Versatile Applications of Metal Single-Atom @ 2D Material Nanoplatforms

Au-Decorated Ce-Ti Mixed Oxides for Efficient CO Preferential Photooxidation

We investigated the photocatalytic behavior of gold nanoparticles supported on CeO2-TiO2 nanostructured matrixes in the CO preferential oxidation in H2-rich stream (photo-CO-PROX), by modifying the electronic band structure of ceria through addition of titania and making it more suitable for interacting with free electrons excited in gold nanoparticles through surface plasmon resonance. CeO2 samples with different TiO2 concentrations (0-20 wt %) were prepared through a slow coprecipitation method in alkaline conditions. The synthetic route is surfactant-free and environmentally friendly. Au nanoparticles (<1.0 wt % loading) were deposited on the surface of the CeO2-TiO2 oxides by deposition-precipitation. A benchmarking sample was also considered, prepared by standard fast coprecipitation, to assess how a peculiar morphology can affect the photocatalytic behavior. The samples appeared organized in a hierarchical needle-like structure, with different morphologies depending on the Ti content and preparation method, with homogeneously distributed Au nanoparticles decorating the Ce-Ti mixed oxides. The morphology influences the preferential photooxidation of CO to CO2 in excess of H2 under simulated solar light irradiation at room temperature and atmospheric pressure. The Au/CeO2-TiO2 systems exhibit much higher activity compared to a benchmark sample with a non-organized structure. The most efficient sample exhibited CO conversions of 52.9 and 80.2%, and CO2 selectivities equal to 95.3 and 59.4%, in the dark and under simulated sunlight, respectively. A clear morphology-functionality correlation was found in our systematic analysis, with CO conversion maximized for a TiO2 content equal to 15 wt %. The outcomes of this study are significant advancements toward the development of an effective strategy for exploitation of hydrogen as a viable clean fuel in stationary, automotive, and portable power generators.

Emerging Multifunctional Single-Atom Catalysts/Nanozymes

Single-atom catalysts (SACs), in which the metal active sites are isolated on the support and stabilized by coordinated atoms such as oxygen, nitrogen, sulfur, etc., represent the maximum usage efficiency of the metal atoms. Benefiting from the recent progress in synthetic strategies, characterization methods, and computational models, many SACs that deliver an impressive catalytic performance for a variety of reactions have been developed. The catalytic selectivity and activity are critical issues that need to be optimized and augmented in the areas of nanotechnology and biomedicine. This review summarizes some recent experimental and theoretical progress aimed at clarifying the structure of SACs and how they influence the catalytic performance. The examples described here elaborate on the utility of SACs and highlight the strengths of these catalysts in the applications of biomedicine, environmental protection, and energy conversion. Finally, some current challenges and future perspectives for SACs are also discussed.

Recent advances of two-dimensional materials in smart drug delivery nano-systems

Smart drug delivery nano-systems show significant changes in their physical or chemical properties in response to slight change in environmental physical and/or chemical signals, and further releasing drugs adjusted to the progression of the disease at the right target and rate intelligently. Two-dimensional materials possess dramatic status extend all over various scientific and technological disciplines by reason of their exceptional unique properties in application of smart drug delivery nano-systems. In this review, we summarized current progress to highlight various kinds of two-dimensional materials drug carriers which are widely explored in smart drug delivery systems as well as classification of stimuli responsive two-dimensional materials and the advantages and disadvantages of their applications. Consequently, we anticipate that this review might inspire the development of new two-dimensional materials with smart drug delivery systems, and deepen researchers' understanding of smart nano-carries based on two-dimensional materials.

Solution-processed two-dimensional materials for ultrafast fiber lasers (invited)

Since graphene was first reported as a saturable absorber to achieve ultrafast pulses in fiber lasers, many other two-dimensional (2D) materials, such as topological insulators, transition metal dichalcogenides, black phosphorus, and MXenes, have been widely investigated in fiber lasers due to their broadband operation, ultrafast recovery time, and controllable modulation depth. Recently, solution-processing methods for the fabrication of 2D materials have attracted considerable interest due to their advantages of low cost, easy fabrication, and scalability. Here, we review the various solution-processed methods for the preparation of different 2D materials. Then, the applications and performance of solution-processing-based 2D materials in fiber lasers are discussed. Finally, a perspective of the solution-processed methods and 2D material-based saturable absorbers are presented.

Graphdiyne coordinated transition metals as single-atom catalysts for nitrogen fixation

The reduction of N₂ molecules to NH₃ is a very challenging task in chemistry. The electrocatalytic nitrogen reduction reaction (NRR) is a promising technology for NH₃ synthesis. By using first-principles calculation, a new class of single-atom catalysts (SACs), graphdiyne coordinated single transition metal atoms (TM@GDY, TM = Sc-Zn, Y-Cd, and La-Hg) were designed, and the NRR catalytic character of TM@GDY was systematically investigated. The results demonstrated that some TM@GDY (TM = Ti, V, Fe, Co, Zr, Rh, and Hf) monolayers exhibit better NRR activities than a Ru(0001) stepped surface. There is an obvious linear correlation between the limiting potential and the atomic N adsorption energy, which confirms that the N adsorption energy may be a descriptor for evaluation of the NRR catalytic performance. The V@GDY monolayer possesses the best NRR catalytic character with the lowest limiting potential of -0.67 V and the potential-limiting step (PLS) of *N₂→ *NNH for both alternating and distal mechanisms. Our results highlight a new family of efficient and stable TM@GDY catalysts and provide useful guidelines for SAC development and practical applications.

Palladium Nanoclusters Confined in MOF@COP as a Novel Nanoreactor for Catalytic Hydrogenation

Metal-nanocluster-doped porous materials are attracting considerable research attention due to their specific catalytic performance. In this study, core-shell metal-organic frameworks@covalent organic polymer (MOF@COP) nanocomposites were formed by the covalent linking of chemically stable COP on the surface of size-selective UiO-66-NH₂. Pd nanoclusters with an average diameter of ∼0.8 nm were successfully confined in UiO-66-NH₂@COP, and the obtained nanoreactor, referred to as UiO-66-NH₂@COP@Pd, exhibited abundant porosity, high stability, and large surface area. Notably, the UiO-66-NH₂@COP@Pd nanoreactor exhibited superior catalytic activity and stability for the catalytic reduction of 4-nitrophenol and hydrogenation of other nitroarenes, demonstrating the potential of Pd-cluster-doped MOF@COP hybrid materials as candidates for efficient catalytic hydrogenation. This study may provide new avenues for the construction of MOF@COP-hybrid-material-based heterogeneous catalysts for efficient catalytic applications.

Single Atom on the 2D Matrix: An Emerging Electrocatalyst for Energy Applications

The electrochemical energy conversions play an essential role in the production of sustainable and renewable energy. However, the performance is not up to the mark due to the absence of highly efficient and stable electrocatalysts. Recently, both 2D-matrix and single-atom catalysts (SACs) are two intense research topics in the field of electrocatalysis due to the high activity and stability and to maximize the utilization efficiency. Engineering the materials from 3D to 2D and modification from nanoparticles to single atoms have created a significant enhancement in the electrocatalytic activity. Hybridizing both the 2D matrix and SACs (2DM@SACs) creates a new electronic state in the materials, and that bequeaths with enhancing potentials toward the electrocatalytic activity. The strong covalent interaction between the 2D matrix and SACs tunes the intrinsic activity of the electrocatalysts. In this mini-review, we have discussed the different synthesis methods of 2DM@SACs with a focus on their electrochemical energy applications such as hydrogen evolution, oxygen evolution, oxygen reduction, and carbon dioxide reduction. This mini-review appraises the contribution to the rational proposal for the synthesis of perfect 2DM@SAC catalysts with their electrochemical properties toward energy conversion applications.

Metallic single-atoms confined in carbon nanomaterials for the electrocatalysis of oxygen reduction, oxygen evolution, and hydrogen evolution reactions

Recent advances of single-atom-based carbon nanomaterials for the ORR, OER, HER, and bifunctional electrocatalysis are covered in this review article.