High-pressure is utilized to realize new 2D layered structures in PdSe2 with high-mobility transport anisotropy for photovoltaic applications.

Hydrogen evolution reaction (HER) is vital for sustainable energy production and plays a key role in achieving a hydrogen economy. Herein, density functional theory calculations are used to screen for suitable HER catalysts among 24 twodimensional double transition-metal (TM) carbide MXenes (chemical composition M 2 ′M″C 2 T x and M 2 ′M 2 ″C 3 T x ; M′ and M″ are two different metals, M′ = Cr, V, Ti, or Nb; M″ = Nb, Ta, Ti, or V; and T = O and/or OH) and determine their thermodynamic stability under HER-relevant conditions. The established surface Pourbaix diagrams, describing the chemistry on the basal planes of the MXenes, reveal the most stable terminations under the standard conditions (U = 0, pH = 0, p = 1 bar, and T = 298 K) for Mo 2 M x ″C y , Ti 2 M x ″C y , and Nb 2 Ta 2 C 3 to be O-termination, whereas Cr 2 M x ″C y and V 2 M 2 ″C 3 expose a mixed O-and OHtermination (M″ = Nb, Ta, Ti, or V; x = 1, y = 2, or x = 2, y = 3). Eighteen different carbides are predicted to be active HER electrocatalyst candidates, and Mo 2 NbC 2 O 2 showed the lowest overpotential. The Pourbaix diagrams and free energy diagrams reveal that the stability of the functional groups under HER-relevant conditions and the HER performance of the investigated double metal MXenes are closely related to its outermost TM. In other words, the outermost metal dominates the basal plane chemistry of the double TM carbide MXenes. Bader charge, density of states, and the crystal orbital Hamilton population analyses indicate that the hydrogen binding strength on different functionalized MXenes is related to the initial outer layer metal M′−O bond. A guiding observation is that the weaker the M′−O bond of the MXenes, the stronger the bonding between the terminated O* and the adsorbed H. Overall, this investigation demonstrates that the double TM carbides, as a subfamily of MXenes, provide a plethora of design opportunities, in particular as promising electrocatalysts for HER and other reactions.

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The quasi 2D layered-structure PdSe2 is predicted to be an intrinsic ferroelastic material with a stress-driven 90° lattice rotation, which is a promising material for perspective applications in microelectromechanical and nanoelectronic device.

Calcium carbonate is wildly used in cementitious composites at different scales and can affect the properties of cementitious composites through physical effects (such as the filler effect, dilution effect and nucleation effect) and chemical effects. The effects of macro (>1 mm)-, micro (1 μm–1 mm)- and nano (<1 μm)-sizes of calcium carbonate on the hydration process, workability, mechanical properties and durability are reviewed. Macro-calcium carbonate mainly acts as an inert filler and can be involved in building the skeletons of hardened cementitious composites to provide part of the strength. Micro-calcium carbonate not only fills the voids between cement grains, but also accelerates the hydration process and affects the workability, mechanical properties and durability through the dilution, nucleation and even chemical effects. Nano-calcium carbonate also has both physical and chemical effects on the properties of cementitious composites, and these effects behave even more effectively than those of micro-calcium carbonate. However, agglomeration of nano-calcium carbonate reduces its enhancement effects remarkably.