Hydrogen sulfide (H2S) is a prototype molecular system and a sister molecule of water (H2O). The phase diagram of solid H2S at high pressures remains largely unexplored arising from the challenges in dealing with the pressure-induced weakening of S-H bond and larger atomic core difference between H and S. Metallization is yet achieved for H2O, but it was observed for H2S above 96 GPa. However, the metallic structure of H2S remains elusive, greatly impeding the understanding of its metallicity and the potential superconductivity. We have performed an extensive structural study on solid H2S at pressure ranges of 10-200 GPa through an unbiased structure prediction method based on particle swarm optimization algorithm. Besides the findings of candidate structures for nonmetallic phases IV and V, we are able to establish stable metallic structures violating an earlier proposal of elemental decomposition into sulfur and hydrogen [R. Rousseau, M. Boero, M. Bernasconi, M. Parrinello, and K. Terakura, Phys. Rev. Lett. 85, 1254 (2000)]. Our study unravels a superconductive potential of metallic H2S with an estimated maximal transition temperature of ∼80 K at 160 GPa, higher than those predicted for most archetypal hydrogen-containing compounds (e.g., SiH4, GeH4, etc.).

The search for high-temperature superconductors has been focused on compounds containing a large fraction of hydrogen, such as SiH4(H2)2, CaH6 and KH6. Through a systematic investigation of yttrium hydrides at different hydrogen contents using an structure prediction method based on the particle swarm optimization algorithm, we have predicted two new yttrium hydrides (YH4 andYH6), which are stable above 110 GPa. Three types of hydrogen species with increased H contents were found, monatomic H in YH3, monatomic H+molecular “H2” in YH4 and hexagonal “H6” unit in YH6. Interestingly, H atoms in YH6 form sodalite-like cage sublattice with centered Y atom. Electron-phonon calculations revealed the superconductive potential of YH4 and YH6 with estimated transition temperatures (Tc) of 84–95 K and 251–264 K at 120 GPa, respectively. These values are higher than the predicted maximal Tc of 40 K in YH3.

Hydrogen sulfides have recently received a great deal of interest due to the record high superconducting temperatures of up to 203 K observed on strong compression of dihydrogen sulfide (H2S). A joint theoretical and experimental study is presented in which decomposition products and structures of compressed H2S are characterized, and their superconducting properties are calculated. In addition to the experimentally known H2S and H3S phases, our first-principles structure searches have identified several energetically competitive stoichiometries that have not been reported previously; H2S3, H3S2, and H4S3. In particular, H4S3 is predicted to be thermodynamically stable within a large pressure range of 25-113 GPa. High-pressure room-temperature X-ray diffraction measurements confirm the presence of H3S and H4S3 through decomposition of H2S that emerge at 27 GPa and coexist with residual H2S, at least up to the highest pressure studied in our experiments of 140 GPa. Electron-phonon coupling calculations show that H4S3 has a small T c of below 2 K, and that H2S is mainly responsible for the observed superconductivity of samples prepared at low temperature (<100K).

Conventional cationic and anionic (catanionic) surfactant mixtures tend to form precipitates at the mixing molar ratio of the cationic and anionic surfactant of 1:1 because of the excess salt formed by their counterions. By using OH- and H+ as the counterions, however, excess salt can be eliminated, and salt-free catanionic systems can be obtained. Here, we report the detailed phase behavior and rheological properties of salt-free catanionic surfactant system of tetradecyltrimethylammonium hydroxide (TTAOH)/lauric acid (LA)/H2O. With the variation of mixing molar ratio of LA to TTAOH (rho=nLA/nTTAOH), the system exhibits much richer phase behavior induced by growth and transition of aggregates. Correspondingly, the rheological property of the system changes significantly. Take the series of samples with fixed total surfactant concentration (cT) to be 15 mg.mL(-1), the system only forms a low viscous L 1 phase with a Newton fluid character at the TTAOH-rich side. With increasing rho, first a shear-thickening L1 phase region is observed at 0.70or=1.05, and finally, at rho>or=1.13, the excess LA will separate from the bulk solution and form a white top layer. Investigations were also carried out by varying c T at fixed rho and by changing temperature, respectively. It was found micelle growth would be greatly suppressed at higher temperatures. However, the vesicle phases showed a considerable resistance against temperature rise.

Carbon materials have become a hot topic as potential substitutes for Pt/C catalysts for the oxygen reduction reaction (ORR). However, most of them exhibit their catalytic activities only in alkaline solutions, which severely limits their application in polyelectrolyte membrane fuel cells. To address this issue, here porous boron carbon nitride (BCN) nanosheets are fabricated by a facile and efficient polymer sol−gel method, which involves the annealing of polyvinylic akohol (PVA), boric acid, guanidine, and poly(ethylene oxide-co-propylene oxide) (P123) gel mixtures. The as-prepared porous BCN nanosheets possess a high surface area of 817 m 2 /g and display impressive ORR catalytic performance in both alkaline and acidic media, rivalling that of commercial Pt/C and other recently reported carbon materials. Importantly, the resulting metal-free catalysts exhibit much greater durability and higher methanol tolerance in both alkaline and acidic environments. This study provides a new insight into metal-free ORR catalysts that are practicable in industrial fuel cells.

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Vesicles from salt-free cationic and anionic (catanionic) surfactant aqueous solutions were prepared. The phase behavior and the phase transition in aqueous solutions of 100 mM tetradecyltrimethylammonium hydroxide (TTAOH) with fatty acids (decanoic acid (DA), lauric acid (LA), myristic acid (MA), and palmitic acid (PA)) were investigated. In this case, the solutions do not contain excess salts because the counterions H+ and OH- can form water. For the four systems of TTAOH/DA/H2O, TTAOH/LA/H2O, TTAOH/MA/H2O, and TTAOH/PA/H2O, one finds with increasing concentration of fatty acid a low viscous L1 phase, a viscous L1 phase, a L1/Lα phase in which the birefringent Lα phase is on the top of the viscous L1 phase, and finally a more or less transparent viscoelastic Lα phase with the typical feature of unilamellar and multilamellar vesicles. The microstructures and the rheological properties of the unilamellar and multilamellar vesicles were determined by using freeze-fracture transmission electron microscopy and rheological measurements. Both unilamellar and multilamellar vesicles coexist in the birefringent Lα phase. The unilamellar vesicles have diameters ranging from about 30 nm to more than 200 nm, and the multilamellar vesicles have diameters about 250 nm but are relatively rare. The complex viscosity (|η*|) with 100 mM TTAC n (n = 10, 12, 14, and 16) at a frequency (ν = 0.01 Hz) was found to be increasingly linear with the carbon number of the fatty acids.

Flexible optoelectronic devices attract considerable attention due to their prominent role in creating novel wearable apparatus for bionics, robotics, health care, and so forth. Although bulk single-crystalline perovskite-based materials are well-recognized for the high photoelectric conversion efficiency than the polycrystalline ones, their stiff and brittle nature unfortunately prohibits their application for flexible devices. Here, we introduce ultrathin single-crystalline perovskite film as the active layer and demonstrate a high-performance flexible photodetector with prevailing bending reliability. With a muchreduced thickness of 20 nm, the photodetector made of this ultrathin film can achieve a significantly increased responsivity as 5600A/W, 2 orders of magnitude higher than that of recently reported flexible perovskite photodetectors. The demonstrated 0.2 MHz 3 dB bandwidth further paves the way for high-speed photodetection. Notably, all its optoelectronic characteristics resume after being bent over thousands of times. These results manifest the great potential of single-crystalline perovskite ultrathin films for developing wearable and flexible optoelectronic devices.