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Nanoscale Kirkendall effect has been widely used for rationally fabricating high-quality hollow nanocrystals, but often requires the intrinsic diffusion coefficient of out-diffusion materials higher than that of in-diffusion components. Here we demonstrate an unexpected Kirkendall effect that occurs in diffusing intrinsically faster Cu atoms into Pd icosahedra, leading to the formation of PdCu alloyed hollow nanocrystals. The control experiment with Pd octahedra replacing icosahedra indicates the critical role of twin boundaries in facilitating such unexpected Kirkendall effect. In addition, geometric phase analysis and density functional theory calculation show that out-diffusion of Pd atoms in the icosahedra is faster than in-diffusion of Cu atoms, particularly through the twin boundaries, upon the strain gradient with an inward distribution from tensile to compressive strains. The unexpected Kirkendall effect is also found in the interdiffusion of Ag and Pd atoms in Pd icosahedra. Our finds break the limitation of the intrinsic diffusion coefficient for the synthesis of hollow nanocrystals through Kirkendall effect and are expected to enormously enrich the family of hollow nanocrystals which have shown great potential in broad areas, such as fine chemical production, energy storage and conversion, and environmental protection. This work also provides a deep understanding in the diffusion behavior of atoms upon the strain gradient.

Native defects can degrade the performance and stability of an optoelectric device. Herein, a zeolitic imidazolate framework-8 (ZIF-8)@H:ZnO core–shell nanorod arrays/Si heterojunction self-powered photodetector is demonstrated. Via the combined process of hydrogenation and ZIF-8 passivation, the photoresponsive characteristics of this photodetector could be significantly enhanced; the photodetector exhibited the superior detectivity of ∼2.14 × 1016 Jones (1 Jones = 1 cm Hz1/2 W−1), the high responsivity of ∼7.07 × 104 mA W−1, the prominent sensitivity of ∼2.08 × 1012 cm2 W−1 and broadband photodetection in the region ranging from ultraviolet to near infrared. As a critical figure of merit, its responsivity increased by nearly 5 orders of magnitude than that of the pristine ZnO nanorod arrays/Si heterojunction photodetector. More importantly, the comprehensive performance of the ZIF-8@H:ZnO core–shell nanorod arrays/Si heterojunction photodetector was significantly higher than that of ZnO-based photodetectors reported to date, and it was comparable to those of two-dimensional (2D) materials, zero-dimensional (0D) materials, topological insulators, perovskites and other oxide-based self-powered photodetectors. This novel post-treatment strategy has significant potential for application in the development of high-performance self-powered photodetectors. Moreover, this study may be extended to other oxide-based optoelectronic devices.

Two-dimensional (2D) ultrathin and mesoporous Co3O4 nanomeshes modified by ZnO nanocrystals (ZnO/Co3O4) was achieved via in-situ growth of ZIF-8 polyhedral particles on ultrathin ZIF-67 nanosheets, which was synthesized by a salt-template strategy followed by calcination. The ZnO/Co3O4-2 (the molar ratio of Zn/Co is 2) ultrathin nanomeshes heterostructure based sensor exhibits a much higher response (Ra/Rg) of 3.2–5 ppm triethylamine with a shorter response/recovery time of 30/55 s at a low operating temperature of 100 °C compared with pristine Co3O4 and ZnO. The enhanced gas sensing performances of ZnO/Co3O4-2 are attributed to its ultrathin mesoporous nanostructure, the presence of rich oxygen vacancy defects and unique heterojunctions at the ZnO/Co3O4 interface. This research not only provides a solid and universal design strategy for the synthesis of 2D ultrathin nanomeshes but also offers valuable guidance to optimize their gas sensing properties by heterostructure design.

Metal-organic frameworks (MOFs) derived metal oxides, as a promising gas-sensing material, have received extensive attention because of its unique advantages of abundant pore structures, high surface areas and diverse compositions. In this work, hierarchical flower-like Zn-doped Co3O4 derived from Zn/Co-MOF is prepared through a process of solvothermal synthesis and annealing. 3 mol% Zn-doped Co3O4 exhibits the highest response of 52.3 toward 100 ppm acetone at 200 °C, which is nearly 20 times higher than that of pure Co3O4. Additionally, 3 mol% Zn-doped Co3O4 based sensor displays a ppb-level detection limit for acetone, outstanding selectivity, excellent long-term stability and good independence to humidity. The probable reasons for the improved sensing properties are attributed to the changes of carrier concentration, specific surface area and chemisorbed oxygen amount due to Zn doping.

The membrane fouling is the obstruction that limits the efficiency and durability of the nanofibrous membrane in oil-water emulsion separation. Herein, a polyvinyl alcohol (PVA) hydrogel nanofibrous membrane could continuously separate emulsion by using squeezing coalescence demulsification (SCD). During this SCD process, the small oil droplets can coalesce into bigger ones under the mandatory compression of the pore wall. And then the coalesced oil droplets will pass through the nanofibrous membrane and form an oil layer, and the oil layer can be easily removed. Furthermore, a two-phase numerical model is proposed to demonstrate this coalescence process of oil droplets in the pore. The numerical results also indicate that the pore size, wettability of membrane, and the interface tension coefficient between oil and water can control the efficiency of SCD. This SCD method endows PVA hydrogel nanofibrous membrane with a sustained separation capability, which implys that this method has great potential in practical application.

Biomass derived nitrogen-doped porous carbon (NPC) has received considerable interest in various fields, resulting from that biomass is abundant, renewable, fast generation and low cost. However, few studies were focused on the gas sensing properties of NPC. In this work, by simple calcination and hydrothermal method, chrysanthemum tea derived NPC is originally introduced on ZIF-8 derived ZnO polyhedrons to form ZnO/NPC heterostructure as a novel acetone sensor. Remarkably, the response of ZnO/NPC towards 100 ppm acetone (25.47) is 8 times more than that of ZnO (3.33) and 14 times higher than that of NPC (1.85). DFT calculations results demonstrate that the adsorption energy between acetone molecules and ZnO/NPC heterostructure is higher than the adsorption energy between acetone molecules and pure NPC (or pure ZnO). These experimental and theoretical results can be adopted for the fundamental investigation of other gas sensing materials based on biomass derived NPC.

Efficiently purifying metal ions contained emulsified oily wastewater has become a great challenge. Herein, a novel charged hydrogel nanofibrous membrane (CHNM) with negative surface charges, favorable mechanical ability and anti-fouling capacity was prepared via electrospinning and twice crosslinking process. The obtained CHNM with a surface potential of -32.9 mV at pH = 7.2 possesses outstanding oil-in-water emulsion separation performance with high permeation flux of around 10800 L h-1 m-2 bar-1 for surfactant free emulsion and high permeation flux of around 7100 L h-1 m-2 bar-1 for surfactant stabilized emulsion. The excellent oil/water emulsion separation performance can be attributed to the surface charge demulsification (SCD), which is theoretically demonstrated by a finite element analysis simulation. In addition, CHNM can absorb many kinds of metal ions (Fe3+, Ni2+ and Al3+) due to the chelation effect between high-density phosphate groups and metal ions. With outstanding separation performance for oil/water emulsion and good adsorption capacity for metal ions, the multifunctional CHNM may offer a promising approach to develop a new-generation membrane for treating metal ions contained oily wastewater.

One-dimensional cobalt oxide (Co3O4) exhibits great potential in gas sensing, but the application is severely hindered because of the agglomeration or close packing during the normal synthesis process. Herein, 3D radial Co3O4 assembled with nanorods is fabricated by employing cobalt-based layered hydroxide metal salt (Co-LHMS) as a self-template. Meanwhile, Co3O4 with flake floral and dumbbell radial structure obtained by tuning the hydrothermal reaction time of precursors is used as a control group for gas sensing tests. The sensitivity of 3D radial Co3O4 to sub-ppm acetone is 4 times higher than other structures and the detection limit is 100 ppb at a relatively low working temperature of 200 ℃, which is excellent among intrinsic acetone sensors based on pure Co3O4. The satisfactory gas sensing performance can be ascribed to the large specific surface, numerous transmission pathways, and low grain boundary resistance of the material. Furthermore, this work provides an idea for the further improvement of one-dimensional gas sensors.

Design and preparation of materials with superhydrophilicity/underwater superoleophobicity are of great significance for high-efficiency oil/water separation. In this work, we prepared Ni-doped brochantite@copper hydroxide hierarchical structures on copper mesh using chemical oxidation and hydrothermal synthesis, in which the interleaved embedded hydrophilic Ni-doped brochantite nanosheets distributed around copper hydroxide nanoarrays on copper mesh. The Ni-doped brochantite@copper hydroxide hierarchical structures endowed the mesh with pore size of 10 μm and excellent superhydrophilicity/underwater superoleophobicity and ultrahigh oil resistance with great oil rejection (>99.99%) and extremely high flux of 28,543 L m−2 h−1 for various oil-water mixtures. Importantly, the mesh showed excellent cyclic performance for 20 times separation of pump oil/water mixture with stable high flux and oil rejection and survived conspicuous selective affinities to oil and water from harsh conditions. It is indicated that the hierarchical Ni-doped brochantite@copper hydroxide coated copper mesh (NBCHCM) is a potential membrane material for high-efficient and sustainable oil/water separation.

The demand of acetone gas sensors with great response value and fast response/recovery time to low concentration acetone is increasing in practical diagnosis for diabetes. In this work, metal-organic frameworks (MOFs) derived flower-like ZnO/Co3O4 heterojunctions based sensor is prepared by a simple solution mixing method at room temperature. The sensor shows high acetone response value, short response/recovery time, excellent selectivity and good stability. For example, the response value of ZnO/Co3O4 heterojunctions based sensor is up to 120 % towards 500 ppb acetone at 300 ℃, which is about 30-fold enhancement than these of intrinsic ZnO and Co3O4. Moreover, its minimal detection limitation is 100 ppb acetone and it shows ultra-fast response/recovery time for ppb-level acetone (22 s/27 s@500 ppb). The excellent acetone sensing properties of flower-like ZnO/Co3O4 heterojunctions based sensor is attributed to the sharp resistance variation of heterojunction interface between ZnO and Co3O4 and the ultra-fast gas transport channel provided by ZnO nanosheets. This finding provides important insights and useful guidance for the detection of ppb-level acetone.

It is challenging for current metal oxides based acetone sensors to realize excellent ppb-level acetone sensing properties at low working temperature. Herein, the hollow ZnO/MoS2 nanosheets (HZnO/MoS2) core/shell heterogeneous structures are originally produced. Fast gas transport channels and n–p heterojunctions are produced by the decoration of MoS2 nanosheets on HZnO surface, resulting in high acetone response and fast response/recovery speed. Ultraviolet (UV) light is introduced to further improve acetone response and drastically reduce working temperature. The light diffraction and reflection caused by the decoration of hierarchical MoS2 nanosheets could significantly enhance light harvesting. Hence, the excellent acetone sensing properties are obtained by the synergistic effect of UV light and HZnO/MoS2 core/shell heterogeneous structures. Exactly, at 100 °C, the HZnO/MoS2 possesses a stable response (1.52) to 100 ppb acetone with UV irradiation while it exhibits no response to 100 ppb acetone without UV. Even at room temperature (30 ℃), UV activated HZnO/MoS2 still exhibits a stable response (∼1.33) and fast recovery/recovery time (19 s/97 s) to 50 ppm acetone. Furthermore, the DFT calculations are performed to demonstrate the underlying acetone sensing mechanism. The results demonstrate that UV activated HZnO/MoS2 heterostructure could achieve trace-acetone detection at low temperature.

The development of efficient oil/water separation membranes is highly significant for reducing severe environmental pollution nowadays. Constructing hierarchical structures with double-scale roughness and inverted trapezoidal geometry is an alternative rational design to fabricate low oil-adhesive membranes. Based on this design concept, a Cu mesh vertically wrapped with novel microphone-like Cu-CAT-1 hierarchical microstructures with double-scale roughness was tactfully prepared by a local concentration effect, which was referred to as CCMOF. The CCMOF possessed superior anti-oil-fouling performance and an outstanding water-absorbing capacity (180.3% larger than that of the pristine Cu mesh) due to its ingenious microphone-like structures and its high affinity for water. Therefore, it could separate various types of oil/water mixtures with a considerably high flux of over 25,000 L m−2 h−1 and an extremely low oil content in filtrate of below 10 ppm. The design concept of this work offered a simple idea to design low oil-adhesive membranes for efficient oil/water separation.

It is imperative to explore an accurate ppb-level acetone sensor for noninvasive detection of diabetes. In this work, two dimensional p-type MoS2 nanosheets are introduced on the surface of p-type ZnO derived from metal-organic frameworks (MOFs) to produce ZnO@MoS2 core/shell heterojunctions as a novel acetone sensor, showing a great enhancement of acetone response, about two orders of magnitude than that of pure ZnO derived from MOFs. For example, the ZnO@MoS2 exhibits about 80 times enhancement in response to 100 ppb acetone than that of pure ZnO. More importantly, this ZnO@MoS2 heterojunctions sensor exhibits an ultra-fast response/recovery to ultra-low concentration acetone (60 s/40 s @ 5 ppb), which is the best acetone sensing performance for the metal oxide-based materials reported to date. Moreover, the acetone sensing performances are negligibly affected by humidity and other gas, which is suitable for exhaled acetone detection. Finally, it is elucidated that the sharp increase of negative heterojunction interface resistance, ultra-fast gas diffusion rates in MoS2 nanosheets and strong interaction energy are key factors for the excellent acetone sensing properties of ZnO@MoS2. This work opens up a novel and efficient way for ultra-low concentration gas detection.

For the first time, Co-MOF-74 derived Co3O4/graphene heterojunction nanoscrolls (Co3O4/GNS) structure is synthesized by a simple one-step hydrothermal strategy and subsequent calcination process. Theoretically, the growth mechanism of Co3O4/GNS is revealed by molecular dynamic simulation, demonstrating that a high loading of Co-MOF-74 nanoparticles is needed for effective rolling of GO. The sensor based on Co3O4/GNS exhibits high response of 58.1 (34-fold enhancement compared with that of Co3O4), short response/recovery time of 12 s/66 s (157 s/200 s for Co3O4) toward 1 ppm acetone at 190 °C and superior selectivity, which are much better than those of most reported acetone sensors. Moreover, the Co3O4/GNS can even detect as low as 50 ppb acetone with a notable response (1.24). The appreciable response together with rapid response/recovery of Co3O4/GNS is attributed to the large heterojunction interface between Co3O4 and graphene nanoscroll and high-speed gas transfer pathways provided by nanoscroll. This unique structure design is believed to offer guidance for generalizable synthesis of metal oxide/GNS that can be effectively applied in the fields of ppb-level gas detection.