A combined 3D-atomic/nanoscale comprehension and ab initio computation of iron carbide structures tailored in Q&P steels via Si alloying.

The essences of the quenching and partitioning (Q&P) process are to stabilize the finely divided retained austenite (RA) via carbon (C) partitioning from supersaturated martensite during partitioning. Competitive reactions, i.e., transition carbide precipitation, C segregation, and decomposition of austenite, might take place concurrently during partitioning. In order to maintain the high volume fraction of RA, it is crucial to suppress the carbide precipitation sufficiently. Since silicon (Si) in the cementite θ (Fe3C) is insoluble, alloying Si in adequate concentrations prolongs its precipitation during the partitioning step. Consequently, C partitioning facilitates the desired chemical stabilization of RA. To elucidate the mechanisms of formation of transition η (Fe2C) carbides as well as cementite, θ (Fe3C), besides the transformation of transition carbides to more stable θ during the quenching and partitioning (Q&P) process, samples of 0.4 wt% C steels tailored with different Si contents were extensively characterized for microstructural evolution at different partitioning temperatures (TP) using high resolution transmission electron microscopy (HR-TEM) and three-dimensional atom probe tomography (3D-APT). While 1.5 wt% Si in the steel allowed only the formation of η carbides even at a high TP of 300 °C, reduction in Si content to 0.75 wt% only partially stabilized η carbides, allowing limited η → θ transformation. With 0.25 wt% Si, only θ was present in the microstructure, suggesting a η → θ transition during the early partitioning stage, followed by coarsening due to enhanced growth kinetics at 300 °C. Although η carbides precipitated in martensite under paraequilibrium conditions at 200 °C, θ carbides precipitated under negligible partitioning local equilibrium conditions at 300 °C. Competition with the formation of orthorhombic η and θ precipitation further examined via ab initio (density functional theory, DFT) computation and a similar probability of formation/thermodynamic stability were obtained. With an increase in Si concentration, the cohesive energy decreased when Si atoms occupied C positions, indicating decreasing stability. Overall, the thermodynamic prediction was in accord with the HR-TEM and 3D-APT results.

Effective tailoring of the MoS2 layer number on the surface of CdS nanorods for boosting hydrogen production rate.

Hydrogen fuel plays a ubiquitous role in empowering the sustainable green energy economy. As an eco-friendly production method for hydrogen, photo-assisted water splitting is accepted to be the most reliable. However, the fabrication of stable and efficient photocatalysts is challenging. To overcome this difficulty, here we present a novel and inexpensive oxidant-promoted ultrasonic-assisted liquid phase layer exfoliation technique to fabricate a CdS/H-MoS2 nano hybrid. The newly fabricated CdS/H-MoS2 shows a hydrogen evolution rate of 162.4 mmol g-1h-1, which is 16 times higher compared to that of CdS/Pt and 67 times higher compared to that of bare CdS. Theoretical results clearly demonstrate a built-in electrostatic potential in the heterostructure junction, and that a shift in water reduction potential plays a key role in the enhancement of hydrogen production rate. We believe that the proposed experimental strategies and theoretical studies will open up a new avenue to develop new photocatalysts with high hydrogen evolution efficiency.

Regulating the charge carrier transport rate via bridging ternary heterojunctions to enable CdS nanorods' solar-driven hydrogen evolution.

Solar-driven hydrogen generation using single-semiconductor photocatalysts for hydrogen evolution seems to be challenging due to their poor solar to fuel conversion efficiency because of their fast charge carrier recombination. The ternary heterostructure was prepared by an advanced approach to suppress the recombination of photogenerated charge carriers and has contributed a new platform for designing highly efficient photocatalytic systems. Herein, we fabricated a ternary heterojunction with ultrathin WS2-SnS2 nanosheets and CdS nanorods, and the photocatalytic activity was studied. The optimized CdS/SnS2-WS2 (6 wt%) nanostructures were found to be highly stable and exhibited the highest hydrogen evolution rate of 232.45 mmol g-1 h-1, which was almost 93-fold higher than that of the pristine CdS nanorods. Also, Density Functional Theory (DFT) calculations confirmed that the favorable band alignment for charge transport and superior catalytic activity of the newly fabricated ternary nanostructures make them a potential candidate for solar-driven hydrogen production.