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Highly sensitive flexible piezocapacitive (PC) pressure sensor demonstrates wide applications in wearable electronics. In this paper, we first theoretically proposed an effective strategy to improve the sensitivity of the PC pressure sensor, by constructing a porous dielectric layer composted of inorganics with high dielectric constant (εH) and organics with low dielectric constant (εL). By using CaCu3Ti4O12 (CCTO) nanocrystals with giant ε as the dopant and polydimethylsiloxane (PDMS) with a low ε as the matrix, an ultrasoft CCTO-PDMS dielectric sponge was fabricated, via a simple porogen-assisted process. The CCTO-PDMS composited sponge exhibits an ultralow compression modulus of 6.3 kPa, a highly enhanced sensitivity with the gauge factor of up to 1.66 kPa–1 in a range of 0–640 Pa, and a response time of 33 ms, and the sensitivity outperforms that of pure PDMS and other PC sensors reported recently. This sensitivity enhancement is attributed to the hybridization of two phases of εH/εL in the composites, which provides an effective route to other novel flexible PC sensors. In practical applications, CCTO-PDMS-based PC sensor demonstrates potential applications, such as recording wrist pulse wave with fine accuracy and fidelity, bending and twisting detection, and Moss code simulating. The low-cost fabrication process in conjunction with its superior sensitivity, robustness of the functional versatility, and mechanical flexibility make the CCTO-PDMS-based pressure sensor widely promising for applications in wearable devices, flexible electronics, robotics, etc.

A Pd/Y heterobimetallic MOF (denoted as Pd/Y-MOF) catalyst is synthesized by coordination of Pd(II) and Y(III) with 2,2′-bipyridine-5,5′-dicarboxylate acid (bpydc) under microwave irradiation condition and is characterized by XRD Rietveld refinement, FTIR, Raman, TG-DTA, and XPS. It is shown that the 3D extended framework is constructed by linking Pd(bpydc)Cl 2 building blocks via Y(III) coordinating to carboxylic groups. Pd/Y-MOF exhibits higher catalytic activity than Pd(bpydc)Cl 2 in Suzuki-Miyaura coupling reaction and Sonogashira reaction owing to the highly dispersed Pd(II) sites in the layered structure of Pd/Y-MOF and the cooperative effect between Pd(II) and Y(III). The heterogeneity studies provide mechanistic evidence that the reaction proceeds on the surface of Pd(II) ions in the crystal framework. Thus, Pd/Y-MOF exhibits impressive size selectivity toward substrates. With the small-sized reactants, it displays comparable activities with Pd(OAc) 2 homogeneous catalyst. However, extremely poor activity in Suzuki-Miyaura coupling reaction with bulk substrates 1-iodonaphthalene and 4-(tert-butyl) iodobenzene is observed due to the inhibition of diffusion into the micropore channels. In addition, Pd/Y-MOF can be easily recycled and reused owing to the high stability of the framework formed by coordination of Y(III) with carboxylic group. The incorporation of Pd(II) into the crystal framework of Pd/Y-MOF prohibits the leaching of Pd(II) active species.

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The effect of alkali-metal ions on the local structure and luminescence properties for alkali-metal europium double tungstate compounds AEu(WO(4))(2) (A = Li, Na, K) has been investigated by a dual-space structural technique, atomic pair distribution function (PDF) analysis, and the Rietveld method of powder X-ray diffraction. The compounds AEu(WO(4))(2) (A = Li, Na) crystallize in the isostructure with the tetragonal space group I41/a (No. 88) and show the same luminescence properties in spite of the different doped alkali metals. However, KEu(WO(4))(2) crystallizes in monoclinic symmetry with the space group C2/c (No. 15). Compared with the two other counterparts, KEu(WO(4))(2) exhibits a more effective charge-transfer excitation, a larger Stokes shift, and a broader 612 nm emission band. This phenomenon is ascribed to the lower crystal symmetry in KEu(WO(4))(2), which influences bond distances and the coordination number of Eu(3+). Two complementary methods, the Rietveld method and PDF analysis, reveal that both LiEu(WO(4))(2) and NaEu(WO(4))(2) afford the same local surroundings of Eu(3+). The local structure determined by the Rietveld and PDF methods well account for the observed luminescent properties.

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