insights into the TiO₂/Ce₂S₃ S‑Scheme Photocatalyst for Enhanced Aniline Production with Water as a Proton Source

Exploring heterostructured photocatalysts for the photocatalytic hydrogenation reaction with water as a proton source and investigating the corresponding intrinsic step-by-step mechanism are of great interest. Here, we develop an S-scheme heterojunction through theoretical design and carried out solvothermal growth of Ce2S3 nanoparticles (NPs) onto electrospun TiO2 nanofibers. The low-dimensional (0D/1D) heterostructure unveils enhanced photocatalytic activity for aniline production by nitrobenzene hydrogenation with water as a proton source. Density functional theory (DFT) calculations indicate the electrons transfer from Ce2S3 to TiO2 upon hybridization due to their Fermi level difference and creates an internal electric field at the interface, driving the separation of the photoexcited charge carriers, which is authenticated by in situ X-ray photoelectron spectroscopy along with femtosecond transient absorption spectroscopy. The step-by-step reaction mechanism of the photocatalytic nitrobenzene hydrogenation to yield aniline is revealed by in situ diffuse reflectance infrared Fourier transform spectroscopy, associated with DFT computational prediction

Upconversion-Agent Induced Improvement of g‑C₃N₄ Photocatalyst under Visible Light

Herein, we report the use of upconversion agents to modify graphite carbon nitride (g-C₃N₄) by direct thermal condensation of a mixture of ErCl₃·6H₂O and the supramolecular precursor cyanuric acid-melamine. We show the enhancement of g-C₃N₄ photoactivity after Er³⁺ doping by monitoring the photodegradation of Rhodamine B dye under visible light. The contribution of the upconversion agent is demonstrated by measurements using only a red laser. The Er³⁺ doping alters both the electronic and the chemical properties of g-C₃N₄. The Er³⁺ doping reduces emission intensity and lifetime, indicating the formation of new, nonradiative deactivation pathways, probably involving charge-transfer processes

Liquid-Based Growth of Polymeric Carbon Nitride Layers and Their Use in a Mesostructured Polymer Solar Cell with <i>V</i>_oc Exceeding 1 V

Herein we report a general liquid-mediated pathway for the growth of continuous polymeric carbon nitride (C₃N₄) thin films. The deposition method consists of the use of supramolecular complexes that transform to the liquid state before direct thermal condensation into C₃N₄ solid films. The resulting films exhibit continuous porous C₃N₄ networks on various substrates. Moreover, the optical absorption can be easily tuned to cover the solar spectrum by the insertion of an additional molecule into the starting complex. The strength of the deposition method is demonstrated by the use of the C₃N₄ layer as the electron acceptor in a polymer solar cell that exhibits a remarkable open-circuit voltage exceeding 1 V. The easy, safe, and direct synthesis of carbon nitride in a continuous layered architecture on different functional substrates opens new possibilities for the fabrication of many energy-related devices

Reversible Switching of the Amphiphilicity of Organic–Inorganic Hybrids by Adsorption–Desorption Manipulation

Surfactants are of great significance due to their wide use in fundamental research, industrial production, and daily lives. It remains a grand challenge to design and synthesize surfactants exhibiting reversible amphiphilicity switching. Here, we report on a “hybrid surfactant” prepared by combining an oil-soluble molecule, stearic acid, with water-dispersible Al2O3 nanofibers via chemisorption at the oil–water interface. The long carbon chain of stearic acid functions as the hydrophobic tail of the surfactant, while the inorganic nanofibers can act as the hydrophilic head. This hybrid surfactant exhibits reversible switching between hydrophilic and lipophilic states by manipulating the adsorption–desorption volume of stearic acid attached to the Al2O3 nanofibers. Therefore, the emulsions stabilized by this organic–inorganic hybrid can reversibly transform between oil-in-water and water-in-oil type. Unlike conventional approaches, no other external stimulus is needed to set the amphiphilic properties of the hybrid surfactant. As a bonus, organic–inorganic three-dimensional solid foams can be readily prepared based on the emulsion system, which demonstrates potential applications for remediation of oil spills in the environment

Crystal Transformation from the Incorporation of Coordinate Bonds into a Hydrogen-Bonded Network Yields Robust Free-Standing Supramolecular Membranes

In this work, we report on the synthesis of a free-standing, macroscopic robust supramolecular membrane by introducing silver–nitrogen coordinate bonding into preorganized, supramolecular hydrogen-bonded cyanuric acid-melamine (CAM) crystals. With the assistance of ammonia, silver ions competitively replace two of the three hydrogen atoms from cyanuric acid resulting in the transformation from short CAM nanorods to long CAM-Ag nanofibers (length over 1000 μm), accompanied by tautomerization of cyanuric acid. The single crystal structure of the CAM-Ag nanofibers is solved in the space group P1, with the asymmetric unit containing eight silver atoms, four melamine and four cyanuric acid molecules, which generate 1D coordination polymer chains consisting of alternating melamine and dianionic cyanurate ligands linked via silver–nitrogen bonds. The presence of interchain hydrogen bonds results in the expansion of the supramolecular network into undulating 2D sheets, which then stack into a 3D network via a series of intersheet hydrogen bonds and π–π interactions. Significantly, the CAM-Ag nanofibers spontaneously assemble into a free-standing membrane, with lateral size up to square centimeters and thickness of 30 μm. The membrane shows high flexibility and mechanical strength, owing to the improved flexibility of the CAM-Ag nanofibers with bonded chain structure, and can be reversibly and repeatedly bent over 90 degrees. Remarkably, the CAM-Ag membrane demonstrates distinct optical transmittance being shortwave IR transmissive but impenetrable to UV and visible light

Large-Scale Silver Sulfide Nanomesh Membranes with Ultrahigh Flexibility

The growth of flexible semiconductor thin films and membranes is highly desirable for the fabrication of next-generation wearable devices. In this work, we have developed a one-step, surface tension-driven method for facile and scalable growth of silver sulfide (Ag2S) membranes with a nanomesh structure. The nanomesh membrane can in principle reach infinite size but only limited by the reactor size, while the thickness is self-limited to ca. 50 nm. In particular, the membrane can be continuously regenerated at the water surface after being transferred for mechanical and electronic tests. The free-standing membrane demonstrates exceptional flexibility and strength, resulting from the nanomesh structure and the intrinsic plasticity of the Ag2S ligaments, as revealed by robust manipulation, nanoindentation tests and a pseudo-in situ tensile test under scanning electron microscope. Bendable electronic resistance-switching devices are fabricated based on the nanomesh membrane