Pentagonal C_mX_nY_6–m–n (m = 2, 3; n = 1, 2; X, Y = B, N, Al, Si, P) Monolayers: Janus Ternaries Combine Omnidirectional Negative Poisson Ratios with Giant Piezoelectric Effects

Abstract

Two-dimensional (2D) materials composed of pentagon and Janus motifs usually exhibit unique mechanical and electronic properties. In this work, a class of ternary carbonbased 2D materials, C m X n Y 6−m−n (m = 2, 3; n = 1, 2; X, Y = B, N, Al, Si, P), are systematically studied by first-principles calculations. Six of 21 Janus penta-C m X n Y 6−m−n monolayers are dynamically and thermally stable. The Janus penta-C 2 B 2 Al 2 and Janus penta-Si 2 C 2 N 2 exhibit auxeticity. More strikingly, Janus penta-Si 2 C 2 N 2 exhibits an omnidirectional negative Poisson ratio (NPR) with values ranging from −0.13 to −0.15; in other words, it is auxetic under stretch in any direction. The calculations of piezoelectricity reveal that the out-of-plane piezoelectric strain coefficient (d 32 ) of Janus panta-C 2 B 2 Al 2 is up to 0.63 pm/V and increases to 1 pm/V after a strain engineering. These omnidirectional NPR, giant piezoelectric coefficients endow the Janus pentagonal ternary carbonbased monolayers as potential candidates in the future nanoelectronics, especially in the electromechanical devices.

Recent Progress on Durable Metal‐N‐C Catalysts for Proton Exchange Membrane Fuel Cells

“…In the context of MÀ NÀ C catalysts, it is widely accepted that the MÀ N x moiety is the origin of the ORR catalytic activity, as substantiated by specific experimental findings. [3,[32][33][34] In practice, the stability of the active MÀ N coordination is dubious, as metal ions can detach from their initial coordination, resulting in the inactivation of active sites. Therefore, it is essential to strengthen the resistance of these active sites to dementalization, thereby enhancing the overall stability of the catalyst.…”

“…However, the commercialization of PEMFCs on a significant scale is hampered by their performance, cost, and durability. [3][4][5][6][7][8][9] At the cathode of PEMFCs, the ORR is a sluggish process characterized by multiple electron and proton transfers linked to diverse intermediates (•H, •OH, and •OOH). Traditionally, platinum group metal (PGM) catalysts serve as cathode catalysts, mitigating the electrochemical activation overpotential associated with oxygen reduction reaction (ORR) and assuring stability in oxidative and acidic environments.…”

“…[6,[13][14][15] Although these catalysts exhibit superior performance under certain conditions, their durability in PEMFCs is far from satisfactory. [3,16,17] Commonly, chronoamper-ometry at a cell voltage between 0.4 and 0.7 V vs. RHE is used to determine the stability of FeÀ NÀ C. Initial fuel cell testing typically reveals a 40 % to 80 % performance decline within the first 100 hours. This quick falls far short of the DOE's durability requirements for light-duty vehicle applications (8,000 hours with 10 % performance degradation).…”

A multifunctional hydrogel dressing with high tensile and adhesive strength for infected skin wound healing in joint regions

“…4,5 As an emerging modern dressing, hydrogels have received extensive attention in wound healing and are expected to become an ideal wound dressing. [6][7][8] Hydrogels can effectively absorb tissue secretion at the wound site and maintain a moist environment surrounding the wound. [9][10][11] Moreover, by inserting functional groups or loading medicines, new functionalities can be obtained.…”

High out-of-plane negative Poisson's ratios and strong light harvesting in two-dimensional SiS2 and its derivatives

“…[14][15][16][17] Benefiting from the rapid advancement in the semiconductor industry, an increasing array of 2D materials with NPR have been proposed, including GeSe 2 , 18 tinselenidene, 19 Ag 2 S, 20 Pd-decorated borophene 21 and so on. [22][23][24][25][26][27][28][29][30][31][32][33][34][35] These reported 2D NPR materials can be broadly categorized into four distinct groups with inplane, 18,24,[26][27][28][29][30][31][33][34][35] out-of-plane, 19,25,32 both in-and out-ofplane, 20,23,36 and half auxetic properties. 21 Despite the tremendous potential of two-dimensional materials with NPR across diverse fields, the existing repertoire of materials displaying auxetic behavior remains relatively limited, and those exhibiting both in-and out-of-plane directional auxetic phenomena are even rarer.…”