Two-dimensional (2D) metals have received considerable attention in recent years because of their fascinating physical and chemical properties, as well as potential applications in electrocatalysis, sensors, plasmonics, etc. However, the fabrication of 2D metals, especially atomically thin ones with large lateral size, remains a significant challenge because of the strong and isotropic metallic bonds. Here, we use Pt as a model system and demonstrate a general way to fabricate freestanding, high-quality 2D metals with giant aspect ratios (as large as ∼10 7 ) and controllable thickness down to 1 nm via a combination of room-temperature physical deposition and chemical etching. The cool deposition could suppress the Volmer−Webber growth mode, resulting in the formation of continuous ultrathin Pt with smooth surface and high conductivity. Moreover, the ultrathin 2D Pt exhibit outstanding hydrogen evolution reaction activity with a mass activity of 8.06 mA μg −1 at 0.06 V, ∼18 times higher than that of the commercial Pt/C catalyst. Additionally, the freestanding Pt-based strain sensor exhibits a high gauge factor of up to ∼4643, which is 3 orders of magnitude higher than that of conventional constantan wire-based strain gauges. Our studies pave the way for further developing wafer-scale 2D metal-based devices for various applications.

Ceramics from porous Si 3 N 4 and its derivatives SiAlON and Si 2 N 2 O were once considered the most promising high-temperature wave-transmitting materials. However, their large-scale application in the field of radomes is greatly restricted due to their poor oxidation resistance, high preparation costs, and expensive raw materials. Therefore, the development of low-cost porous oxide ceramics remains of significant interest to the field of high-temperature wave transmission. Surprisingly, mullite ceramics, which are representative of the Al 2 O 3 -SiO 2 -system of ceramics, are ultra-low-cost materials with the potential to replace ceramics from Si 3 N 4 and its derivatives. In this paper, integrated porous Al 2 O 3 -SiO 2 -system ceramics were successfully prepared for load-bearing/wavetransmitting applications, using inexpensive calcined kaolin and alumina powder as the main raw materials. Calcined kaolin can provide seeds for the growth and development of mullite crystals in the ceramic system. High-strength and high-porosity ceramics were obtained with the mullite morphology controlled through the molar ratio of Al 2 O 3 to SiO 2 and the resulting content of mullite seeds. With increasing of mullite seed content, the length and radial width of mullite whiskers with "interlocking structure" gradually change from rodshaped "long and thick" to needle-like "short and thin." The prepared porous Al 2 O 3 -SiO 2 ceramics have high flexural strength, fracture toughness, and good dielectric properties.