Ultrathin Co（Ni）-doped MoS2 nanosheets as catalytic promoters enabling efficient solar hydrogen production

The design of efficient artificial photosynthetic systems that harvest solar energy to drive the hydrogen evolution reaction via water reduction is of great importance from both the theoretical and practical viewpoints. Integrating appropriate co-catalyst promoters with strong light absorbing materials represents an ideal strategy to enhance the conversion efficiency of solar energy in hydrogen production. Herein, we report, for the first time, the synthesis of a class of unique hybrid structures consisting of ultrathin Co（Ni）-doped MoS2 nanosheets （co-catalyst promoter） intimately grown on semiconductor CdS nanorods （light absorber）. The as-synthesized one-dimensional CdS@doped-MoS2 heterostructures exhibited very high photocatalytic activity （with a quantum yield of 17.3%） and stability towards H2 evolution from the photoreduction of water. Theoretical calculations revealed that Ni doping can increase the number of uncoordinated atoms at the edge sites of MoS2 nanosheets to promote electron transfer across the CdS/MoS2 interfaces as well as hydrogen reduction, leading to an efficient H2 evolution reaction.

High-yield synthesis and liquid-exfoliation of two- dimensional belt-like hafnium disulphide

Producing environmentally stable monolayers and few-layers of hafnium disulphide （HfS2） with a high yield to reveal its unlocked electronic and optoelectronic applications is still a challenge. HfS2 is a layered two-dimensional material of group-IV transition metal dichalcogenides. For the first time, we demonstrate a simple and cost-effective method to grow layered belt-like nanocrystals of HfS2 with a notably large interlayer spacing followed by their chemical exfoliation. Various microscopic and spectroscopic techniques confirm that these as-grown crystals exfoliate into single or multiple layers in a few minutes using solvent assisted ultrasonification method in N-cyclohexyl-2- pyrrolidone. The exfoliated nanosheets of HfS2 exhibit an indirect bandgap of 1.3 eV with high stability against surface degradation. Furthermore, we demonstrate that these nanosheets hold potential for electronic applications by fabricating a field-effect transistor based on few-layered HfS2, exhibiting a field-effect mobility of 0.95 cm2/（V.s） with a high on/off current modulation ratio of 10,000 in ambient conditions. The method is scalable and has a potential significance for both academic and industrial purposes.

Aqueous and mechanical exfoliation, unique properties, and theoretical understanding of MoO3 nanosheets made from free-standing a-MoO3 crystals： Raman mode softening and absorption edge blue shift

Crystalline α-MoO3 belts consisting of nanosheets stacked along their [010] axes were synthesized via thermal vapor transport of MoO3 powders at elevated temperatures. The MoO3 belts were millimeters in length along their [001] axes and tens to hundreds of micrometers in width along their [100] axes. Mechanical and aqueous exfoliations of the belts to form two-dimensional （2D） nanosheets were processed via the scotch-tape and bovine serum albumin （BSA） assisted methods, respectively. Upon scotch-tape exfoliation, the Raman features of MoO3 exhibited monotonic decreases in intensity as the thickness was gradually fell to approach that of a 2D nanosheet. Most Raman features eventually disappeared when a monolayer nanosheet was produced, except for the Mo-O-Mo stretching mode （Ag） at - 818 cm^-1, which was accompanied by mode-softening of up to 5 cm^-1 This mode softening, hitherto not reported for 2D α-MoO3 nanosheets, can be attributed to lattice relaxations that are validated here via theoretical density functional perturbation theory calculations. The BSA-assisted exfoliation products exhibited a blueshift in the α-MoO3 nanosheet absorption edge; they also revealed an absorption peak at 3.98 eV that can be attributed to their intrinsic exciton absorptions. These observations, together with the facile synthesis of high-purity α-MoO3 crystals, illuminate the possibility of further 2D α-MoO3 nanosheet production and lattice dynamic studies.