Stable and Highly Efficient Photocatalysis with Lead‐Free Double‐Perovskite of Cs2AgBiBr6

Composition engineering of halide perovskite allows the tunability of the band gap over a wide range so that photons can be effectively harvested, an aspect that is of critical importance for increasing the efficiency of photocatalysis under sunlight. However, the poor stability and the low photocatalytic activity of halide perovskites prevent use of these defect‐tolerant materials in wide applications involving photocatalysis. Here, an alcohol‐based photocatalytic system for dye degradation demonstrated high stability through the use of double perovskite of Cs₂AgBiBr₆. The reaction rate on Cs₂AgBiBr₆ is comparable to that on CdS, a model inorganic semiconductor photocatalyst. The fact of fast reaction between free radicals and dye molecules indicates the unique catalytic properties of the Cs₂AgBiBr₆ surface. Deposition of metal clusters onto Cs₂AgBiBr₆ effectively enhances the photocatalytic activity. Although the stability (five consecutive photocatalytic cycles without obvious decrease of efficiency) requires further improvements, the results indicate the significant potential of Cs₂AgBiBr₆‐based photocatalysis.     

Water‐Catalyzed Oxidation of Few‐Layer Black Phosphorous in a Dark Environment

Black phosphorus (BP) shows great potential in electronic and optoelectronic devices owing to its semiconducting properties, such as thickness‐dependent direct bandgap and ambipolar transport characteristics. However, the poor stability of BP in air seriously limits its practical applications. To develop effective schemes to protect BP, it is crucial to reveal the degradation mechanism under various environments. To date, it is generally accepted that BP degrades in air via light‐induced oxidation. Herein, we report a new degradation channel via water‐catalyzed oxidation of BP in the dark. When oxygen co‐adsorbs with highly polarized water molecules on BP surface, the polarization effect of water can significantly lower the energy levels of oxygen (i.e. enhanced electron affinity), thereby facilitating the electron transfer from BP to oxygen to trigger the BP oxidation even in the dark environment. This new degradation mechanism lays important foundation for the development of proper protecting schemes in black phosphorus‐based devices.     

Tuning the Electrochemical Performance of Titanium Carbide MXene by Controllable In Situ Anodic Oxidation

MXenes are a class of two‐dimensional (2D) transition metal carbides, nitrides and carbonitrides that have shown promise for high‐rate pseudocapacitive energy storage. However, the effects that irreversible oxidation have on the surface chemistry and electrochemical properties of MXenes are still not understood. Here we report on a controlled anodic oxidation method which improves the rate performance of titanium carbide MXene (Ti₃C₂T_x, T_x refers to ‐F, =O, ‐Cl and ‐OH) electrodes in acidic electrolytes. The capacitance retention at 2000 mV s^?1 (with respect to the lowest scan rate of 5 mV s^?1) increases gradually from 38 % to 66 % by tuning the degree of anodic oxidation. At the same time, a loss in the redox behavior of Ti₃C₂T_x is evident at high anodic potentials after oxidation. Several analysis methods are employed to reveal changes in the structure and surface chemistry while simultaneously introducing defects, without compromising electrochemically active sites, are key factors for improving the rate performance of Ti₃C₂T_x. This study demonstrates improvement of the electrochemical performance of MXene electrodes by performing a controlled anodic oxidation.     

The Covalent Functionalization of Layered Black Phosphorus by Nucleophilic Reagents

Layered black phosphorus has been attracting great attention due to its interesting material properties which lead to a plethora of proposed applications. Several approaches are demonstrated here for covalent chemical modifications of layered black phosphorus in order to form P−C and P‐O‐C bonds. Nucleophilic reagents are highly effective for chemical modification of black phosphorus. Further derivatization approaches investigated were based on radical reactions. These reagents are not as effective as nucleophilic reagents for the surface covalent modification of black phosphorus. The influence of covalent modification on the electronic structure of black phosphorus was investigated using ab initio calculations. Covalent modification exerts a strong effect on the electronic structure including the change of band‐gap width and spin polarization.     

On‐Surface Synthesis of Ethynylene‐Bridged Anthracene Polymers

Engineering low‐band‐gap π‐conjugated polymers is a growing area in basic and applied research. The main synthetic challenge lies in the solubility of the starting materials, which precludes advancements in the field. Here, we report an on‐surface synthesis protocol to overcome such difficulties and produce poly(p‐anthracene ethynylene) molecular wires on Au(111). To this aim, a quinoid anthracene precursor with =CBr₂ moieties is deposited and annealed to 400 K, resulting in anthracene‐based polymers. High‐resolution nc‐AFM measurements confirm the nature of the ethynylene‐bridge bond between the anthracene moieties. Theoretical simulations illustrate the mechanism of the chemical reaction, highlighting three major steps: dehalogenation, diffusion of surface‐stabilized carbenes, and homocoupling, which enables the formation of an ethynylene bridge. Our results introduce a novel chemical protocol to design π‐conjugated polymers based on oligoacene precursors and pave new avenues for advancing the emerging field of on‐surface synthesis.     

Atom für Atom

Using the tip of a scanning tunnelling microscope a metal‐free phthalocyanine has been transformed into a silver‐phthalocyanine on a surface. The individual reactions have been performed atom by atom. Each product has been imaged with the scanning tunnelling microscope. Along with the transformation of the molecule, controlled tautomerization and H hopping have been induced by injection of electrons and demonstrate the precision a state‐of‐the‐art scanning tunnelling microscope has reached.