For steady electroconversion to value-added chemical products with high efficiency, electrocatalyst reconstruction during electrochemical reactions is a critical issue in catalyst design strategies. Here, we report a reconstruction-immunized catalyst system in which Cu nanoparticles are protected by a quasi-graphitic C shell. This C shell epitaxially grew on Cu with quasi-graphitic bonding via a gas–solid reaction governed by the CO (g) - CO2 (g) - C (s) equilibrium. The quasi-graphitic C shell-coated Cu was stable during the CO2 reduction reaction and provided a platform for rational material design. C2+ product selectivity could be additionally improved by doping p-block elements. These elements modulated the electronic structure of the Cu surface and its binding properties, which can affect the intermediate binding and CO dimerization barrier. B-modified Cu attained a 68.1% Faradaic efficiency for C2H4 at −0.55 V (vs RHE) and a C2H4 cathodic power conversion efficiency of 44.0%. In the case of N-modified Cu, an improved C2+ selectivity of 82.3% at a partial current density of 329.2 mA/cm2 was acquired. Quasi-graphitic C shells, which enable surface stabilization and inner element doping, can realize stable CO2-to-C2H4 conversion over 180 h and allow practical application of electrocatalysts for renewable energy conversion.

Cu acetate/PAN nanofibers were transformed into porous C nanofibers with doped N and Cu particles, via O2 partial pressure-controlled calcination. N atoms next to Cu trigger the CO2RR by increasing the amount of CO* on the Cu, lowering the energy needed for CO dimerization.

The effect of local atomic arrangement of CuZn alloys was demonstrated on enhanced ethanol selectivity from CO2RR and supported by density functional theory (DFT) calculations.

Molybdenum disulfide (MoS2) has attracted much attention as a material to replace the noble-metal-based hydrogen evolution reaction catalyst. Polymorphism is an important factor in improving the catalytic performance of transition-metal dichalcogenides (TMDs) including MoS2. Several methods have been proposed to synthesize the 1T/1T′ phase with high catalytic efficiency, and a gas–solid reaction has recently been proposed as one of the alternative methods. However, the atomic-scale reaction mechanism between gas molecules and MoS2 has not been clarified. Here, we report a detailed atomic-scale mechanism of structural phase transition of MoS2 nanocrystals under reaction with CO gas molecules using density functional theory calculations. We confirm that the evaporation of S atoms at the edge is much faster than the evaporation at the basal plane of MoS2 nanocrystals. It is found that the S evaporation at the edge induces the structural change from 2H to 1T/1T′ in the basal plane of nanocrystals. The structural change is also attributed to the chain reaction due to the sequential migration of S atoms to the octahedral sites, which is energetically favorable. The present results provide a guideline for the gas–solid reaction-based phase control of TMDs including MoS2 to synthesize a high-performance catalyst.

Daily sleep monitoring is limited by the needs for specialized equipment and experts. This study combines a mask-shaped triboelectric nanogenerator (M-TENG) and machine learning for facile daily sleep monitoring without the specialized equipment or experts. The fabricated M-TENG demonstrates its excellent ability to detect respiration, even distinguishing oral and nasal breath. To increase the pressure sensitivity of the M-TENG, the reactive ion etching is conducted with different tilted angles. By investigating each surface morphology of the polytetrafluoroethylene films according to the reactive ion etching with different tilted angles, the tilted angle is optimized with the angle of 60° and the pressure sensitivity is increased by 5.8 times. The M-TENG can also detect changes in the angle of head and snoring. Various sleep stages can be classified by their distinctive electrical outputs, with the aid of a machine learning approach. As a result, a high averaged-classification accuracy of 87.17% is achieved for each sleep stage. Experimental results demonstrate that the proposed combination can be utilized to monitor the sleep stage in order to provide an aid for self-awareness of sleep disorders. Considering these results, the M-TENG and machine learning approach is expected to be utilized as a smart sleep monitoring system in near future.

Polymorph conversion of transition metal dichalcogenides (TMDs) offers intriguing material phenomena that can be applied for tuning the intrinsic properties of 2D materials. In general, group VIB TMDs can have thermodynamically stable 2H phases and metastable 1T/T′ phases. Herein, we report key principles to apply carbon monoxide (CO)-based gas–solid reactions for a universal polymorph conversion of group VIB TMDs without forming undesirable compounds. We found that the process conditions are strongly dependent on the reaction chemical potential of cations in the TMDs, which can be predicted by thermodynamic calculations, and that polymorphic conversion is triggered by S vacancy (VS) formation. Furthermore, we conducted DFT calculations for the reaction barriers of VS formation and S diffusion to reveal the polymorph conversion mechanism of WS2 and compared it with that of MoS2. We believe that phase engineering 2D materials via thermodynamically designed gas–solid reactions could be functionally used to achieve defect-related nanomaterials.

With recent rapid increases in Cu resistivity, RC delay has become an important issue again. Co, which has a low electron mean free path, is being studied as beyond Cu metal and is expected to minimize this increase in resistivity. However, extrinsic time-dependent dielectric breakdown has been reported for Co interconnects. Therefore, it is necessary to apply a diffusion barrier, such as the Ta/TaN system, to increase interconnect lifetimes. In addition, an ultrathin diffusion barrier should be formed to occupy as little area as possible. This study provides a thermodynamic design for a self-forming barrier that provides reliability with Co interconnects. Since Cr, Mn, Sn, and Zn dopants exhibited surface diffusion or interfacial stable phases, the model constituted an effective alloy design. In the Co-Cr alloy, Cr diffused into the dielectric interface and reacted with oxygen to provide a self-forming diffusion barrier comprising Cr2O3. In a breakdown voltage test, the Co-Cr alloy showed a breakdown voltage more than 200% higher than that of pure Co. The 1.2 nm ultrathin Cr2O3 self-forming barrier will replace the current bilayer barrier system and contribute greatly to lowering the RC delay. It will realize high-performance Co interconnects with robust reliability in the future.

Electrochemical reduction reaction of CO2 (CO2 RR) to valuable products offers potential for storing excessed renewable energy as well as chance to close the carbon loop, displacing the fossil fuel in chemical industries. Major efforts have been made to direct conversion of CO2 to ethylene which is widely used building blocks for chemical industries with large market size. Recently, tandem electrocatalysts composed of Cu and promoter metals which produces CO such as Au, Ag, and Zn have been investigated to enhance ethylene selectivity by increasing local concentration of CO*, overcoming limitation of pure Cu. However, these catalysts fabricated by sequential deposition of promoter metals on copper have limits in difficulty of controlling proximity between activated CO production sites and copper surfaces. Carbon is known for good supporter for the electrocatalyst due to the good electrical conductivity and electrochemical stability. In addition, Doped nitrogen in the pyridinic or pyrrolic configurations acts as the active site of CO2 reduction to CO. However, many researches have been reported carbon as supporter or catalyst producing CO gas, not as CO supplying co-catalyst with Cu. This may be caused by difficulty of composing nitrogen uniformly in carbon and in the proximity of Cu. Here, we have developed a new organic-metal hybrid tandem catalyst composed of Cu particles embedded in N-doped carbon nanofiber (Cu/N-CNF) and N atoms located on the periphery of Cu particles via “one-pot selective oxidation”. In order to fabricate Cu/N-CNF, electrospun copper acetate (CuAc: Cu(CO2CH3)2)/polyacrylonitrile (PAN: (C3H3N)n) nanofibers were calcinated at thermodynamically designed oxygen-controlled conditions based on Ellingham diagram. Cu-embedded undoped carbon nanofiber (Cu/CNF) catalysts were also synthesized by same process as Cu/N-CNF using polyvinyl alcohol (PVA : (C2H4O)n) instead of PAN. During calcination, temperature was maintained at 800 ºC for 5 h under 50 mTorr of pO2 at which the carbon is combusted and the copper is reduced, besides, the nitrogen hardly reacted with oxygen during calcination. It was confirmed that metallic Cu particles are successfully formed in all catalysts by SEM and TEM analysis. Also, Cu/N-CNFs contains pyridinic N predominantly than other N species by SEM and XPS analysis. The activity and faradaic efficiency (FE) of each product were examined by flow cell reactor with 5 M KOH electrolyte. Cu/N-CNFs show higher current densities compared to Cu/CNF catalyst at measured potential range (-0.2 ~ -0.8 V vs. RHE). Furthermore, Cu/N-CNF shows maximum C2H4 selectivity (62%) at much lower overpotential of -0.57 V vs. RHE compared to undoped Cu/CNF (-0.75 V vs. RHE). It can be inferred that increased CO production by doped N promotes CO* dimerization, whereas, without doped N, lower CO* generation limits the maximum amount of CO* dimerization. Furthermore, decrease of the CO dimerization energy barrier by CO production of doped N around Cu particles also will be discussed by DFT calculations.