Keng Xu

Unique gas-sensing properties of semiconducting hybrids that are mainly related to the heterogeneous interfaces have been considerably reported. However, the effect of heterogeneous interfaces on the gas-sensing properties is still unclear, which hinders the development of semiconducting hybrids in gas-sensing applications. In this work, SnO2-SnS2 hybrids were synthesized by the oxidation of SnS2 at 300 °C with different times and exhibited high response to NH3 at room temperature. With the increasing oxidation time, the relative concentration of interfacial Sn bonds, O-Sn-S, among the total Sn species of the SnO2-SnS2 hybrids increased first and then decreased. Interestingly, it can be found that the response of SnO2-SnS2 hybrids to NH3 at room temperature exhibited a strong dependence on the interfacial bonds. With more chemical bonds at the interface, the lower interface state density and the higher charge density of SnO2 led to more chemisorbed oxygen, resulting in a high response to NH3. Our results revealed the real roles of the heterogeneous interface in gas-sensing properties of hybrids and the importance of the interfacial bonds, which offers guidance for the material design to develop hybrid-based sensors.

Recently, tin sulfide (SnS2) with different structures has been widely used for NH3 detection. However, the low sensitivity and high operating temperatures severely limit its potential application. Here, we demonstrate enhanced room temperature NH3 sensing using scalable SnS2 nanosheets, which are facily synthesized by chemical exfoliation. Structural and morphological characterization revealed that the as-prepared two-dimensional (2D) SnS2 is composed of 1–3 layers (0.6 nm–1.8 nm). Compared with the bulk SnS2 that shows none response to NH3, the gas sensors based on the as-prepared 2D SnS2 exhibit excellent ammonia gas sensing performance at room temperature. When exposed to 500 ppm ammonia, the response time, i.e., 16 s, is the shortest among all the NH3 gas sensors based on transition metal dichalcogenides (TMDs). We attribute this enhanced sensitivity mainly to the effective NH3 gas adsorption is dominated by the high energy defect sulfur vacancies on the surface of 2D SnS2. The easily synthesized 2D SnS2 with sulfur vacancies are expected to find a vast application in gas sensing.

Coupling graphene-based materials with SiC nanostructures is one of the most feasible strategies to improve the photocatalytic CO2 reduction performance. Here, we synthesized the vertical graphene (VG) sheets on the SiC nanowires achieved from rice husks to form the [email protected]/C composite, in which the VG sheets possess high surface area, excellent electrical conductivity, and chemical stability, and the SiC nanowires have a suitable bandgap for sunlight absorption. The resulting [email protected]/C composite provides an enhanced photocatalytic reduction of CO2 to CO and CH4, yielding CO and CH4 evolution yields of 25.5 and 2.3 μmol g−1 h−1, respectively. The achieved CO yield is maximal so far among the SiC-based photocatalysts. The defective VG sheets promote the absorption of sunlight, enhance the CO2 adsorption and activation, increase the separation of photogenerated electron-hole pairs, improve the activity of photocatalyst, and thereby prompt the photocatalytic reduction of CO2 to CO with high yield and selectivity.

Graphene (GR)-wrapped WO3 nanosphere composite was synthesized by using a facile sol–gel method. The morphology and structural properties of the GR-WO3 nanocomposites were characterized by field-emission scanning electron microscopy (FESEM), X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and Raman spectroscopy. The GR-wrapped WO3 nanospheres composite exhibits p-type gas sensing behavior and the response of GR-WO3 sensor toward NO2 shows a linear increase with an increase in the concentration from 7 to 56 ppm at room temperature. Upon exposure to 56 ppm NO2, its response value becomes 40.8%, but there is no responsiveness for the sensors based on pure WO3 and graphene sensors. The effective charge transfer through chemically bonded interfacial contact between graphene and WO3 nanospheres is proposed to be responsible for the room temperature sensing performance. This work may provide a new insight into the structural design of GR-base nanocomposites and has a potential prospect in the environment monitoring or disease detection of NO2 at room temperature.

Hierarchical porous (HP) nanostructures of metal oxide have been attracting increasing attention due to its fast response and high sensitivity in sensors application. However, the controllable synthesis of HP structures is rather complex and these fragile structures can be easily destroyed during fabrication process of sensors. To solve this problem, a novel integration of materials synthesis and sensors manufacture was successfully realized by introducing the topological transformation approach (TTF) on basis of a facile, low-cost, conventional process including screen printing and calcination. By employing this method, HP-SnO2 micro-rods assembled by nanoparticles were prepared in situ on the co-planar sensors' surface. The formation mechanism of HP-SnO2 was mainly attributed to a decomposition reaction followed by gas escaping process. As expected, the as-prepared HP-SnO2 sensor exhibited not only fast response (∼4.3 s), which was one-tenth of response time of the gas sensor based on SnO2 nanoparticles, but also high sensitivity (Ra/Rg = 3.86) to formaldehyde at 1 ppm. The excellent gas-sensing properties can be indeed ascribed to the HP structure which was favorable for gas diffusion and sensing reactions. This work renders great potential in the fabrication process of gas sensor with HP structure simply by a TTF method which can be further applied in indoor pollution detection.

Array-based sensors are considered as potential candidates for gas detection due to their low cost and great miniaturization potential. However, the fabrication process of array-based sensors is complex and time-consuming since it usually contains the formation and growth processes of seed layers on the surface of alumina substrates. In addition, the gas-sensing materials for the fabrication of array-based sensors are mainly confined to n-type semiconductors such as ZnO, TiO2 and WO3. In this work, a kind of p-type heterostructures arrays composed of NiMoO4 nanosheets and Co3O4 nanowire (Co3O4@NiMoO4) was fabricated in-situ on flat alumina substrates via a simple hydrothermal method without seed layers. SEM and TEM characterizations revealed that the Co3O4 nanowire arrays were fully covered with NiMoO4 nanosheets. The gas-sensing measurements revealed that the Co3O4@NiMoO4 composite arrays showed the highest response (Rg/Ra = 17.12) towards 100 ppm trimethylamine at its optimal operating temperature of 250 °C. This response value was 3.91 times higher than that of Co3O4 arrays (Rg/Ra = 4.39) and 9.25 times higher than that of NiMoO4 nanosheets (Rg/Ra = 1.85) at their optimal operating temperatures of 250 and 350 °C, respectively. Meanwhile, the enhanced sensing mechanism of the Co3O4@NiMoO4 composite arrays was also discussed. It could be explained by the special heterojunction structure of the Co3O4@NiMoO4 composite arrays, which offered a high surface area and an additional modulation in resistance. Our studies shed a new light to design p-type heterostructure arrays in-situ for fabricating sensing device by a facile method. Moreover, the as-designed Co3O4@NiMoO4 composite arrays are potential candidates in the fabricating of high performance trimethylamine sensors.

Here we report the synthesis of two-dimensional (2D) ultra-thin WO3 nanosheets (∼4.9 nm) with dominant {002} crystal facets through a facile surfactant-induced self-assembly method. It was found that the ultra-thin WO3 nanosheets showed remarkably enhanced xylene sensing performance and methyl orange photocatalytic degradation performance, which could be ascribed to the high percentage of reactive {002} crystal facets (>90%) and high specific surface area (121 m2/g). The mechanism of gas sensing and photocatalysis was systematically studied. This work will be intriguing for designing high-performance metal oxides-based gas sensing and photocatalytic materials through 2D structural modulation and crystal facets engineering, which is important to promote their practical applications in environmental issues.

Array-based sensors have been regarded as excellent candidates for the gas-sensing elements owing to their low-cost preparation, great miniaturization potential as well as stable performance. However, it still remains a great challenge to prepare porous arrays and regulate their porosity to increase pores for gas transport and active sites for surface reactions. To solve this problem, the porosity of metal oxide arrays (Co3O4) that had grown directly in-situ on the surface of ceramic substrates was regulated by the constructing and oxidizing of MOFs (ZIF-67) that coated on the surface cobalt-containing precursor arrays. It was found that the porosity and surface area of Co3O4 nanowire arrays increased with the content of ZIF-67. The porous Co3O arrays with the highest porosity and surface area exhibited the highest response value (Rg/Ra = 16.7), the lowest optimal operating temperature (200 °C) as well as the fastest response/recovery time (4/39 s) towards acetone. These porous Co3O4 arrays are thus promising gas-sensing materials for acetone detection with excellent performances. This novel strategy to enhance the porosity of arrays can pave a new way to improve the gas-sensing properties of array film-based sensors.

Resistive gas sensors based on metal oxides have aroused great interest in the sensing of NO₂ gas due to their low cost, good stability, and easy fabrication.

The discovery of gas sensing properties of single-crystalline nanostructures comparable or even better than their polycrystalline counterparts has triggered the attention of the gas sensor research community. To eleborate the sensing mechanisms of single-crystalline and polycrystalline nanostructures, the single-crystalline (SC) NiO hexagonal nanosheet and the nanoparticle self-assembled polycrystalline (PC) NiO nanosheet were prepared for room-temperature NO2 sensing studies. The gas sensing studies revealed that the SC NiO exhibited a remarkably higher response to NO2 at room temperaure than the PC NiO. The correlation between the structural features of NiO and the room temperature NO2 sensing performances was discussed in detail via the grain boundary scattering theory. It was found that the scattering potential played a vital role in the sensing process. Because the absence of grain boundary in the SC NiO reduced the NO2 chemisorption-induced carrier scattering at interfaces, the SC NiO performed the largely enhanced sensitivity. Here we hope that this work can help us to further understand the gas sensing mechanism and open up a new generation of gas sensors.

Hierarchical porous (HP) nanostructures of p-type metal oxide have attracted great attention in gas-sensing application due to their less agglomerated configurations and the advantages for gas diffusion. However, the contacts between grain boundaries around pores of HP nanostructures are usually too fragile to resist the shear force caused by ultrasonic treatment during sensor fabrication processes and the thermal stress produced from high operating temperature. To solve this problem, uniform Co3O4 microspheres assembled by single-crystalline porous nanosheets were synthesized by an ethylene glycol (EG) mediated solvothermal method with the co-assistant of water and polyvinyl pyrrolidone (PVP). Due to the structural stability of single-crystalline nanosheets, the as-synthesized HP Co3O4 is well kept during the sensor fabrication processes. Moreover, the Co3O4 shows a high response (Rg/Ra = 74.5–100 ppm) to xylene at the optimized temperature (150 °C), which is almost 4 times larger than that of commercial Co3O4 (19.1). It also exhibits excellent long-term stability with small deviations (less than 6%) for two months, which can be ascribed to the structural stability of single-crystalline nanosheets. Combined with their high selectivity and low detection limit, the as-prepared HP Co3O4 microspheres assembled by single-crystalline porous nanosheets are highly promising gas-sensing materials for xylene detection.

In recent years, there has been increasing interest in synthesis of reduced graphene oxide (rGO)-metal oxide semiconductor (MOS) nanocomposites for room temperature gas sensing applications. Generally, the sensitivity of a MOS can be obviously enhanced by the incorporation of rGO. However, a lack of knowledge regarding how rGO can enhance gas-sensing performances of MOSs impedes its sensing applications. Herein, in order to get an insight into the sensing mechanism of rGO-MOS nanocomposites and further improve the sensing performances of NiO-based sensors at room temperature, an rGO-NiO nanocomposite was synthesized. Through a comparison study on room temperature NO2 sensing of rGO-NiO and pristine NiO, an inverse gas-sensing behavior in different NO2 concentration ranges was observed and the sensitivity of rGO-NiO was enhanced obviously in the high concentration range (7-60 ppm). Significantly, the stimulating effect of rGO on the recovery rate was confirmed by the sensing characteristics of rGO-NiO that was advantageous for the development of NO2 sensors at room temperature. By comprehending the electronic interactions between the rGO-MOS nanocomposite and the target gas, this work may open up new possibilities for further improvement of graphene-based hybrid materials with even higher sensing performances.

In this paper, novel flower-like Co3O4 structures have been fabricated by an ethylene glycol (EG) mediated solvothermal with the co-assistant of water and polyvinyl pyrrolidone (PVP). It was found that the water and PVP played a key role in the formation process. The structure and morphology of the as-prepared products were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM). The results revealed that the flower-like Co3O4 was assembled by single-crystalline porous nanosheets. Due to the structural stability of single-crystalline nanosheets, the flower-like hierarchical porous structures can be well kept during the sensor-fabrication processed. Notably, gas-sensing tests revealed that the flower-like Co3O4 showed a high response (Rg/Ra = 79.8–100 ppm) to xylene at its optimized temperature (150 °C), which was 4 times higher than that of commercial Co3O4 (Rg/Ra = 19.1). Moreover, the flower-like Co3O4 also exhibited excellent long-term stability with small deviations (3.1%) for two months. The enhanced gas-sensing performances of flower-like Co3O4 were mainly attributed to the structural stability of single-crystalline porous nanosheets as building blocks. Combined with its high selectivity and low detection limit, the as-prepared novel flower-like Co3O4 assembled by single-crystalline porous nanosheets is a highly promising gas-sensing material for xylene detection.

Developing high-efficiency electrocatalysts based on non-precious transition metal compounds alternative to noble metal for oxygen evolution reaction (OER) is crucial for renewable energy storage technologies. Herein we designed a novel OER electrocatalyst by incorporating transition metal oxide (TMO) and sulfide (TMS) into a hierarchical nanoporous structure. Hierarchical porous Co3O4 nanosheet-assembled microspheres (HPMS Co3O4) were fabricated by a facile solvothermal method followed by an annealing process, and the Co3O4/CoS2 composite was easily obtained by a simple subsequent sulfurization process, which maintained the HPMS structure. Due to rapid surface diffusion and mass transfer, increased active sites, improved conductivity and strong synergetic coupling effects, the prepared HPMS Co3O4/CoS2 composite showed significantly enhanced electrocatalytic OER activities with a current density of 10 mA cm−2 at a low overpotential of 280 mV, a small Tafel slope of about 63 mV dec−1 and satisfactory stability at 20 mA cm−2. Notablely, it was non-durable at high current density. We expect that more active TMS functionalized hierarchical porous TMO will open a new avenue to innovate non-precious OER electrocatalysts.

Novel one-dimensional shish-kebab structures composed of active (110)-faceted α-MnO2 nanowires as backbones and (111)-faceted Co3O4 nanocrystals as shells were prepared by a hydrothermal method followed by an in-situ epitaxial attachment growth strategy. A series of characterization revealed the formation of Co3O4@MnO2 heterostructures with closely integrated hetero-interfaces. The growth mechanism study suggested that the multiple functions of hexamethylenetetramine were crucial for the the successful growth of the Co3O4@MnO2 heterostructures. The gas sensing performance of the Co3O4@MnO2 heterostructures was studied for the first time. It was shown that the Co3O4@MnO2 heterostructures exhibited obvious enhanced triethylamine sensing performance compared with bare MnO2 nanowires and Co3O4 nanocrystals, suggesting a strong synergistic effect. The unique one-dimensional shish-kebab structures, increased surface active oxygen species and closely-attached hetero-interface were the key factors to improve the triethylamine sensing performance. The work not only provide an in-situ strategy for the synthesis of one-dimensional heterostructures, but also provide an effective way to improve the inherently low gas response of α-MnO2 through comprehensive design of morphology, crystal facet and hetero-interface. Moreover, thanks to the unique morphology, surface and interface characteristics of the Co3O4@MnO2 heterostructures, they are expected to be used in more fields, such as batteries, supercapacitor and catalysis.

Because of their ultra-high surface area, large porosity and excellent structural tailorability, metal-organic frameworks (MOFs) are considered as outstanding candidates among sensing materials for hazardous gas detection. However, most of MOFs-based sensing films show weak film adhesion and low conductivity due to the poor formation ability of MOFs films by traditional sensor fabrication methods as well as the intrinsic insulating character of these MOFs. In this work, we propose a novel strategy to directly grow robust gas-sensing films based on pristine MOFs arrays (ZIF-67 nanosheets) in-situ on the surface of ceramic substrates. To improve the conductivity of MOFs arrays, anion-exchange method is applied to couple Prussian blue analogue (PBA) on the surface of ZIF-67 arrays. Structural characterization revealed that this permutation reaction can significantly improve the conductivity of the MOF films while their sheet-like structures can be mainly remained. Benefiting from the robust structure and improved conductivity, the as-designed ZIF-67/PBA films exhibited superior sensing performances such as good reproducibility, high response value (Rg/Ra = 11.7), and fast response/recovery speed (5/182 s) towards triethylamine. This work provides a new strategy to fabricate MOFs gas sensors and paves a new way to modulate the conductivity of MOF films.

The formation of p–n junctions has attracted much attention for improving the gas-sensing performances in the field of gas detection. Although the p–n heterojunctions with a defined interfacial contact are crucial in investigating their roles on gas-sensing properties, the design of such well-defined p–n heterojunctions has been sparse until now. Herein, a p–n heterostructure composed of well-defined {001}-faceted Co3O4 as backbones and Fe2O3 nanorods as shells was synthesized using a simple hydrothermal method without the requirement of seed crystals. During the reaction, it was demonstrated that the acetic acid holds the key to the epitaxial growth of Fe2O3 nanorods on the surface of Co3O4 nanocubes. The response of this as-designed Fe2O3@Co3O4 composite (Ra/Rg = 134.9) toward 100 ppm triethylamine was about 6 times higher than that of pristine Co3O4 (Ra/Rg = 23.1) and 5 times higher than that of pristine Fe2O3 (Ra/Rg = 28.9). The highly boosted gas-sensing performances were attributed to the positive effects of the heterointerface on the gas adsorption process that were revealed through the first-principles method based on the defined heterointerfacial contacts, as well as the enlarged signal of gas–solid reactions through the heterojunctions. This work not only provides a strategy to improve gas-sensing performances but also provides guidance for the investigation of the effects of p–n heterojunctions on gas-sensing properties by designing interfacial contact with defined crystal facets.

Light assistance and construction of heterojunctions are both promising means to improve the room temperature gas sensing performance of MoS2 recently. However, enhancing the separation efficiency of photo-generated carriers at interface and adsorption ability of surface have become the bottleneck problem to further improve the room temperature gas sensing performance of MoS2-based heterojunctions under light assistance. In the present study, a novel direct Z-scheme MoS2/SnO2 heterojunction was designed through crystal facets engineering and its room temperature gas sensing properties under light assistance was studied. It was found that the heterojunction showed outstanding room temperature NO2 sensing performance with a high response of 208.66 toward 10 ppm NO2, together with excellent recovery characteristics and selectivity. The gas sensing mechanism study suggested that high-energy {221} crystal facets of SnO2 and MoS2 directly formed Z-scheme heterojunction, which could greatly improve the separation efficiency of photo-generated carriers with high redox capacity. Moreover, {221} facets greatly enhanced adsorption ability towards NO2. This work not only opens up the application of Z-scheme heterojunctions in gas sensing, which will greatly promotes the development of room temperature light-assisted gas sensors, but also provides a new idea for the construction of direct Z-scheme heterojunctions through crystal facets engineering.

MnO2 is considered to be one of the promising electrode materials for supercapacitors thanks to its ultra-high theoretical capacitance value, but its actual electrochemical performance is not ideal due to its low electrical conductivity and poor stability. Herein, we find that the supercapacitor performance of graphene quantum dots (GQDs)@MnO2 composite is superior to that of pure MnO2 electrode. The GQDs@MnO2 composite is obtained by a highly efficient one-step hydrothermal method, in which KMnO4 reacts with graphene oxide to produce MnO2 nanosheets anchored with GQDs in a short time. The GQD@MnO2 electrode presents high specific capacitance of 246 F g−1 at a scan rate of 1 mV s−1 in Na2SO4 electrolyte, and the as-assembled asymmetric supercapacitor (GQDs@MnO2//activated carbon) exhibits superior energy density of 29.9 Wh kg−1 at power density of 538.0 W kg−1, and good cycling performance (81.3 % retention after 8000 cycles) that was far better than that of pure MnO2-based supercapacitor. The excellent supercapacitor performance of GQDs@MnO2 composite results from its enhanced electrical conductivity, good wettability and abundant available contact sites for aqueous electrolyte, which are ascribed to the intrinsic high electrical conductivity as well as the quantum confinement and edge effects of GQDs.

Metal oxide quasi-one-dimensional (quasi-1D) nanostructures have a very good gas-sensing performance due to their large surface area and porous structures with a less agglomerated configuration. However, the well-designed fragile nanostructures could be easily destroyed during the conventional fabrication process of gas sensors. Herein, we presented a novel materials-sensor integration fabrication strategy: on basis of screen printing (SP) technology and calcination, micro-injecting (MI) was introduced into the fabrication process of sensors, which was named as SPMIC, to obtain In2O3 nanowire-like network directly on the surface of coplanar sensors array by structure replication from sacrificial carbon nanotubes (CNTs). The obtained In2O3 nanowire-like network exhibited an excellent response (electrical resistance ratio Ra/Rg), about 63.5, for100 ppm formaldehyde at 300 °C, which was about 30 times larger than that of compact In2O3 nanoparticles film (non-network film). The enhanced gas-sensing properties were mainly attributed to the high surface-to-volume ratio and the nanoscopic structural properties of materials. Furthermore, the SPMIC could be employed not only in the preparation of other metal oxide nanowire-like network, but also in the fabrication of coplanar gas sensors arrays on the required sites with different materials.

In this work, gas-sensing materials based on Co3O4/Fe2O3 nanocubes were prepared via a MOF hybrid-assisted approach with the aid of ion-exchange. Structural characterizations proved the as-prepared Co3O4/Fe2O3 nanocubes possessed hollow porous structures. The formation mechanisms were also investigated. Gas-sensing measurements revealed that the Co3O4/Fe2O3 nanocubes showed high response (Rg/Ra = 3.27) towards acetone, which was 3.06 times higher than that of Co3O4 nanocubes (Rg/Ra = 1.07). The enhanced sensing mechanism of Co3O4/Fe2O3 nanocubes were also discussed, which could be explained by synergistic effects aroused by the as-formed p-n heterojunctions and hollow porous structures that offered numerous diffusion channels and an additional modulation in resistance. Our studies shed a new light to design p-n naoncomposites by a facile MOF hybrid-assisted method. Moreover, the as-designed Co3O4/Fe2O3 nanocubes have potential application for fabricating high performance acetone sensors.

Low conductivity of metal-organic frameworks limits their applications in electrochemical energy storage and conversion. To overcome this shortcoming, we synthesized the zeolitic imidazolate framework-67 on the vertical graphene (VG) surface with abundant defects grown on carbon cloth (ZIF-67-VG-CC). Based on "one-for-two" strategy, the Co3O4 nanoparticles (Co3O4-VG-CC) and nanoporous carbon (NC-VG-CC) can be derived from ZIF-67-VG-CC via selective pyrolysis in the controlled atmospheres. In the hierarchically structured Co3O4-VG-CC and NC-VG-CC, the Co3O4 nanoparticles with a size of 20 nm and N-doped porous carbons were uniformly dispersed on the VG surface, as confirmed by electron microscopies. Both the Co3O4-VG-CC and NC-VG-CC yield high specific capacitance and excellent rate capability. After assembling these two electrodes to make an asymmetric supercapacitor, the Co3O4-VG-CC//NC-VG-CC delivered a maximum energy density of 43.75 Wh/kg at a power density of 5.2 kW/kg as well as excellent cycling stability with 91.5 % specific capacitance retained after 20,000 charge-discharge cycles. All these results should be ascribed to that the VG sheets possess their high electrical conductivity and 3D network structure, and provide the hierarchical supports for the uniform distribution of the ZIF-67-dervied Co3O4 nanoparticles and NC materials, which are beneficial to the fast electron and ion transfer in the electrodes.

Vertical graphene (VG) sheets, which consist of few-layer graphene vertically aligned on the substrate with three-dimensionally interconnected porous network, make them become one of the most promising energy storage electrodes, especially for SCs. Nevertheless, the intrinsic hydrophobic nature of pristine VG sheets severely limited its application in aqueous SCs. Here, electrochemical oxidation strategy is adopted to increase the hydrophilicity of VG sheets by introducing oxygen functional groups so that the aqueous electrolyte can fully be in contact with the VG sheets to improve charge storage performance. Our work demonstrated that the introduction of oxygen functional groups not only greatly improved the hydrophilicity but also generated a pseudocapacitance to increase the specific capacitance. The resulting capacitance of electrochemically oxidized VG for 7 min (denoted as EOVG-7) exhibited three orders of magnitude higher (1605 mF/cm2) compared to pristine VG sheets. Through assembled two EOVG-7 electrodes, a symmetric supercapacitor demonstrated high specific capacitance of 307.5 mF/cm2, high energy density of 138.3 μWh/cm2 as well as excellent cyclic stability (84% capacitance retention after 10000 cycles). This strategy provides a promising way for designing and engineering carbon-based aqueous supercapacitors with high performance.

Controllable crystal facets exposing is an efficient approach for enhancing gas sensing performances of semiconductor nanomaterials. In order to study the crystal-facets-dependent gas-sensing properties of TiO2 systematically and exactly, a series of anatase TiO2 nanocrystals with different percentage of high-energy {001} crystal facets exposure (from 6% to 91%) were designed and synthesized by a simple hydrofluoric acid-assisted hydrothermal method. Gas sensing tests suggested that gas response of the TiO2 nanocrystals increased with the percentage of high-energy {001} crystal facets, although the specific surface areas gradually decreased. The results unquestionably confirmed the enhanced gas sensing activity of anatase TiO2 high-energy {001} crystal facets. Moreover, it was found that surface fluorination of high-energy {001} crystal facets played a negative role on the gas sensing activity. The relevant sensing mechanism was discussed in detail based on combination of experimental characterization and first-principle calculations, which showed that gas adsorption properties made a great contribution to gas sensing performance. By providing a comprehensive understanding of the crystal-facets-dependent gas-sensing properties of TiO2, the present work will be helpful for the designing of TiO2-based gas-sensing materials.

As inspired by a novel materials-sensor integration fabrication strategy, nanowire-like network (NWN) structured ZnO, Co3O4, In2O3 and SnO2 films are synthesized directly onto an alumina substrate by the screen printing combined with micro-injecting and calcination (SPMIC) to fabricate a flat-type coplanar gas sensor array based on these four sensors. The gas-sensing properties of the sensor array are investigated at different operating temperatures (150–350 °C), and the best selectivity is interestingly found among the four indoor pollution gases including toluene (C7H8), formaldehyde (HCHO), acetone (CH3COCH3) and ammonia (NH3) at 300 °C. This is also confirmed by the principal component analysis (PCA) result that the four gases can be discriminated and recognized by the sensor array. The enhancement in the selectivity is attributed to the combination of special nanostructures with large surface area and sensor array based on the four different metal oxides. Furthermore, the SPMIC method is demonstrated to be an effective route not only to synthesize NWN structured metal oxides directly onto an alumina substrate without being destroyed, but also to fabricate coplanar sensor array based on various NWN structured metal oxide films.

Nanocomposite materials with excellent receptor and transducer functions are promising in ameliorating their gas sensing properties. However, due to the abrupt changes of receptor and transducer functions when different components are combined together, structural engineering that considers both the receptor and transducer functions to design such desirable sensing materials still remains a great challenge. Here, a nanocomposite material composed of 1D ZnO nanorods and 3D Co3O4 microspheres assembled by single-crystalline porous nanosheets has been designed, which was inspired by the high-efficiency receptor-transducer-response structure of venus flytraps. The as-designed ZnO/Co3O4 composite exhibited high response (Ra/Rg = 125 to 100 ppm ethanol) which was 84 times and 8 times higher than those of Co3O4 (Rg/Ra = 1.43) and ZnO (Ra/Rg = 15). The excellent sensing properties are ascribed to the as-designed flytrap-like structure which possesses a super receptor function from 1D ZnO with a large surface area, p-n heterojunctions with an amplified response signal, as well as excellent transducer functions from single-crystalline porous Co3O4 with fast charge transport channels. This strategy provides us with new guidance on the exploration of high-performance gas sensors which could further extend to other bio-structures that are abundant in nature.

Design and control of crystal facets is a promising way to optimize the gas sensing activity of metal oxides semiconductors. To study the crystal facets effects on the gas sensing properties, a series of anatase TiO2 with designed crystal facets were synthesized by hydrothermal routes. The gas sensing properties of the three most important low-index crystal facets ({010}, {001} and {101} facets) of anatase TiO2 were systematically studied and compared for the first time. It was found that {010} facets exhibited a surprisingly higher sensing activity in comparison with that of {101} facets and {001} facets, and the gas sensing activity order were determined to be {010} facets >{001} facets >{101} facets. The crystal facets-dependent gas sensing mechanism was investigated based on theory calculations from the atomic and electronic scale. It was found that the excellent gas adsorption properties and electron transfer properties both facilitated the gas response of {010} facets. The present results not only provide a new way to improve the gas response of TiO2 by exposing of {010} facets, but also deepen the understanding of crystal facets-dependent gas sensing properties of TiO2, which will be useful for the design of metal oxides gas sensing materials.

α-MoO₃@NiO nanocomposite with well-defined core@shell P–N heterojunction nanobelts was prepared which exhibited heterointerface-determined acetone sensing characteristics.

In this work, porous NiCo2O4 nanosheets anchored on the surface of 1D WO3 nanofibers were synthesized via a hydrothermal route followed by a facile chemical deposition treatment. The as-obtained products were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and Brunauer-Emmett-Teller analysis. The porous NiCo2O4 nanosheets grew on the surface of WO3 nanofibers with an intimate contact between their interfaces, leading to the formation of p-n heterojunctions and an increase of surface area. Compared with the pristine WO3 nanofibers and NiCo2O4 nanosheets, the NiCo2O4/1D WO3 nanocomposite exhibited the highest response to xylene (Rg/Ra = 15.69 toward 100 ppm). Moreover, the NiCo2O4/1D WO3 nanocomposite also demonstrated outstanding selectivity, excellent stability and low detection limitation (~ 5 ppm) to xylene vapor. The main mechanism for the enhanced gas-sensing properties of NiCo2O4/1D WO3 nanocomposite was attributed to the unique porous structures and p-n heterojunctions between NiCo2O4 nanosheets and WO3 nanofibers. The excellent gas-sensing properties of NiCo2O4/1D WO3 nanocomposite make it a good candidate as a gas-sensing material for the practical application in xylene detection.

Anatase TiO2 hierarchical microspheres were synthesized by a facile hydrothermal method. Characterization results indicated that the microspheres were consisted of massively aggregative nanothorns with truncated tips, which had exposed {001} and {101} crystal facets, and there were abundant mesoporous structures in the microspheres. A possible growth model of the TiO2 hierarchical microspheres was put forward based on a series of experimental analysis. When used for gas sensing, it was found that the TiO2 hierarchical microspheres showed a distinct high sensitivity towards acetone, the optimal response to 100 ppm acetone was 14.6, which was much higher than that of other materials reported in previous works. Moreover, the TiO2 hierarchical microspheres exhibited a fast response/recovery speeds (<10 s), a low detection limit (the response still reaches 6.1 even at a low acetone concentration of 10 ppm) and an excellent selectivity. Through systematical analysis, it can be concluded that the enhancement of gas sensing performance of the TiO2 hierarchical microspheres was ascribed to its unique structural features (perfect hierarchical mesoporous structure as well as specific crystal facets exposing), which could be described as structurally enhanced gas sensing performance. The present results encourage us to further investigate crystal facets-dependent gas sensing properties of TiO2 for the designing of conductometric gas sensors with better sensing performance.

Recently, graphene nanomesh (GNM) has attracted great attentions due to its unique porous structure, abundant active sites, finite band gap and possesses potential applications in the fields of electronics, gas sensor/storage, catalysis, etc. Therefore, diverse GNMs with different physical and chemical properties are required urgently to meet different applications. Herein we demonstrate a facile synthetic method based on the famous Fenton reaction to prepare GNM, by using economically fabricated graphene oxide (GO) as a starting material. By precisely controlling the reaction time, simultaneous regulation of pore size from 2.9 to 11.1 nm and surface structure can be realized. Ultimately, diverse GNMs with tunable band gap and work function can be obtained. Specially, the band gap decreases from 4.5-2.3 eV for GO, which is an insulator, to 3.9-1.24 eV for GNM-5 h, which approaches to a semiconductor. The dual nature of electrophilic addition and oxidizability of HO(•) is responsible for this controllable synthesis. This efficient, low-cost, inherently scalable synthetic method is suitable for provide diverse and optional GNMs, and may be generalized to a universal technique.

Gas-sensing properties of metal-oxide nanocomposites are largely depended on their structures and compositions. It remains great challenges to design well-defined heterostructures to improve gas-sensing properties and establish the relationship between these properties with heterostructures. In this paper, an extremely sensitive ethanol sensing material via decoration of NiCo2O4 nanosheets onto 1D α-MoO3 nanorods by a facile chemical deposition method was developed. The structures and compositions of the as-prepared sensing materials were characterized by XRD (X-ray diffraction), SEM (scanning electron microscopy) and TEM (transmission electron microscopy). It was revealed that uniform porous NiCo2O4 nanosheets grown on the surface of α-MoO3 nanorods backbones with a low lattice mismatch at the interface of heterostructures. Gas-sensing tests showed that this novel NiCo2O4/α-MoO3 nanocomposite exhibited superior sensing performances for ethanol with high response, negligible cross-responses to other interfering gases and excellent stability. The response to 1 ppm ethanol was about 20 (Rg/Ra), whereas pure NiCo2O4 and α-MoO3 showed almost no response. Notably, the detection limit for ethanol was as low as 50 ppb (Rg/Ra = 2.7). The enhanced sensing properties were attributed to the unique structures and heterojunctions between NiCo2O4 nanosheets and α-MoO3 nanorods. The results above indicated the potential of this as-designed NiCo2O4/α-MoO3 nanocomposite to detect sub-ppm level ethanol in a highly sensitive manner.

Crystal plane engineering is considered to be an effective means to improve the surface activity of catalysts recently. Nevertheless, study related to crystal plane dependent electrocatalytic activity for hydrogen evolution reaction (HER) is still scarce. Here, two NiS2 nanocrystals with different crystal planes ({1 1 1} planes and {1 0 0} planes) were designed and controllably prepared. It was demonstrated that the NiS2 nanocrystals with {1 1 1} planes showed an enhanced HER activity (an overpotential of 138 mV to achieve a current density of 10 mA/cm2) compared with the NiS2 nanocrystals with {1 0 0} planes (an overpotential of 302 mV to achieve a current density of 10 mA/cm2). Through first-principles calculation based on density functional theory, mechanism of the crystal plane dependent electrocatalytic activity was studied in detail. It was found that the {1 1 1} planes of NiS2 had much higher surface energy and better H adsorption capacity compared with that of {1 0 0} planes, which both contributed to the enhanced HER activity. The present study provides a typical case for crystal plane dependent HER electrocatalytic activity, which is of great significance for deep understanding the HER mechanism and accurate designing of high efficiency HER electrocatalysts.

Well-oriented Co3O4 nanowire arrays were synthesized in-situ on Al2O3 substrates via a simple hydrothermal method without seed layers. The phase structure and array morphology of Co3O4 nanowire arrays were investigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was revealed that the array density of Co3O4 nanowire arrays could be controlled by the concentration of ammonium fluoride. The gas-sensing measurement revealed that the array structure on the surface of gas sensors exerted great impact on their performances. It was found the response value enhanced and then decayed with the increased array density of Co3O4 nanowires. We concluded that the Co3O4 nanowire arrays with moderate array density can possess highly exposed effective surface area to provide more pathways for gas diffusion than other samples. Our studies can provide a significant guidance on array design to fabricate high performance gas sensors. Moreover, the as-designed Co3O4 nanowire arrays have potential application in trimethylamine (TEA) detection.

In this work, novel n-type heterostructures of ZnSnO3 modified p-type Co3O4 (Co3O4–ZnSnO3) nanowire arrays were fabricated in-situ on flat alumina substrates via a simple hydrothermal method without seed layers. SEM and TEM characterizations revealed that the Co3O4 nanowire arrays were fully covered by ZnSnO3 nanosheets. The gas-sensing measurement revealed that the Co3O4–ZnSnO3 composite arrays showed the highest response 5.57 (Rg/Ra) towards ethanol, which was 2.04 times higher than that of Co3O4 nanowire arrays (Rg/Ra = 2.73). Meanwhile, the enhanced sensing mechanism of Co3O4–ZnSnO3 composite arrays was also discussed, which could be explained by the special heterojunction structure arrays that offered an additional modulation in resistance. Our studies shed a new light to design p-n heterostructure arrays in-situ sensing device by a facile method. Moreover, the as-designed Co3O4–ZnSnO3 composite arrays have potential application for fabricating high performance ethanol sensors.

In this work, Co3O4 nanowire arrays are prepared in-situ on the flat alumina substrates via a simple hetero-epitaxial growth without adding seed layers. It is found that the density of Co3O4 nanowires can be regulated by varying the concentration of NH4F that acts as substrate activation promoting the formation of nuclei on alumina substrate. In order to improve the gas-sensing performance, porous NiO nanosheets are anchored on the surface of Co3O4 nanowire arrays to form a novel heterostructure (Co3O4/NiO). Gas-sensing tests indicate a higher response value of these array composites towards ethanol (Rg/Ra = 4.26) than that of pristine Co3O4 nanowire arrays (Rg/Ra = 1.27) and NiO nanosheets (Rg/Ra = 1.89). The improved gas-sensing performances resulting from the special array structures and novel heterojunctions can provide abundant diffusion channels for gas molecules as well as a synergistic effect between Co3O4 and NiO.

The development of high-performance carbon-based anodes for Na-ion batteries is highly desired but still remains challenging because of carbon materials with a low reversible capacity and poor cyclic performance. Herein, novel S-doped carbon nanosheets (SCNs) were prepared by a hydrothermal self-assembly process in the presence of graphene oxide (GO) as the matrix, starch as the carbon source, and dibenzyl disulfide as the sulfur source. The obtained SCNs with hierarchical pores and a sandwich-like structure were utilized as anode materials for Na-ion batteries, exhibiting a high reversible discharging capacity of 207.3 mAh g–1 after 100 cycles at 50 mA g–1. When the current density is up to 1 A g–1, a reversible discharge capacity of 118.8 mAh g–1 can also be acquired. Moreover, the prominent long-term cycling stability of more than 500 cycles can be obtained at 200 mA g–1. The outstanding electrochemical property (high reversible capacity, high rate performance, and long-term cycling stability) of the SCN electrode may be due to the synergistic effect of S doping, hierarchical pores, and the sandwich-like structure. Furthermore, electrochemical kinetic analysis also confirmed that the sodium storage mechanism of the SCN electrode reinforced pseudocapacitive-control behavior. The present study not only shows a high-performance anode material for Na-ion batteries but also provides a new method to prepare S-doped carbon materials for various applications.

Crystal facet engineering and graphene modification are both effective means to improve the gas sensing performance of metal oxide semiconductors (MOSs) currently. However, research on the crystal facet effect and synergistic effect of graphene modification of p-type MOS sensors is relatively lacking. Here, p-type Co3O4 nanocrystals with controlled crystal facets ({112} and {100}) were in situ wrapped in the two-dimensional (2D) nanosheet network of graphene. It was found that bare {112} facets showed a significantly higher triethylamine sensing performance than {100} facets, implying a strong crystal facet effect. Interestingly, the triethylamine sensing performance of {112} facets was significantly improved after rGO modification, while the performance improvement of Co3O4 {100} was limited after rGO modification. Further study suggested that {112} facets contained more active Co3+ species and chemically adsorbed oxygen species than {100} facets, which promoted the adsorption of triethylamine and the subsequent sensing reaction. In addition, the strong electronic interaction between Co3O4 {112} crystal facets and rGO promoted efficient charge exchange through the heterogeneous interface. This work provides a new way to improve the gas sensing performance of Co3O4 through the synergistic effect of crystal facet engineering and graphene modification.

In this work, Co3O4 enclosed with specific facets that anchored with Fe2O3 nanorods was prepared by a facile hydrothermal method. To reveal the role of heterojunctions on their gas-sensing performances, the exposed facets of Co3O4 crystals were varied from nanocubes enclosed with six {1 0 0} (denoted as Fe2O3/C-Co3O4) to truncated nanocubes enclosed with six {1 0 0} and eight {1 1 1} (denoted as Fe2O3/TC-Co3O4). It revealed that the growth of Fe2O3 on the surface of Co3O4 nanocrystals can remarkably improve their gas-sensing performances. The response value of Fe2O3/TC-Co3O4 can reach as high as 318.7 (Ra/Rg) towards 100 ppm triethylamine (TEA) which is 6–7 and 1.6 times higher than its pristine counterparts and Fe2O3/C-Co3O4, respectively. The highly facet-dependent gas-sensing performances of these p-n heterostructures were contributed to the effect of hetero-contact on charge transfer as well as gas adsorption which were confirmed by First-principles calculation. This study opens an avenue to investigate the effect of p-n heterojunctions on gas-sensing properties by designing interfacial contact with defined crystal facets, as well as guidance for the preparation of sensing materials with super performances.

Due to the special three-dimensional structures and excellent physicochemical properties, vertical graphene (VG) has been extensively investigated as potential material for supercapacitors. However, achieving VG-based supercapacitors with high-energy density and high-power density is a still tremendous challenge. Here we attempt to synthesize large-scale VG films on flexible substrates and design a novel battery-like supercapacitor (BSCs) with the VG on Ni foil (VG@Ni) as a binder-free electrode operating in the KOH electrolyte with the redox additives of K3Fe(CN)6 and/or K4Fe(CN)6. The VG@Ni electrodes demonstrate an ultrahigh areal capacitance of 1453 mF cm−2 at 5 mA cm−2 in 1 M KOH electrolyte with adding 0.07 M K3Fe(CN)6 and 0.07 M K4Fe(CN)6, coulombic efficiency greater than 90%, and long-life cycling stability (capacitance retention is 99.3% after 20000 charge-discharge cycles). The BSCs, which are assembled with two identical VG@Ni electrodes, deliver the areal capacitance of 231 mF cm−2 with energy density of 32 μWh cm−2 and power density of 2498 μW cm−2 at 1 mA cm−2. These outstanding performances can be ascribed to the special feature and excellent properties of VG materials, high electronic conductivity of binder-free VG@Ni electrode, faradaic properties of redox electrolyte, and synergistic effects between the VG film and redox electrolyte.

In this paper, 1D ZnO nanofibers were selected as the backbones, which were prepared by a hydrothermal route. Afterward, porous NiCo2O4 nanosheets with large surface area were further grown on the surface of single-crystal ZnO nanofibers via a chemical bath deposition. The structures and compositions of the as-prepared gas-sensing materials were characterized. It was found that uniform porous NiCo2O4 nanosheets were anchored on the surface of ZnO nanofibers backbones with a low lattice mismatch at the interface of heterostructures. Gas-sensing tests showed that this novel [email protected]2O4-shell nanocomposite exhibited improved gas-sensing performances towards methanol, such as high response, low detection limit and excellent stability. The response to 5 ppm methanol was about 1.96 (Rg/Ra), whereas pristine NiCo2O4 and ZnO showed negligible response. The enhanced gas-sensing properties were attributed to the unique core-shell heterojunctions between NiCo2O4 nanosheets and ZnO nanofibers resulting in an improved receptor function as well as transducer function.

Gas sensing performance of hierarchical porous (HP) nanostructured metal oxide can be improved by microstructure modulation. However, these microstructure-modulated HP structures are too delicate to stand the destruction from conventional fabrication processes of gas sensors. To solve this problem, effective surface area and ratio between different phases of HP structures based on topological transformation were modulated by sintering treatment in situ on coplanar sensors’ surface. A strong correlation between effective surface area, crystal structures and gas-sensing properties was revealed. During sintering temperature from 400 °C to 650 °C, gas response increased to formaldehyde with increasing effective surface area, while decreased from 650 °C to 800 °C since the influence of phase transformation from orthorhombic SnO2 to rutile SnO2 overcame the change of effective surface area. A conclusion that the gas sensing performance of orthorhombic SnO2 was superior to that of rutile SnO2 was thus carried out. This was mainly attributed to the loose building block ([SnO6]8−) of orthorhombic SnO2 which could produce much oxygen vacancies, as verified by XPS, PL and EPR. In brief, this work shed a light on the process of sensor design to obtain gas sensing material with microstructure-modulated HP structure.

Carbon-based materials are the most advantageous candidates for anode in sodium-ion batteries (SIBs). Nevertheless, their practical utilization has been hampered by their low reversible capacity and poor cyclic performance. In this study, novel double-shell and hierarchical porous N-doped carbon nanocages (DHNCNs) were prepared by a soft template-assisted strategy in the presence of ZIF-8 as the matrix, dopamine hydrochloride as the nitrogen and carbon source, and F127/TMB as the soft template. The study found that the carbonization temperature has a significant impact on the surface area, pore volume, interlayer distance, defect concentration, and N content of DHNCNs, thereby altering their electrochemical performance. The DHNCNs-900 synthesized at 900 °C exhibit the best sodium-ion storage performance, including high discharge capacity of 235.5 mAh g−1 after 100 cycles at 50 mA g−1, outstanding rate performance of 209.4 mAh g−1 at 2.0 A g−1, and excellent long-term cycling stability of 171.3 mAh g−1 after 900 cycles at 500 mA g−1. The exceptional electrochemical performance of the DHNCNs-900 electrode is induced by synergistic effects of the double-shell, hierarchical porous, N-doped, and hollow structure. Additionally, the electrochemical kinetic analysis indicates that DHNCNs-900 electrode possesses a reinforced sodium-ion storage mechanism by the pseudo-capacitive-control behavior. In conclusion, this study presents a superior anode material for SIBs and exhibits a simple method for synthesizing N-doped carbon nanocage materials, holding promise for advanced energy storage systems.

In vivo 3D ultrasound imaging with 2D-array transducers is of great importance for both clinical application and biomedical research, but it is complicated in fabrication and also very expensive in hardware due to thousands of electronic channels. In this work, we demonstrate a new fabrication process of 7-MHz 128 + 128 elements row-column-array (RCA) transducer with relaxor ferroelectric PMN-0.28PT single crystal. With piezoelectric single crystal and improved acoustic matching, the optimized performance of -6 dB bandwidth of ∼82 % and insertion loss of -44.6 dB is achieved. The axial and lateral imaging resolutions at different depth of the RCA transducer are quantified by the point spread function (PSF), and the results are respectively 0.20 mm and 0.41 mm at the depth of 7.7 mm, and 0.22 mm and 0.47 mm at the depth of 16.7 mm. The transducer is validated experimentally on a hyperechoic phantom, and 3D view and slices of B-mode images are obtained. The experimental results indicate that our developed RCA transducer can obtain high-quality 3D ultrasound images, demonstrating great potential on ultrafast 3D and functional imaging.

Fabrication of a self-cleaning anti-reflection coating that can degrade gaseous pollutants and applied contaminants to maintain long-term transmittance property.

Proper morphology, surface and interface structure designing are required to obtain efficient gas sensing and photocatalytic materials. In the present work, ultra-thin NiO single-crystalline porous nanosheets with dominant {1 1 1} crystal facets (denoted as SP-NiO) and hierarchical NiO porous microspheres (denoted as HP-NiO) supported highly dispersed Pt nanoparticles with controllable sizes were designed and synthesized. Their gas sensing and photocatalytic performance were investigated. It was found that both the formaldehyde sensing and methyl orange photocatalytic degradation performance were greatly enhanced by decorating Pt nanoparticles on SP-NiO, while Pt nanoparticles decoration contributed little to the improved photocatalytic performance of HP-NiO. The results indicated that surface structure of the NiO support could also produce significant impact on the activity of Pt/NiO heterojunctions. Moreover, Pt decorated SP-NiO with stable structure showed a marked long-term stability with negligible attenuation of gas sensitivity (less than 5%) for 45 days, while Pt decorated HP-NiO exhibited obvious attenuation of gas sensitivity (more than 30%) due to the structural collapse. The work not only offers promising materials for gas sensing and photocatalytic application, but also brings new dawn for the designing of efficient p-type metal oxides gas sensing and photocatalytic materials through the synergistic effect of single-crystalline porous structures modulation, crystal facets engineering and facet-selective deposition of highly-dispersed Pt nanoparticles.

One-dimensional NiS2 nanotube arrays and nanorod arrays are controllably grown on Ni foam surface. The electrocatalytic test shows that the NiS2 nanotube arrays require competitive overpotentials of 209 mV for HER and 367 mV for OER (to achieve a current density of 50 mA/cm2), respectively, which are much lower than the NiS2 nanorod arrays and other NiS2 nanostructures reported before. Specifically, the NiS2 nanotube arrays can be employed as an efficient bi-functional catalyst for overall water splitting, with a low cell voltage (1.58 V) to deliver a current density of 10 mA/cm2. The outstanding performance can be attributed to the special structural characteristics of nanotubes, which have high specific surface areas along with abundant active sites. The present study not only enriches the morphology of NiS2 nanostructures for highly efficient electrocatalytic reaction, but also provides an interesting self-assembly path for the synthesis of one-dimensional NiS2 nanostructures.

Crystal facets engineering and graphene hybridizing have been proved to be effective means to improve the photocatalytic activities of semiconductor photocatalysts in recent years. However, most of these efforts are concentrated in metal oxides. In the present study, crystal facets effect on the photocatalytic activity of metal sulfide NiS2 was studied for the first time. It was found that the {111}-faceted NiS2 nanocrystals showed improved photocatalytic activity in the degradation of various typical pollutants in water compared with {100}-faceted NiS2 nanocrystals. Moreover, through hybridizing with rGO nanosheets, the photocatalytic activity of the {111}-faceted NiS2 nanocrystals can be further improved, resulting in the complete degradation of heavy metal hexavalent chromium and organic dyes. The photocatalytic mechanism was studied in detail through theory calculation and experimental characterization. It was found that both the surface energies of Ni-terminated and S-terminated {111} facets were much higher than that of {100} facets, indicating that {111} facets were more active. Besides, rGO hybridizing can realize the effective separation of photogenerated electrons and holes. The results provide important guidance for the further development of efficient metal sulfide photocatalysts.

Recently, developing economical electrocatalysts with high performance in water decomposition has become a research hotspot. Herein, two kinds of cobalt-hybridized Cu3P nanostructure array electrocatalysts (including highly mesoporous 2D nanosheets and sugar gourd-like 1D nanowires) were controllably grown on a nickel foam substrate through a simple hydrothermal method combined with a subsequent phosphating treatment method. An electrocatalytic test indicated that the as-prepared 2D nanosheet array exhibited excellent activity and stability toward hydrogen evolution reaction under alkaline conditions, which offered a low overpotential of 99 mV at 10 mA/cm2 and a small Tafel slope of 70.4 mV/dec, whereas a competitive overpotential of 272 mV was required for oxygen evolution reaction. In addition, the 2D nanosheet array delivered a low cell voltage of 1.66 V at 10 mA/cm2 in a symmetric two-electrode system, implying its huge potential in overall water decomposition. The electrocatalytic performance is superior to the as-prepared 1D nanowire array and most of the Cu3P-related electrocatalysts previously reported. Experimental measurements and first-principles calculations show that the excellent performance of the 2D nanosheet array can be attributed to its unique 2D mesoporous structure and hybridization of cobalt, which not only provide a large electrochemically active surface and fast electrocatalytic reaction kinetics but also weaken the binding strength of electrocatalytic reaction intermediates. The present study provides a simple and controllable approach to synthesize Cu3P-based bimetallic phosphide nanostructures, which can be used as boosting Janus electrocatalysts for water decomposition.

Understanding the physics that correlate strain and physical properties of quantum dots (QDs) is crucial for technology applications. In this paper, GaAs QDs confined in Al2O3 matrix are synthesized using the pulsed laser deposition method and rapid thermal annealing technique. It is revealed that the confined GaAs QDs experience compressive strain during the growth process. The strain can be used to improve and tailor the optical properties of confined GaAs QDs by engineering the bandgap and thus the photoluminescence emission band to a distinct wavelength. These findings presented here can engineer the properties of GaAs QDs for potential application in optoelectronic and photonic devices.

Hydrogen passivation can be used to improve and tailor the optical properties of confined Au nanoparticles by engineering the strain and interfacial defects of the confined Au nanoparticles.

In the present work, SnS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> single layer was first synthesized by Li-intercalation method and well characterized. TEM results showed the graphene-like structure of the as-obtained SnS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and XRD results showed well c-oriented thin film. For room temperature sensing of NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , the ultrathin SnS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> exhibited rapid response and high sensitivity at room temperature. The potential gas sensing mechanism is also discussed.

Selective synthesis of Fe2O3 nanomaterials with desired phase-structure is of great importance for broadening and improving their industrial applications. In this paper, the structure-phase transformation in the as-prepared samples with different annealing process is studied, trying to find a facile route to fabricate α- and γ-Fe2O3 nanoparticles by inducing external strain in a well control mode, which is of great importance for the design and applications of the iron oxide. The high-resolution transmission electron microscope, X-ray photoelectron spectra and Raman spectra analysis clearly demonstrate that the long-term thermal annealing and rapid thermal annealing can result in the formation of α- and γ-Fe2O3 confined in Al2O3 matrix, respectively. The magnetic properties display that the residual magnetization and coercive force values for the γ-Fe2O3 nanoparticles are much higher than those for the α-Fe2O3 nanoparticles, which supplies an enhanced magnetic route by selective synthesis of confined Fe2O3 nanoparticles with desired strain-induced phase-structure and paves the ways for the applications of Fe2O3 nanoparticles in magnetic nano-devices.

We describe a simple, cost-effective approach for surface modification of various substrates with SnO2 nanoparticles by controlled hydrolysis, which is not confined on specific chemical states of the surface. Specifically, these SnO2 nanoparticles were loaded onto the surfaces of carbon-based materials (graphene), ionic crystals (tin sulfide), polymer materials (silk fiber) and biological materials (yeast) with uniform distribution, despite the great differences in surface chemistries. Moreover, the formation mechanism of the SnO2 nanoparticles has been discussed, confirming that the process described can be easily implemented and adapted to other systems. These as-synthesized nanocomposites are expected to have wide applications in the fields of gas sensing, photocatalysis and biomaterials.

It is well known that the face-centered cubic (fcc) Fe is thermodynamically unstable at ambient conditions. In this paper, we theoretically and experimentally demonstrate that thermodynamically stable fcc Fe nanocrystals can be induced by external strain at room temperature. Fe nanocrystals confined in non-magnetic Al2O3 matrix are fabricated using pulsed laser deposition method and rapid thermal annealing technique. During the growth process, the confined Fe nanocrystals experience net deviatoric strain from the Al2O3 matrix, which can modify the microstructures of the confined Fe nanocrystals and lead to the formation of thermodynamically stable fcc Fe nanocrystals (space group Fm-3m) at room temperature. First-principles calculations also demonstrate that the strained Fe nanostructure with fcc phase is more thermodynamically stable. A typical behavior of weakly interacting Fe nanocrystals is observed, characterized by a superparamagnetic regime and a blocking of particle moments centered at TB (∼9 K).

In this study, one-dimensional (1D) zinc oxide was loaded on the surface of cobalt oxide microspheres, which were assembled by single-crystalline porous nanosheets, via a simple heteroepitaxial growth process. This elaborate structure possessed an excellent transducer function from the single-crystalline feature of Co3O4 nanosheets and the receptor function from the zinc oxide nanorods. The structure of the as-prepared hybrid was confirmed via a Scanning Electron Microscope (SEM), X-ray diffraction (XRD), and a Transmission Electron Microscope (TEM). Gas-sensing tests showed that the gas-sensing properties of the as-designed hybrid were largely improved. The response was about 161 (Ra/Rg) to 100 ppm ethanol, which is 110 and 10 times higher than that of Co3O4 (Rg/Ra = 1.47) and ZnO (Ra/Rg = 15), respectively. And the as-designed ZnO/Co3O4 hybrid also showed a high selectivity to ethanol. The superior gas-sensing properties were mainly attributed to the as-designed nanostructures that contained a super transducer function and a super receptor function. The design strategy for gas-sensing materials in this work shed a new light on the exploration of high-performance gas sensors.

Abstract In this paper, a consensus based fully distributed optimization algorithm is proposed for solving economic dispatch problem (EDP) in smart grid. Since the incremental cost of all buses reach consensus when the optimal solution is achieved, it is selected as a consensus variable. An additional variable at each bus, called “surplus” is added to record the local power mismatch, which is used as a feedback variable to purse the balance between power supply and demand. Different from most of the existing distributed methods which require the communication network to be balanced, the algorithm uses a row random matrix and a column random matrix to precisely steer all the agents to asymptotically converge to a global optimal solution over a time‐varying directed communication network. Due to the use of a fixed step size, the proposed algorithm also outperforms other algorithms in terms of convergence speed. The graph and eigenvalue perturbation theories are employed for the algorithm convergence analysis, and the upper bound of the parameters required for convergence is given theoretically. Finally, the performance and scalability of the proposed distributed algorithm are verified by several case studies conducted on the IEEE 14‐bus power system and a 200‐node test system.

The construction of composites based on metal oxides with exposed high-energy facets is very significant in a wide range of applications including gas detection, catalysis, and energy storage. However, the synthesis of such composites is always hindered by the smooth exposed surface of metal oxides, which is difficult to nucleate and grow a second component. To solve this problem, a novel metal–organic framework-assisted method was proposed to anchor ZnO nanoparticles on {221} facets of SnO2 octahedrons simply by a coating and oxidation process of ZIF-8 on the smooth {221} surface. It was found that a high nucleation energy results from an appropriate ratio of Zn2+ and 2-methylimidazole, giving priority to the heterogeneous nucleation and growth process for ZIF-8 on the surface of SnO2 octahedral nanoparticles. The coverage of ZnO on the smooth surface can also be modulated by the ZIF-8 film. Thanks to the newly designed composites with special structure, the gas-sensing performances of {221} SnO2 were improved extensively, whose response toward 100 ppm triethylamine (TEA) can be increased more than triple times from 5.68 to 18.37 (Ra/Rg) by the combination of ZnO nanoparticles. This intensively improved gas-sensing performance was attributed to the special structure with extra sensitive depletion layers at the heterojunction as well as the single-crystalline feature of SnO2 octahedral nanoparticles. These composites are thus promising gas-sensing materials for TEA detection with excellent performances. More significantly, it can also pave a new way to combine metal oxides with exposed high-energy facets, providing a unique and effective means for enhancing the properties and functionality of materials in a range of fields.

Multi-energy microgrid (MMG) is a promising form of energy system, which can integrate various forms of energy and provide an important means to accommodate renewable energy sources. However, its internal complex energy structure and equipment coupling relationship also bring challenges to its operation optimization. To minimize operating costs, an energy management strategy is proposed in this paper considering integrated demand response. On the supply side, energy storage devices are used to realize the flexible transfer of energy. On the demand side, a integrated demand response model including interruptible loads and shiftable loads is constructed, and a response compensation mechanism is proposed. On this basis, with the goal of mini-mizing the sum of the system operating cost and the response compensation cost, comprehensively considering the schedulable resources on both sides of supply and demand, a multi-energy complementary MMG optimization model is established. The simulation results show that the proposed energy management strategy can effectively improve the energy supply flexibility and operational economy of the system.

To avoid the destruction of hierarchical porous (HP) structures during fabrication process of sensors, a novel integration of materials synthesis and sensors manufacture was successfully realized by introducing the topological transformation approach (TTF) on basis of a facile, low-cost, conventional process including screen printing and calcination. By employing this method, HP-SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> micro-rods assembled by nanoparticles were prepared in situ on the co-planar sensors' surface. The as-prepared HP-SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> sensor exhibits not only fast response (~4.3 s), which is one-tenth of response time of the commercial one, but also high sensitivity (Ra/Rg = 3.86) to formaldehyde at 1 ppm.

Electric vehicles (EVs) have become potential re-sources on demand-side. This paper constructs a charge and discharge management platform for collecting EV data, fitting to probabilistic function, and instructing EVs' charging/discharging by 3 different strategies. The strategies are: disordered charging, ordered charging, ordered charging and discharging. In these strategy models, the peak-valley-shoulder periods' division and EV owners' expected charging state are taken into consideration. Markov Chain Monte Carlo method is applied to simulate the models and results show that the management of ordered charging/discharging is helpful for peak regulation, expenses reduction, and energy saving.

With the development of smart grid, power grid security monitoring is becoming more and more important. Non-intrusive load monitoring is helpful to understand the operating status of electrical equipment and is of great significance to the economic and safe operation of the power grid.This paper provide a non-intrusive load safety monitoring method for smart grid based on clustering algorithm. Based on improved generalized likelihood ratio detection, this paper introduces the voting window to establish an event detector model, and introduces event detection metrics to determine the value of related parameters and obtain the best event detector. In view of the difficulty in distinguishing electrical appliances with similar power in load decomposition, this paper uses DFT to extract the harmonic characteristics of bus current signals, and establishes a load feature library combined with active power. Then affinity propagation clustering algorithm is used to establish the load feature library to realize the load decomposition. Finally, the effectiveness of the proposed method is verified on REDD data sets.