Single crystals of purely organic free-standing two-dimensional woven polymer networks.

The aesthetic and practicality of macroscopic fabrics continue to encourage chemists to weave molecules into interlaced patterns with the aim of providing emergent physical and chemical properties when compared with their starting materials. Weaving purely organic molecular threads into flawless two-dimensional patterns remains a formidable challenge, even though its feasibility has been proposed on several occasions. Herein we describe the synthesis of a flawless, purely organic, free-standing two-dimensional woven polymer network driven by dative B-N bonds. Single crystals of this woven polymer network were obtained and its well-defined woven topology was revealed by X-ray diffraction analysis. Free-standing two-dimensional monolayer nanosheets of the woven polymer network were exfoliated from the layered crystals using Scotch Magic Tape. The surface features of the nanosheets were investigated by integrated low-dose and cryogenic electron microscopy imaging techniques. These findings demonstrate the precise construction of purely organic woven polymer networks and highlight the unique opportunities for the application of woven topologies in two-dimensional organic materials.

Ion adsorption promotes Frank-van der Merwe growth of 2D transition metal tellurides.

Reliable synthesis methods for high-quality, large-sized, and uniform two-dimensional (2D) transition-metal dichalcogenides (TMDs) are crucial for their device applications. However, versatile approaches to growing high-quality, large-sized, and uniform 2D transition-metal tellurides are rare. Here, we demonstrate an ion adsorption strategy that facilitates the Frank-van der Merwe growth of 2D transition-metal tellurides. By employing this method, we grow MoTe2 and WTe2 with enhanced lateral size, reduced thickness, and improved uniformity. Comprehensive characterizations confirm the high quality of as-grown MoTe2. Moreover, various characterizations verify the adsorption of K+ and Cl- ions on the top surface of MoTe2. X-ray photoelectron spectroscopy (XPS) analysis reveals that the MoTe2 is stoichiometric without K+ and Cl- ions and exhibits no discernable oxidation after washing. This top surface control strategy provides a new controlling knob to optimize the growth of 2D transition-metal tellurides and holds the potential for generalized to other 2D materials.

Scalable Synthesis of High-Quality Ultrathin Ferroelectric Magnesium Molybdenum Oxide.

The development of ultrathin, stable ferroelectric materials is crucial for advancing high-density, low-power electronic devices. Nonetheless, ultrathin ferroelectric materials are rare due to the critical size effect. Here, a novel ferroelectric material, magnesium molybdenum oxide (Mg2Mo3O8) is presented. High-quality ultrathin Mg2Mo3O8 crystals are synthesized using chemical vapor deposition (CVD). Ultrathin Mg2Mo3O8 has a wide bandgap (≈4.4 eV) and nonlinear optical response. Mg2Mo3O8 crystals of varying thicknesses exhibit out-of-plane ferroelectric properties at room temperature, with ferroelectricity retained even at a 2 nm thickness. The Mg2Mo3O8 exhibits a relatively large remanent polarization ranging from 33 to 52 µC cm- 2, which is tunable by changing its thickness. Notably, Mg2Mo3O8 possesses a high Curie temperature (>980 °C) across various thicknesses. Moreover, the as-grown Mg2Mo3O8 crystals display remarkable stability under harsh environments. This work introduces nolanites-type crystal into ultrathin ferroelectrics. The scalable synthesis of stable ultrathin ferroelectric Mg2Mo3O8 expands the scope of ferroelectric materials and may prosper applications of ferroelectrics.

Controlled Synthesis of a Two-Dimensional Non-van der Waals Ferromagnet toward a Magnetic Moiré Superlattice.

Two-dimensional (2D) magnetic materials provide an ideal platform for spintronics, magnetoelectrics, and numerous intriguing physical phenomena in 2D limits. Moiré superlattices based on 2D magnets offer an avenue for controlling the spin degree of freedom and engineering magnetic properties. However, the synthesis of high-quality, large-grain, and stable 2D magnets, much less obtaining a magnetic moiré superlattice, is still challenging. We synthesize 2D ferromagnets (trigonal Cr5Te8) with controlled thickness and robust stability through chemical vapor deposition. Single-unit-cell-thick flakes with lateral sizes of tens of micrometers are obtained. We observe the layer-by-layer growth mode for the crystal formation in non-van der Waals Cr5Te8. The robust anomalous Hall signal confirms that Cr5Te8 of varying thickness have a long-range ferromagnetic order with an out-of-plane easy axis. There is no obvious change of the Curie temperature when the thickness of Cr5Te8 decreases from 52.1 to 7.2 nm. Here, we construct diverse 2D non-van der Waals/van der Waals vertical heterostructures (Cr5Te8/graphene, Cr5Te8/h-BN, Cr5Te8/MoS2). A uniform moiré superlattice is formed in the heterostructure through a lattice mismatch. The successful growth of 2D Cr5Te8 and a related moiré superlattice introduces 2D non-van der Waals ferromagnets into moiré superlattice research, thus highlighting prospects for property investigation of a non-van der Waals magnetic moiré superlattice and massive applications which require a scalable approach to magnetic moiré superlattices.

Scalable and Versatile Transfer of Sensitive Two-dimensional Materials.

Damage-free transfer of large-area two-dimensional (2D) materials is indispensable to unleash their full potentials in a wide range of electronic, photonic, and biochemical applications. However, the all-surface nature of 2D materials renders many of them vulnerable to surrounding environments, especially etchants and water involved during wet transfer process. Up to now, a scalable and damage-free transfer method for sensitive 2D materials is still lacking. Here, we report a general damage-free transfer method for sensitive 2D materials. The as-transferred 2D materials exhibit well-preserved structural integrity and unaltered physical properties. We further develop a facile TEM sample preparation technique that allows direct recycling of materials on TEM grids with high fidelity. This recycling technique provides an unprecedented opportunity to precisely relate structural characterization with physical/chemical/electrical probing for the same samples. This method can be readily generalized to diverse nanomaterials for large-area damage-free transfer and enables in-depth investigation of structure-property relationship.

Metal Selenides as Efficient Counter Electrodes for Dye-Sensitized Solar Cells.

Solar energy is the most abundant renewable energy available to the earth and can meet the energy needs of humankind, but efficient conversion of solar energy to electricity is an urgent issue of scientific research. As the third-generation photovoltaic technology, dye-sensitized solar cells (DSSCs) have gained great attention since the landmark efficiency of ∼7% reported by O'Regan and Grätzel. The most attractive features of DSSCs include low cost, simple manufacturing processes, medium-purity materials, and theoretically high power conversion efficiencies. As one of the key materials in DSSCs, the counter electrode (CE) plays a crucial role in completing the electric circuit by catalyzing the reduction of the oxidized state to the reduced state for a redox couple (e.g., I3-/I-) in the electrolyte at the CE-electrolyte interface. To lower the cost caused by the typically used Pt CE, which restricts the large-scale application because of its low reserves and high price, great effort has been made to develop new CE materials alternative to Pt. A lot of Pt-free electrocatalysts, such as carbon materials, inorganic compounds, conductive polymers, and their composites with good electrocatalytic activity, have been applied as CEs in DSSCs in the past years. Metal selenides have been widely used as electrocatalysts for the oxygen reduction reaction and light-harvesting materials for solar cells. Our group first expanded their applications to the DSSC field by using in situ-grown Co0.85Se nanosheet and Ni0.85Se nanoparticle films as CEs. This finding has inspired extensive studies on developing new metal selenides in order to seek more efficient CE materials for low-cost DSSCs, and a lot of meaningful results have been achieved in the past years. In this Account, we summarize recent advances in binary and mutinary metal selenides applied as CEs in DSSCs. The synthetic methods for metal selenides with various morphologies and stoichiometric ratios and deposition methods for CE films are described. We emphasize that the in situ growth method exhibits advantages over other methods for fabricating stable and efficient CEs. We focus on the effect of morphology on the electocatalytic and photovoltaic performance. Application of transparent metal selenide CEs in bifacial DSSCs and the superiority of in situ-grown metal selenide nanosheet fiber CEs used for fiber DSSCs are presented. In addition, we show that metal selenides with a hollow sphere structure can function not only as an efficient electrocatalyst but also as a light-scattering layer. Finally, we present our views on the current challenges and future development of metal selenide CE materials.

Hybridization of Cd0.2Zn0.8S with g-C3N4 nanosheets: a visible-light-driven photocatalyst for H2 evolution from water and degradation of organic pollutants.

Novel visible-light-driven Cd0.2Zn0.8S/g-C3N4 inorganic-organic composite photocatalysts were synthesized by a facile hydrothermal method. The prepared Cd0.2Zn0.8S/g-C3N4 composites were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), ultraviolet-visible diffuse reflection spectroscopy (DRS), photoluminescence (PL) spectroscopy and photoelectrochemical (PEC) experiments. Under visible-light irradiation, Cd0.2Zn0.8S/g-C3N4 photocatalysts displayed a higher photocatalytic activity than pure g-C3N4 and Cd0.2Zn0.8S for hydrogen evolution and degradation of pollutants, and the optimal g-C3N4 content was 20 wt%. The optimal composite showed a hydrogen evolution rate of 208.0 µmol h(-1). The enhancement of the photocatalytic activity should be attributed to the well-matched band structure and intimate contact interfaces between Cd0.2Zn0.8S and g-C3N4, which lead to the effective transfer and separation of the photogenerated charge carriers. Furthermore, the Cd0.2Zn0.8S/g-C3N4 photocatalysts showed excellent stability during photocatalytic hydrogen evolution and degradation of pollutants.

A reduced graphene oxide supported Cu(3)SnS(4) composite as an efficient visible-light photocatalyst.

In this study, a visible light responsive Cu3SnS4/reduced graphene oxide (RGO) photocatalyst has been synthesized by a facile one-step solvothermal method. The as-synthesized samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, N2 adsorption-desorption, UV-vis diffuse reflectance spectra (DRS), and photoluminescence (PL) emission spectroscopy. The photocatalytic activity of the Cu3SnS4/RGO composite under visible-light irradiation (λ > 420 nm) was evaluated by measuring the degradation of rhodamine B (RhB) and phenol. The results revealed that the Cu3SnS4 nanoplates dispersed uniformly on the RGO surface. The Cu3SnS4/RGO composite exhibited much higher photocatalytic activity than pure Cu3SnS4. The enhancement in photocatalytic activity is likely to be due to the synergistic effect of an improved adsorptivity of pollutants, an enhanced visible light absorption and an effective charge separation. In addition, the Cu3SnS4/RGO photocatalyst was stable during the reaction and could be used repeatedly.

Graphene sheets grafted three-dimensional BiOBr0.2I0.8 microspheres with excellent photocatalytic activity under visible light.

A series of graphene sheets grafted three-dimensional BiOBr0.2I0.8 microspheres with different graphene contents have been synthesized by a simple one-step solvothermal method. The BiOBr0.2I0.8 microspheres were composed of numerous nanoplates with a thickness of about 10nm and dispersed uniformly on the surface of graphene. The assembled BiOBr0.2I0.8/graphene composites exhibited excellent photocatalytic activity in the degradation of rhodamine B (RhB) and phenol under visible light irradiation (λ>420 nm). The optimal graphene content was found to be 10.0 wt.%, and the corresponding photocatalytic activity in degradation of RhB and phenol was 3.19 and 3.27 times that of pure BiOBr0.2I0.8, respectively. The enhanced photocatalytic activity could be attributed to more effective charge transportations and separations, larger specific surface areas and the increased light absorption. A possible photocatalytic mechanism of the BiOBr0.2I0.8/graphene composites was also proposed.